AP Physics Lab

Using the Law of Reflection to Measure Heights



small bowl

meter stick


small mirror

dark colored paper

measuring tape


It is very easy to use the Law of Reflection and some simple geometry to measure the height of an object that would be very difficult to measure directly. Please study the diagram below.

In this diagram, a person observes the image of the top of a flagpole, using the surface of a bowl of water as the reflecting surface. The Law of Reflection guarantees that the two triangles formed are similar (Do you see why?)

Measuring Heights Diagram 1


In the diagram below, "h" is the height of the person's eyes above the surface of the water, and "d" is the horizontal distance from the person to the center of the bowl. "H" represents the height of the flagpole, and "D" is the horizontal distance from the flagpole to the bowl of water. The 3 distances h, d, and D are easy to measure.

Similar Triangles & Derivation Diagram

Notes & Suggestions:

Your instructor will pick a tall object for you to measure. When you finish your calculations, you can compare your value for the height with the value obtained by the other lab groups in the class.

If you have trouble seeing the image of the object in the surface of the water:

You should make two sets of measurements - one with a meter stick and one with a long measuring tape. You need to decide:

If you are so inclined, a spreadsheet program can be used to perform your calculations:

In your lab book, be sure to:


adapted from IHETS AP Physics Lab #2 - September 1992

and Measuring Heights Activity

last update August 14, 1997 by JL Stanbrough