Snell's law

Here's a graphical way of constructing Snell's law of refraction. The black line separates two media, the outside medium has a red light ray entering and inpinging on the boundary. The speed of light in this medium is indicated by the radius of the red circle.

In the second medium, the speed is different, equal to the radius of the blue circle. The geometrical construction drawn constructs the outgoing direction such that Snell's law is satisfied.

That law is usually expressed as: the ratios of the sine of the angle of the incident ray and the sine of the refracted ray is the ratio of the speeds of light in those media.

Please enable Java for an interactive construction (with Cinderella).

You can change the speed of light in both media (by changing the radii), the angle of the incident ray, and the boundary surface. The construction should be clear when you play with it -- although you may need to prove to yourself that this is Snell's law in action!

Created with Cinderella