What Is the Minimum Radius Needed to Paint on a Cube to Contain Light?

[firyace]

Lets say you got a glass cube and a small light source embedded at the exact center of the glass cube.
Now if you want to paint a circle on each face of the cube so that the light is prevented from leaving the cube, what is the minimum radius of the circle that you need to paint?

Let say each of the sides of the cube is 16.0cm.

Thanks!

 

[berkeman]

You need to show us your work so far in order for us to help you. Here's a starting point:

http://en.wikipedia.org/wiki/Total_internal_reflection