Lectures on Physics has been derived from Benjamin Crowell's Light and Matter series of free introductory textbooks on physics. See the editorial for more information....

Snell's Law

Question: A submarine shines its searchlight up toward the surface of the water. What is the angle θ shown in the figure?

Solution: The tricky part is that Snell's law refers to the angles with respect to the normal. Forgetting this is a very common mistake. The beam is at an angle of 30 with respect to the normal in the water. Let's call the air medium 1 and the water medium 2. Solving Snell's law for θ1, we find

As mentioned above, air has an index of refraction very close to 1, and water's is about 1.3, so we find θ1=40. The angle a is therefore 50.

Last Update: 2010-11-11