The circle of light: A new lab on total internal reflection

Abstract

Shine a laser into a pan of shallow water, but on the bottom of the pan place a piece of paper. The result will be an interesting perfect circle of light (Fig. 1). This is caused by internal reflection of the diffusely reflected light. The diameter of the circle depends on both the depth of the water and the index of refraction of the liquid. In this article, I investigate this very simple activity and show examples of how students and teachers can use this phenomenon as a laboratory or demonstration to investigate concepts of the critical angle and total internal reflection.