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Virtual Images

What happens when we move the object inside the focal point?

The principal ray which comes in horizontally from the left and goes through the right focal point is easy:

The principal ray which goes through the near focal point and then the lens is more difficult because to go through the focal point would require the light to go away from the lens. The trick here is that the light can go from the object to the lens in the same direction as if it had come from the focal point.

There is one more problem: the two rays emerging on the right hand side go apart or diverge instead of coming together. They do not form a real image as in all the previous examples. It does, however, appear as if the light came from a point on the other side of the lens.

This is an example of a virtual image. Unlike a real image, if we put a piece of paper where the virtual image is we would not see anything. However, if we look through the lens from the right hand side it appears as if the object is at the position of the virtual image.


Can you guess how a magnifying glass works?

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Selman Hershfield /
Last modified: January 1, 1995
This work is supported by NSF grant DMR9357474.