Why is an image formed when two light(one to the focus and one to the lens perpendicularly from the top of the object) rays coming from an object intercept?

  • 2
    $\begingroup$ You are using these two specific rays of light because you can easily construct the edge of object from these two. The image itself is formed from many rays "starting" from each point of the object. $\endgroup$
    – npojo
    Mar 7 '18 at 17:49

I am assuming you are talking about either lenses or mirrors. In order for one of these systems to form a real image, all of the light rays coming from a single point on the object must converge at a corresponding point in space. Like @npojo said, there are certain rays coming from the object that are easy to draw by hand in diagrams, so we use those. But to form in image we must have light rays from each point on the object converging at a corresponding point in space.

This applet is fun to play around with this. It shows only 3 rays from different points of the object, but in fact every ray from each point is converging at the corresponding image point.


  • $\begingroup$ If all light rays converge at a single point then how do we get the whole image? It should be a point. $\endgroup$
    – Time rub
    Mar 7 '18 at 18:06
  • 1
    $\begingroup$ I am not saying every light ray from the object converges at a single point. I am saying for each point on the object the light rays coming from that point needs to converge at a corresponding point in space. This is what forms the image. Point 1 converges at point 1', 2 at 2', etc. $\endgroup$ Mar 7 '18 at 18:09
  • $\begingroup$ Since in the system of lens and object two rays come from a single point and converge. Would it spoil the image? $\endgroup$
    – Time rub
    Mar 7 '18 at 18:11
  • $\begingroup$ In order to have an image you need multiple rays converging at a point in space from a corresponding point on the object. If this is not happening then you do not have an image. In the applet move the screen around and notice how you do not get a well focused image if your screen is not at the point in space where the light rays converge. $\endgroup$ Mar 7 '18 at 18:17
  • $\begingroup$ Look here.physics.stackexchange.com/q/197039 $\endgroup$ Mar 7 '18 at 18:18

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