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Ray Diagrams

We are going to be drawing a lot of pictures, called ray diagrams. In fact, that is really all we are going to be doing today. Let me show you how I draw my diagrams.

First let us put down two dashed lines which are guides to the eye.

Next at the intersection of two dashed lines we will place a lens.

The lens drawn is a particular kind of lens called a converging lens. It has the property that light coming from very far away will be focused down to a point, called the focal point. We draw this as:

This is our first ray diagram. The solid lines represent rays of light.

Many of you may have experienced the phenomena shown in this picture first hand. When I was a little kid, I used to play with a magnifying glass. By focusing the light from the sun, I was able to make a very bright spot at the focal point. If I held my hand very steady, I could ignight a dried leaf -- or at least get it to smoke.

The whole process works in reverse, too. Namely, if light goes through the focal point, it will come out horizontally on the other side of the lens.

Notice that we have labelled the focal point by F.

Finally, there is a focal point on the left hand side as well as the right hand side. If we send in light horizontally from the right, it will converge to the focal point on the left.

If light starts out at the focal point on the left, then it will come out horizontally on the right.

Thus, a converging lens has the property that:

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