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Next we are going to put something in front of our lens. We might put an ant, a newspaper, or a flower -- whatever object you would like. It doesn't matter. Whenever you see something, your eyes are detecting light coming from that object. We will be drawing our objects as arrows:

Principal Rays

Light goes in all directions from the object, but we are only interested in the light which goes through the lens. In particular we know that there are special rays, traditionally called principal rays, which we can use to locate the image produced by our lens.

A ray coming in horizontally from the tip of the arrow, goes through the focal point.

A ray going from the tip of the arrow to the near focal point will come out horizontally.


If we combine the two we get the following picture.

Notice that the rays intersect on the right side. At the point where they intersect, I have drawn another arrow. This is the image of our object. It is called a real image because if you put your hand or a piece of paper at that position, you would actually see an inverted arrow created out of light. This image of the arrow is like the image created by a slide projector that is focused on a screen.

We have only followed the principal rays because they are the easiest. Other rays coming from the object and going through the lens, will also be focused to form the image of the arrow.


Before going to the next page, try the following. In the pictures shown above the object has been placed twice as far from the lens as the focal point. The distance from the lens to the focal point is called the focal length.

Draw diagrams yourself, moving the object a little further from the lens:

and then a little closer to the lens:

For both of these cases note whether the image gets closer or further from the lens and whether it gets larger or smaller.

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