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$S$ is a light source, $E$ is the eye of an spectator. There are two points $A$ and $B$ on a plane. Since incident angle and reflection angle is equal for all cases, how can the light reflected from point $A$ reach the eye? If it doesn't, how can we see the point?enter image description here

Riz222
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3 Answers3

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Since incident angle and reflection angle is equal for all cases. . . .

True for a mirror but not so if the light is scattered by the surface.

Have a look at the following post - Difference between the reflection and the scattering of light.

Farcher
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Yes. Given the flat surface it isn’t possible for a light that is reflected from point A that originated at the source to reach your eye.

However, considering prefect reflections from A, the light can get scattered from elsewhere for example from the red line in the following image. The light is scattered off of a wall that is to the left of the source that reflects from A to reach your eye. This can happen with multiple scattering before reflection from A as well. enter image description here

Superfast Jellyfish
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    You are using the term "scattered" with the law of reflection (equal incident and reflected angles). But light scattered from a diffuse surface leaves the surface in a range of angles. If A is on a diffuse surface, some of the light from S to A will reach E. – Not_Einstein Jan 16 '20 at 14:54
  • Yes you are right. I have edited mentioned the domain of validity of my answer. – Superfast Jellyfish Jan 16 '20 at 16:25
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It is very important to understand the difference between specular and diffuse reflection.

  1. Spacular (mirror like) reflection:

enter image description here

In this case the reflected light's angle will be the same as the incident angle. The light will most probably not reach the eye in this case from point A (only from point B).

Specular reflection, also known as regular reflection, is the mirror-like reflection of waves, such as light, from a surface. In this process, each incident ray is reflected at the same angle to the surface normal as the incident ray, but on the opposing side of the surface normal in the plane formed by incident and reflected rays. The result is that an image reflected by the surface is reproduced in mirror-like (specular) fashion.

https://en.wikipedia.org/wiki/Specular_reflection

  1. Diffuse reflection:

enter image description here

In this case the surface is so, that the light from the source light will be reflected (or absorbed and re-emitted) at random angles.

Diffuse reflection is the reflection of light or other waves or particles from a surface such that a ray incident on the surface is scattered at many angles rather than at just one angle as in the case of specular reflection. An ideal diffuse reflecting surface is said to exhibit Lambertian reflection, meaning that there is equal luminance when viewed from all directions lying in the half-space adjacent to the surface.

https://en.wikipedia.org/wiki/Diffuse_reflection

In this case the light from the source will reach the eye through both A and B points.

Árpád Szendrei
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