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I am a high school student and I am very confused in one thing in optics (ray optics) which I think is the most basic thing but didn't find any answer on internet, before I ask let me present one thing because only if this is true my question will be understood.

this image shows a room having a single point source of light

My understanding :

I think when we switch on a light bulb in a dark room,

The light rays from it gets bounced off from walls and they crosses each other at several points in space of the room and these points are behaving like point sources and light rays are not coming directly from source but from these points in space that's why a room gets illuminated, I mean not only the walls but the space of the room also gets brighten up.

And that's only because of these points in space where light rays coming from many objects crosses each other.

If my understanding is correct then let's take a look at my query:

this image shows two point source of light

As we can see, in above image there are two point sources (for practical purposes I am considering it as point sources because all the light rays from them are diverging) you can see that :

There are several points in space where light rays from them are crossing each other in space, it means they are getting mixed (by mixed I mean if the light rays from "different" objects focus on one single point on retina when rays from them will enter eye rather light rays from each object separately), but still we can distinguish both sources separately and at the same time we can also see the illuminated space around them.How is it even possible? How we can see the brighten space as well as distinguish both sources?

I have taken only two sources but in actual life, there are many objects in room which reflects off light and all of their rays are getting mixed in space before entering eyes still our eyes (or brain) is able to distinguish which rays are coming from which object.How?

For me, everything should look like a uniform mess of illumination like this:

uniform mess of brightness

I appreciate the efforts made by people{for me the most important part is discussion ,not the solution} but for those who still doesn't get a clear view of what I am asking(as it seems some people are still not very clear in understanding what I am asking) I am uploading one more picture , I am just asking when we see any object why the ray diagram can't look like the second one in this image [![this image shows rough ray diagrams when we see any object]this image shows ray diagrams of when we see any object

Arun Bhardwaj
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    "and these points are behaving like point sources and light rays are not coming directly from source but from these points in space that's why a room gets illuminated," why do you think places where the light rays intersect are themselves sources? surely the light just passed through each other like any other linear wave. – shai horowitz Jun 12 '22 at 12:03
  • I already know what u are talking about, but we can see brighten space i.e the points in space where light rays are crossing each other just like we see any real image. so by saying that light has to come directly from a physical object then only we will be able to see it is wrong..........unfortunately u didn't understand what my question really is, please try to think because this is the best I can present my question – Arun Bhardwaj Jun 13 '22 at 12:11
  • I have called them as point source, just so that people can imagine the picture I have in my brain'the light rays are just crossing each other{I have shown two but in reality there are many from different -different sources) and if my eyes are pointing in that direction these rays will enter into my eyes ,,,so we see those collection of points which appears continuous[illuminate space}my whole point is how our brain know which rays are coming from which object?or how does it decides in what way should it focus light rays in order to distinguish objects?as we know we see all objects separately – Arun Bhardwaj Jun 13 '22 at 16:20
  • to be more precise, how does our brain know that it should focus the light rays only in that way so that the images of those two point sources will form separately over the retina? also how we can see both the brighten space and at the same time identify two objects/sources separately? because that can only happen if our brain already knows there are two objects there and it has to focus light in such a way that we will see the brighten space as well as those two objects – Arun Bhardwaj Jun 13 '22 at 16:26
  • because if my eyes are focusing the points present in space{i,e focusing light rays coming from different points,} then how it can simultaneously focus the light rays coming from two sources seperately? i,e they can either get focused combinedly or they can focus on the retina separately , how both are happening? – Arun Bhardwaj Jun 13 '22 at 16:28
  • please focus on my last line,,,,I think if the light rays coming from two objects are combinedly focusing on the retina{ which is happening here }, we should only see the point from which they are crossing and then each object should lose its individual identity .....but what we actually see is both the illuminated space as well as both objects separately – Arun Bhardwaj Jun 13 '22 at 16:32
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    You say "the light rays from it gets bounced off from walls [true] and cross each other at several points in space and these points are behaving like point sources [false] and light rays are not coming directly from source but from these points in space that's why a room gets illuminated [false], I mean not only the walls but the space of the room also gets brighten up [false] ." and similar "we can see brighten space i.e the points in space where light rays are crossing each other just like we see any real image." but what everybody here tries to tell you: This is just wrong. – Koschi Jun 14 '22 at 15:51
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    Drop this conclusion of emerging point sources due to 'ray crossing' or whatever, and your question resolves. Down to a certain resolution your retina can resolve the original source of the light ray, and it does not matter how many other rays this one crossed on its way. Also remember that you cannot see light rays 'from the side' (sorry for this sloppy formulation)... At night we only see the sunlight reflecting at the Moon and the other planets, we do not see the rays that travel from the Sun to these objects. – Koschi Jun 14 '22 at 15:51
  • I know we can't see light rays from the side simply because they are NOT entering our eye,if u want me to drop the conclusion that the space surrounding the objects also gets brighter i.e a rooms gets illuminated,then tell me why do we see my room SPACE{even if there is vaccum and no air,, as brighter if I turn on the bulb ,,when the bulb was off the space was dark and nothing was visible,this observation is not wrong just turn on light bulb and you will see the brightness around in the space and if the source is too intense it will be actually difficult for you to determine bulb size – Arun Bhardwaj Jun 15 '22 at 13:35
  • @Koschi if still u didn't get it,,,,just tell me if those points cannot behaving like point sources then how do we even see a REAL IMAGE OF ANY OBJECT? according to your arguement we shouldn't see it because rays are crossing each other in space,,,,,but we do see it and its a fact,,,,,how can our brain know if the rays are coming from object or those intersections in space?,,,it doesn't matter if there is a physical object there or not,,,if diverging light rays are entering eyes,,they will converge at some points on retina and our brain will produce a output – Arun Bhardwaj Jun 15 '22 at 13:40
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    @ArunBhardwaj Please remain calm and friendly, in some your comments you tell people 'they are not getting it' in a manner to comes across as rude. – Koschi Jun 15 '22 at 14:12
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    I still do not understand why you think intersection points of light rays could be visible in any way. If you turn on a light bulb in a room, you can see the light bulb and reflections from the walls. Nothing else. There is no 'brightness around in the space' that you could see. If the room would have perfectly absorbing walls (perfectly black walls), the room would still be dark and you would only see the bulb. – Koschi Jun 15 '22 at 14:15
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    If your question actually relates to how the eye can distinguish different light sources, since rays from all over the place reach your eye (i.e. light rays from different sources INTERSECT right at your eye, which is true)... then you should read about the basics of optics and lenses. If we would have no lenses than it would be hard to distinguish different sources because the angular resolution of our eyes would be strongly reduced. – Koschi Jun 15 '22 at 14:19
  • @Koschi I am sorry if I sounded rude in any way, Your last comment pretty much relates to what I wanna ask but still I am confused that how our brain already knows that ,these light rays are coming from a single point ,,so I will converge it over a single point on retina and those which are intersecting and coming from different -different point I will not converge them over a single point because I already know which light rays are coming from which part......this is what I am asking as all light rays from all objects are randomly going anywhere – Arun Bhardwaj Jun 15 '22 at 16:00
  • please have a look at not exactly same but similar question which have perfect diagrams to understand what I am asking: https://physics.stackexchange.com/questions/658655/why-does-an-image-only-form-where-light-rays-coming-from-a-single-point-get-refl – Arun Bhardwaj Jun 15 '22 at 16:03
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    Light does not effect other light. It passes through it. It does not collide and propagates the same as if it were not there. – jensen paull Jun 18 '22 at 18:20
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    @jensenpaull please take a look at the link I have given above ,,I am not at all saying light gets collide or anything like that – Arun Bhardwaj Jun 21 '22 at 07:13
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    @ArunBhardwaj take care that our eyes are not just screens. They are screens (retinas) PRECEDED BY lenses. The lenses deconvolve the rays coming from distinct points (at different angles) into unique points on the screens. This is a special property of lenses. – Jagerber48 Jun 21 '22 at 08:41
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    @ArunBhardwaj in the second diagram of the last image you posted in the edit the eye lens does not converge light rays as a lens would do. Lenses converge light from different points to different points. Apart the case the object is out of focus which I discussed below. – Mattia Jun 21 '22 at 13:52
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    @ArunBhardwaj: Yes, as in your hand sketch, if the eye is focused the plane that contains A and B, A and B will be focused to points on the retina, and if the eye is focused to the plane that contains O and P, O and P will be focused on the retina. The difference is, for focus on plane OP, there are many other points on that plane where light rays from A and B will pass through OP and be focused to other points on the retina, ie, a blur. If the eye is focused on the AB plane, their light is focused to two points, and if focused to OP, the light is unfocused. Same light, the lens changes. – tom10 Jun 21 '22 at 19:04

8 Answers8

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The basic point about light, either as described in optics by classical electromagnetic waves, or by quantum physics as a multiplicity of photons, is that light does not interact with light. In optics optical rays showing the direction of light waves go through each other, they are just a geometric model.

Light interacts with electric and magnetic fields of matter, the air in the room for example. If the room is full of smoke you just see the smoke and not the images the light would carry because it has interacted with the molecules of smoke and lost its information of direction.

See the introduction here.

If you study physics further you will see that light waves do not "wave" through a medium, the way water waves move over water. They EM waves are just sinusoidal variations of electric and magnetic fields in the directions of the light ray.

light

The fields making up the wave will not interact with fields of an other ray.

anna v
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    I don't think this answer got the main point of the question. It's not about interaction of light rays but about how the eye can distinguish the direction (and the object) from which a light ray come. – Mattia Jun 19 '22 at 10:22
  • @Mattia, yes you understood the question whereas all the responses already have a picture in mind from the books that the rays only converge like the way they have shown,,,and they don't want to even give a second thought on the question – Arun Bhardwaj Jun 21 '22 at 07:22
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    @Mattia the way the question is stated it talks as if there is intersection of light rays,"How we can see the brighten space ", there is no brightness in space if there are no (or few) molecules for the light rays to interact with. It should be a biophysics tag, perception of light. https://www.thehealthboard.com/what-is-light-perception.htm – anna v Jun 21 '22 at 11:07
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for me everything should look like a uniform mess of illumination like this: . . . shows an incorrect diagram.

A ray is the path taken by light.

Assume that without a change in refractive index light travels in straight lines.
That where light rays cross there is no "interference" between the rays.
In other words one ray is totally ignorant of what the other ray is doing and when rays cross they do not deviate in direction.

I have added to your second diagram to show a selected number of rays from the two sources passing through a convex lens and hitting a screen, which could be your eye with the retina as the screen) to form real images $A',B'$ on the screen of sources $A,B$.

enter image description here

In spite of rays crossing over what is seen on the screen is two distinct images of the sources.
Where you have placed white circles to show where rays cross makes no difference as far as the passage of the rays is concerned, they carry on travelling in the same direction as before.
Rays reflected/scattered off walls will produce similar images of the walls.

The photographs show the result of @EdV using two laser pointer beams, red and green, intersecting without interacting, inside a large plexiglass rod. The green laser is attenuated with a ND 2.0 filter and the plexiglass just facilitates seeing where the beams are.

enter image description here
enter image description here

Farcher
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  • please have a look at this question https://physics.stackexchange.com/questions/658655/why-does-an-image-only-form-where-light-rays-coming-from-a-single-point-get-refl In your ray diagram you have purposely converged the rays from distinct points separately wheras I am saying the points in the space where light rays from two objects are crossing can also converge the same way on the retina right? so if that's the case we would see that light rays COMING FROM TWO DIFFERENT OBJECTS ARE GETTNG CONVERGED OVER A SINGLE POINT ON RETINA AND WE WOULD NOT BE ABLE TO DISTINGUSH – Arun Bhardwaj Jun 21 '22 at 07:26
  • CONTINUING: WE WOULD NOT BE ABLE TO DISTINGUISH WHICH RAYS ARE COMING FROM WHICH OBJECT{ITS DISTANCE, ITS DIRECTION AND ALL INFROMATION ABOUT OBJECT} – Arun Bhardwaj Jun 21 '22 at 07:27
  • with lasers all rays are parallel and light rays from two sources are not intersecting at all,,,that's why we don't see those points in space,,,,,,use the software I have used,,,and track ALL the rays passing from converging lens you will see those points are also getting converged – Arun Bhardwaj Jun 21 '22 at 07:29
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    There is a place on my diagram where one of my "green" rays crosses with one of my "yellow" rays but they are travelling in different directions so end up at different places on the screen. If those rays were to have ended up at they same place on the screen they must have changed direction as they crossed ie by being scattered off a dust particle. – Farcher Jun 21 '22 at 08:39
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    I have written an answer to your other question which might help you understand what is going on? – Farcher Jun 21 '22 at 09:15
  • (positive) Lenses have two very important properties for this to work (and the fact they have those properties depends on their shape): (1) All rays coming from one point passing through a lens will RECONVERGE at another point on the other side of the lens and (2) The convergence point for rays from one point source is at a DIFFERENT POINT IN SPACE than the convergence point for rays from another point source. It is true that rays from the two point sources will cross on both sides of the lens (as shown in the diagram), but the convergence points are always distinct. – Jagerber48 Jun 21 '22 at 13:40
  • Please explain the -1 down vote. – Farcher Jun 21 '22 at 21:29
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enter image description here

In the image above you can see that light coming from different directions is focused in different places on the retina by the lens, so your brain can distinguish different objects in space.

Further references:
http://ffden-2.phys.uaf.edu/211_fall2013.web.dir/jessica_garvin/retina_color.htm
https://en.wikipedia.org/wiki/Retina#Spatial_encoding

Mattia
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    this is completely vague diagram, its just showing those rays from top and bottom part which are not crossing each other. – Arun Bhardwaj Jun 15 '22 at 13:42
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    @ArunBhardwaj If the rays cross (exactly in the lens or before entering the eye) it's the same: the lens (a part of the eye) transmits them in different parts of the retina. Because as pointed out in the answer above two rays of light crossing don't interact. – Mattia Jun 15 '22 at 14:37
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    think logically, your brain doesn't know if there is any physical object or not unless is sees it,,,,,so why it will converge light rays from discrete points onto discrete points over the retina? why can't it take say for e.g two light rays FROM DIFFERENT POINTS and converge it over a single point over the retina? – Arun Bhardwaj Jun 15 '22 at 15:53
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    because it's not the brain the converges the light rays, it's the eye lens that works, well, like a lens. And lenses converge light rays with different directions to different points. – Mattia Jun 15 '22 at 16:15
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    brain decides what I wanna see and what I don't ,,,say for example you are focusing on the wall in front of you,,,light rays from all other parts are still entering eye but are not getting focused over retina{like your nose} but it will still look blurry,....have a look at my this question to know more details: of what I am asking even though its not exactly same its quite similar .https://physics.stackexchange.com/questions/658655/why-does-an-image-only-form-where-light-rays-coming-from-a-single-point-get-refl – Arun Bhardwaj Jun 15 '22 at 16:22
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    I'm not sure to understand what you are asking with this comment. Anyway your brain decides the distance of the object whose light rays are correctly focused by the lens, light rays from objects at other distances are directed to the retina by the lens but not at focus so you see them blurry. – Mattia Jun 15 '22 at 16:33
  • It's hard to imagine a worse image for this question. Artificially and counter-factually, this image is not paying any attention at all to how the rays enter the eye. It shows the rays going outside the aperture of the pupil, with the image floating in space in the eyeball and not on the retina, and, most importantly, the rays entering the eye are not intersecting at the lens (when that's the key element for the OP's question). It's like the artist knew the stop sign should be inverted, and that's the only thing they understood. – tom10 Jun 18 '22 at 16:26
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    @tom10 I think instead that the image shows the key element of the questions that is how rays that reach the eye in the same point are directed to different places on the retina to form the image. Anatomic details are not what this question is about. – Mattia Jun 19 '22 at 09:52
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    @tom10 yes tom,,,you have understood what I wanna ask,,,in the image they have shown they already know which rays "they" have to choose in order to forcefully claim the bookish diagrams to be right – Arun Bhardwaj Jun 21 '22 at 08:03
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    @Mattia,,,,,When rays are not focused on the retina,,,it means they are hitting at different points rights,,,here comes the question: 1)it indirectly means they are converging with the rays coming from different parts i,e the rays from a single point is not converging at a single point but getting mixed with the rays from other points while converging onto retina...2)how we can determine the depth of a point even when the rays are not focused ,,,we see it blurry I know as a fact,,but still we can determine how far it is how? – Arun Bhardwaj Jun 21 '22 at 08:08
  • @ArunBhardwaj I think that is because when you look around you are watching objects whose distances don't differ so much, so when you put your focus at a certain distance rays coming from farther or nearer objects are still sent to different part of the retina. There could be some small overlap just at the edges of the objects but this doesn't interfere with your vision. – Mattia Jun 21 '22 at 13:23
  • @ArunBhardwaj I think you you can determine the distances because despite you see some objects blurry your brain can interpret the image and determine roughly the distances. For example if you see two apples quite aligned on table you find the furthest looking at the order in which they are placed. And moreover you see through two eyes so your mind can do a kind a triangulation to figure out distances. – Mattia Jun 21 '22 at 13:36
  • @ArunBhardwaj To convince you that the change of focal length of the eye lens is not so important for vision, people with cataracts or myopia get implanted a lens with fixed focal length: https://en.wikipedia.org/wiki/Intraocular_lens – Mattia Jun 21 '22 at 14:04
  • @Mattia,,,even when I see with one eye and still rays are not focused ,,I can quite accurately determine which object is far and which is not,, so there are several factors at play. In my last diagram,,I wanna say lens is a lens,,its doesn't know if the light rays are coming from distinct points or not,,,,so the second diagram in last image can be true,,and in that case we should see those points in space and everything should be a mes – Arun Bhardwaj Jun 24 '22 at 11:50
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    I must admit, also referring to the other question you linked, that this is a good question and I know no answer. You could try to ask a new question explaining clearly the question and linking the last answer to the other question asking if it's right. – Mattia Jun 25 '22 at 22:48
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In short: light rays emanating from an object carry two pieces of (mathematical) information. An angle and a position.

When an input light ray hits a lens the angle and position of the output ray depend on both the angle and position of the input ray. For example, a ray colinear with the optical axis passes through undeflected while a ray parallel to the optical axis, but off axis, will deflect by an angle depending on how far off axis it is and the focal length of the lens.

This helps explain why multiple rays emanating from a single object and hitting a lens at different positions and angles can focus all rays down to a single point.

Likewise, it explains why separate rays from two different objects that hit a lens at the same position (but different angles) or that hit your eye with the same angle (but different position) can get focused down to different points behind the lens.

Finally, take care that you recall that the human eye (as well as most cameras) consist of TWO optical elements. A lens followed by a detector screen (the retina in the case of your eye or a ccd or cmos sensor in the case of a digital camera.) If instead you only had a screen then you would only see position information about the rays and indeed any point source would illuminate the entire screen rendering any two point sources entirely indistinguishable.

The answer to your question: we can distinguish objects using light rays because the lens in our eye works to convert the angles and positions of rays from distinct objects into distinct spots on the retinas in our eyes.

Note that this property of lenses (focusing light from distinct objects onto distinct points) only works for certain shaped refractive materials. In particular it works for a lens that has an approximately parabolic shape. Fortunately a spherical shape approximates a parabola well enough for this property to be realized. But if you instead had, for example, a triangular or quartic or something lens this property would be more and more spoiled (this is related to optical aberration). Note also, of course, that the distance between the screen and lens compared to the focal length matters as well.

Jagerber48
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  • Your method of deciphering the information in theory is valid, but the eye definitely doesn't do that. It's a pinhole camera, meaning only light from a certain angle enters at a certain spot. – jensen paull Jun 21 '22 at 10:24
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    @jensenpaull I’ve never heard that before. Do you have a reference? My understanding of a pinhole camera is that (1) you have an aperture that is at most tens or maybe one hundred times the wavelength of light (less is better for sharper images I think?), this would be 10’s of microns, not a few millimeters and (2) there is definitely no lens. Definitely not a tunable focus lens with a focal length close to the distance between entrance pupil and retina… – Jagerber48 Jun 21 '22 at 12:33
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    Just read more about pinhole cameras. Looks like optimal pinhole size for visible light and about 25 mm distance between aperture and screen would be about 150 micron. At least an order of magnitude smaller than the human eye entrance pupil. Pretty sure the eye is not a pinhole camera. https://en.m.wikipedia.org/wiki/Pinhole_camera – Jagerber48 Jun 21 '22 at 12:42
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This is a good question, and I think you may benefit from an answer without a lot of technical or mathematical detail. At a basic level, the mess of color spread all over that you describe is exactly what a near sighted person sees without their glasses, or what a camera sees without its aperture and lens.

The reason your eye can resolve images is that your pupil is very small and blocks the vast majority of the light coming from any object except a narrow slice. Then, the little bit of light that makes it through is further focused to an even smaller point on the retina.

Your point about light rays crossing is not a problem, because the light passes right through other light without interacting.

Here is a video that may help you https://youtu.be/OydqR_7_DjI

EDIT: I think after reading thru all this, I may finally understand better what you are asking: Given, say, your 2nd diagram, how does your brain "know" where the ray of light is coming from, when it could have originated from any point along the ray? The answer lies in neuroprocessing rather than physics. From a purely physics standpoint, it is true there is no way to "know" simply from the incoming light where the object is located. The mind relies heavily on context (comparisons with other objects in the visual field, perspective) and memory to make organize objects in space into a meaningful image. If you stood in a black room or space devoid of anything but one or two generic points of light, it would be impossible to determine how far away from you or from each other they are. You could only tell the angular separation, but it could be two distant stars, or one star and one Christmas light 20 feet away, or two Christmas lights. That is why if you look up at the stars on a very dark night in a remote area, they seem to be at ceiling height, just above our heads (I've found) even though they are light years away. But if you take that second ray image and add a lens, so the refraction is correctly drawn geometrically, instead of your hand drawn sketch, it will help you see how point sources of light can both converge without being conflated. Viz.

enter image description here

RC_23
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  • but how pupil helps in this? I am not able to understand clearly...I have already seen the video you have linked,,,but that also don't clear the question,,,,Please have a look at this question{specially the diagrams }to clearly understand what I wanna ask: https://physics.stackexchange.com/questions/658655/why-does-an-image-only-form-where-light-rays-coming-from-a-single-point-get-refl – Arun Bhardwaj Jun 21 '22 at 07:32
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    @ArunBhardwaj, Re, "how pupil helps...?" There's a compromise at work. A lens with a large aperture gathers a lot of light, but it has limited ability to form an image. (google "depth of field" and "optical aberration"). A tiny aperture (a.k.a., a "pinhole") can form an image that is better in some ways, but only if the aperture is so small that it hardly allows any light in at all. (google "pinhole camera.") Combining the two ideas—a lens and a small, but not "tiny" aperture—can give a better result than either one alone can give. – Solomon Slow Jun 21 '22 at 13:40
  • @SolomonSlow look at my last image,,,I am saying light from these points CAN still enter eyes like this,,,,so we should see these random points,,,but that would be a mess – Arun Bhardwaj Jun 24 '22 at 11:54
  • @ArunBhardwaj, I don't understand what you mean by "these random points." It makes me wonder whether you still believe that when two light rays cross, their intersection becomes a secondary source of light. If you still hold fast to that idea, then you won't be able to understand any of the replies that you have received to your question. That's not how light works. – Solomon Slow Jun 24 '22 at 12:05
  • give me atleast one reason ,,why the points "o" and "P" can't converge at the retina and only A and B should converge?? only then anything would make sense – Arun Bhardwaj Jun 24 '22 at 12:08
  • @ArunBhardwaj Points "o" and "P" in your original drawing are empty points in space. There is nothing there. There is no light emitted from those points. Your reasoning in your original question is correct. IF light behaved the way you expect it to behave, then it would be exactly as if you were surrounded by a heavy fog. You wouldn't see anything that was not right in front of your face, and you would only see that dimly. Your reasoning is correct, but your postulate is wrong. Nothing happens when light rays cross. The rays are completely unaffected by each other. – Solomon Slow Jun 24 '22 at 13:28
  • I edited my answer with some discussion that may help you – RC_23 Jun 25 '22 at 02:42
  • @RC_23 I only understood that neuroprocessing is involved in this,,but still I am unable to clearly understand what will happen if the rays get converged like what I have shown in my 2nd diagram of last picture? will we see points "O" and "p" and nothing else?? if not ,why not? – Arun Bhardwaj Jun 25 '22 at 13:28
  • The answer is that the scenario in your picture cannot happen. The figure in my answer is the geometrically correct version of your figure. Rays from two different objects cannot converge to the same point. If the rays from two objects do end up at the same point, it means the objects are not in focus. It cannot be both "in focus" and "overlapping rays" – RC_23 Jun 26 '22 at 00:37
  • @RC_23 yes I know,,if they are overlapping it means the object is not in focus,,,but I think that if that's happening{objects not in focus and rays from different objects are converging at common points} we should see those random points in space instead of seeing distict objects,,but instead in reality we see the actual distinct objects but they look blurry why?? – Arun Bhardwaj Jun 26 '22 at 13:59
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I think I understand your question, but I don't think your diagrams[or you] explain your issue well.

enter image description here

Here is a picture of an "eye", where white light from one source, and blue light from one source, do infact hit eachother at the same point in the retina, which I have circled in yellow

You would be absolutely correct, that this picture of an eye, would yield completely blurred images and you would not be able to see anything.

This is why the eye has developed a natural pinhole camera.

enter image description here

Here is an image of an eye with a retina. Notice how the case where the light from the 2 sources mix, is now blocked by the retina! [Which I have circled in yellow]

The pinhole shape, means that light entering one spot in the retina, can only enter the retina on that spot, if it comes in from the SAME angle.

Thus, you have a spatial location dependance, on light that enters the retina on a certain spot. Our brains can then make image.

It is the retinas job to make sure that light entering the retina at a certain spot, only comes from a certain angle, thus eliminating the possibility of light "mixing" from 2 different locations.

Is this what you think happens? enter image description here

Edit 2: enter image description here

Here is a better diagram of your situation in your previous question. I can always cherry pick certain rays that "converge" to a single point in space. And then I'm guessing you say "why doesn't the eye say the objects at that convergent location". Simple answer is clear from the diagram, only 1 ray will make it into the eye which registers as light coming from a certain angle.

I can artificially Cherry pick more rays that converge to another location and then only 1 ray survives.

I have highlighted the ray that makes it into the eye in orange.

Doing so for many "convergent" rays, will just say light is coming from all angles associated with the e.g wall, and the brain registers a wall.

The eye isn't perfect. So saying " go really close and then something messes up" well obviously... can you see an object from 0.0001m away from you? No. As a concept, say the pinhole is infinitely small.

jensen paull
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  • if I solely follow the diagram of pinhole camera, the problem appears to be solved,,,but still there is no reason I can think of that the rays from those intersection points in space after entering that that pinhole wouldn't converge at a single point see this link please: https://physics.stackexchange.com/questions/658655/why-does-an-image-only-form-where-light-rays-coming-from-a-single-point-get-refl – Arun Bhardwaj Jun 21 '22 at 10:29
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    Somewhere along your road in optics, you have severely misunderstood something. Why would light, Traveling at a specific angle, entering our pinhole camera in that angle , all change direction, and converge to a single point? A pinhole camera, is not convex lens... – jensen paull Jun 21 '22 at 10:34
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    Look at my edit – jensen paull Jun 21 '22 at 10:36
  • see you are drawing rough diagrams,,,,,but if you will use simulations and other softwares you will see the pinhole is actually bigger than you think,,,,there is no such thing as "only a single ray from a single point will cross the pinhole" ,,,make your sources closer to each other and then draw the ray diagram and then you will see the intersections points can also converge https://en.wikipedia.org/wiki/Pinhole_camera#/media/File:Light_behaviour_through_pinhole.svg – Arun Bhardwaj Jun 21 '22 at 10:40
  • pinhole helps in resolution I KNOW but in this case{my question} I am not sure if that's the correct answer – Arun Bhardwaj Jun 21 '22 at 10:41
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    Look at my edit. – jensen paull Jun 21 '22 at 10:50
  • ....If I believe that's true then I just have one question left with this,,,,for the whole time I used to believe that we feel problem in focussing the eye to see a real image that's why its difficult to see them without screen and this difficulty is because the rays from other sources {all other objects except the object of which real image is forming} are getting mixed with the rays diverging from a real image that's why we can't see it in a lighten up room......but according to your answer we should always see a real image but we don't always see it?why? – Arun Bhardwaj Jul 11 '22 at 09:15
  • also give me some reason why the rays from different objects cannot converge at same point and if they are actually not converging at same point we should always see the real image right? just like we see a virtual image – Arun Bhardwaj Jul 11 '22 at 09:17
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great question. I'm going to try to do this without diagrams, as I don't have the technology to generate them. I'm a neuroscientist and hobbyist physicist.

The blur you drew originally would be exactly what would happened if you walked into a room that was so bright that you could not distinguish any objects -- blinding light. We see within a range of lumens that the brain can decipher. Similarly, if you turn the light intensity down very low, you see decreasing amount of detail and then nothing. The people who pointed out that your pupil functions as an aperture are correct. We see a small amount of the total light rays bouncing off objects in our environment. You can show yourself this experimentally by noting that as you move around in your environment, your view of objects changes slightly; a slightly different set of light rays are going through your pupils and you see this different view. A short answer is that an optimal amount of ambient light is required for the retina and rest of brain to convert a photon entering to an electrochemical signal via a rod (black and white retinal receptor) or cone (color retinal receptor). If you make a diagram of the physics of those rays, you will note that the projection is inverted both up and down, and left and right. That's where a massive amount of neural processing comes in to re-create our perceptual world from our sensory world. The brain reconstructs the world "out there" to a perceptual world. All sorts of transformations must take place to take visual 'snap shots' captured by moments (~200 msec) our eyes are stationary between saccades during which we are blind and reconstructs a dynamic visual world. the first level of processing is simple lines of orientation. You can read more about this on Cate's Science Corner: https://l.facebook.com/l.php?u=https%3A%2F%2Fcatebennett.wixsite.com%2Fwebsite%3Ffbclid%3DIwAR1Vz9VFtbxz7OYa4LQNd18euOAsbYv-hbJqnnPPWVw6gdU0s63Pw31MLSk&h=AT3nUNLqTyvbIaHS27SUir8kYaaOcZzhCnjB1sqn1auG8GWGlVFWKUugB30y3_bUW3ZC5ByBmJO3q6csCxcf8lW-rxHN4LWQWDqGJXbjUG9jGfb5PFGPB8im7y1d9z4ByKw

Cate B
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  • Welcome to SE. First, on this site it is mandatory to disclose that a site or article you're linking to is your own. Second, a Facebook or Wix link is all but stable over time, so you should make sure that anything useful for this answer is here and that the link isn't necessary to understand it. Thank you. – Miyase Sep 11 '22 at 16:40
  • I don't understand but let me try. First, the link is to my own website, as I noted. Second, my response is a stand-alone reply. The article in my website expands on this issue and specifically how we perceive a stable but dynamic world. It goes into the neural processing of vision. I'm not sure how to "prove" that it is my own except to guarantee that it is. – Cate B Sep 25 '22 at 19:53
  • @CateB, hi cate ,I revisited the question after a long time and read your answer but I didn't come to any conclusion on reading this, my only question lies on the last image I have posted in which point "O" and "p" are the points where light rays from two different point coincides and I have shown them converging over the retina but in reality the ray diagram looks the one above it and not the one with converging "o" and "p" so my question is how does our eye "knows" to converge like this only and not like I have shown in the 2nd one? – Arun Bhardwaj Nov 10 '22 at 13:04
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When light is incident at object. The light can either be absorbed, speculary reflected, or scattered.

A green object, Absorbs all light apart from green, and then scatters green light in all directions.

This green light then travels to my eye, and my brain processes it as a "green object".

The light scattered from the green object that does not travel towards me, does not get registered by my brain, and therefore I will not see those rays.

I hope everything upto this point is clear.

You talk about "mixing light rays" and then draw a diagram of those rays and then I presume you think that we see those rays? We do not.

We do not see light that doesn't directly travel from the source to the eye in straight lines.

Light rays do not interact with eachother.

The reason that the "entire room" is lit up, is because the rays that do not directly enter our eye, are incident on something like a wall.

The rays from that green object that miss our eyes, instead reach the Wall, get absorbed and remitted by the molecules the wall.

The light from the wall gets scattered in all directions, the rays from the wall that don't travel directly to our eye We don't see. The rays from the wall that DO travel directly to our eyes, we do see.

The light from the wall is registered by our brains as what ever colour the wall happens to be.

Ofcourse the light directly from the torch ALSO gets directly scattered by the wall aswell (along with the light scattered off the green object)

This is also a reason why if you put a green object close to a wall, the wall appears slightly greener. The light from the green object is scattered by the wall, so that our brains register the wall having a greener tint to it. As there is more greenlight that is now scattered, in addition to the variety of light scattered by the torch.

enter image description here

For simplicity I have only added a single point of scattering

Here we have white light that is incident an object. When it hits the object, light scatters in ALL directions. The light the enters the eye, is the ONLY light that I see. no mention of "the light ray that doesnt enter acts as another source that propagates to the eye," this is wrong.

So as of now, if I were the eye, I would register the only light I see, is a green object(or a single point of green as I have only included a single point of scattering), and the rest of my vision is black. This is similar to the sun in space. In space everything is black, apart from the point at which the sun is, as the rays from the sun move in straight lines towards my eyes.

Now what If we add walls?

enter image description here

For simplicity I have only added a single reflected Ray from the green object. The light that doesn't initially reach my eye, gets scattered by the wall in all directions!, the light that doesn't get scattered in my direction, I don't see, the light that does get scattered in my direction, I do see.

(Including all of the scattering elements from the wall and not just a single one) From this POV, you should technically see a green wall as green light is all that is scattered. However in the diagram I have drawn blue. This is because the white light from the torch is also scattered by the wall, which I have chosen is predominantly blue that is scattered. However in reality, it should be blue + a tint of green.

I hope this clears it up.

In general, white light enters an object, only part of that light is scattered, giving it a specific colour, the light that is scattered directly towards you , you see, and your brain registers it as a coloured object.

Huygens principle is what is confusing you I'm pretty sure. This is already accounted for in the fact that you consider the net EM wave propagating and not a single element, these cancel out the apparent "source" of light from a direction not travelling straight toward you

Mixing elements:

enter image description here

There is no trouble in "unmixing" if the rays Cross paths, light does not interact with light so it doesnt effect anything.

In the diagram you see 2 light rays crossing paths. The eye will not see the rays as coming from the same location.

In the diagram I have drawn a huge eye in black and have circled where the eye registers each ray on the retina in yellow. The eye picks up the light at different locations on the retina and will perceive the light as coming from 2 distinct locations, ie , the places I have drawn on the diagram.

jensen paull
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  • I think I have not said anywhere that light rays interacts on the point of "INTERSECTION" ,,I know they just passes each other without changing direction I am just saying that the points in space where they are "CROSSING" should also converge on our retina and in this way rays CMING FROM TWO DIFFERENT SOURCES gets converged OVER A SINGLE POINT ON RETINA,By "MIXING" i just meant that but everybody took a different meaning out of it ,,,,please take a look at this question to understand in detail – Arun Bhardwaj Jun 21 '22 at 07:46
  • https://physics.stackexchange.com/questions/658655/why-does-an-image-only-form-where-light-rays-coming-from-a-single-point-get-refl – Arun Bhardwaj Jun 21 '22 at 07:46
  • here in your last diagram,,why you have not converged the white light as well as the blue light entering the eye on the same point over retina? If you would be able to answer this in reality then i would think you have answered my question....the point where they are crossing can also be converged over retina right? its like saying because I already know rays are coming from which part i would converge those parts only,,,it isn't fair – Arun Bhardwaj Jun 21 '22 at 07:50
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    I don't think you explain your issue very well. However I have added a new answer. I also think this is a biology not a physics question. – jensen paull Jun 21 '22 at 10:23