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Why don't we consider incident rays which doesn't pass through the center or focus of a spherical mirror while drawing a ray diagram? Do those rays form any image when they intersect? like point a,b,c?

enter image description here

Mehedi
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2 Answers2

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We don't consider these rays for simplicity.As the rays parallel to the axis coverge at the focus and the rays going through the focus become parallel.Ultimately all the rays from a point object will converge at a point to make an image.The other rays are not considered for simplicity as we don't know exactly where they will go.Hope it helps

BlackSusanoo
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  • Thanks for answering, so if those rays intersects , they form a image too ? See points a,b,c of the figure – Mehedi May 31 '20 at 06:55
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    The extended object that you have made,all the rays coming from the each partof the object will converge at one point and many of these points will make an image.For simplicity we only take the top and bottim point objects,all the rays from the top object will converge at a single point.As the rays are converging at one point a person will think that the rays of the object are coming from another point known as image of the object – BlackSusanoo May 31 '20 at 14:24
  • If you are satisfied with my answer please tick my answer.Thanks – BlackSusanoo May 31 '20 at 14:28
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Your ray diagram needs to be re-done. For example, you have two different points labeled as P. And, ray SV is not drawn in the right direction. If you draw the diagram correctly, your question will answer itself.

Begin by drawing just one ray from the top of the object (the left P in your diagram), parallel to the axis until it hits the mirror, and then from the mirror through the focal point F. Next, draw a ray from the same P at the top of the arrow, through the focal point F to the mirror, then back horizontally from the mirror.

The reflected parts of those two rays will intersect somewhere below the axis, between O and F. That point of intersection (in your new diagram, label this point P') is where an image of the top of the arrow will form. An image of the bottom of the arrow (not labeled in your diagram, but it's where the arrow butts up against the axis) forms at a point that is on the axis and directly vertical from the image of the top of the arrow. Note that the image is inverted relative to the object.

If the mirror were ideal, all of the rays that diverge from the tip P of the arrow will converge to the same point P'. Although it's not obvious how to construct rays diverging from the bottom of the arrow, they too will converge to the corresponding image point. So, ray SV will pass through the point P' that you haven't drawn yet.

One important caveat: This method of constructing the rays is only a good approximation if the angles of the rays relative to the axis are small. The formed image always has some aberration, and the aberration gets worse when the angles are large.

S. McGrew
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