Refraction

Refraction happens as a wave changes direction entering a different medium. 

When it meets a boundary, some of the energy of the ray is reflected back and the rest transmitted. If it meets the boundary at an angle to the normal the transmitted ray is refracted, and so the speed changes as optical density does, (ie- decreasing in a more optically dense material as the ray bends towards the normal, and so the wavelength falls for same frequency).

Towards the normal, if the ray bends in this direction, the velocity will slow down as the ray travels to a more optically dense material, and wavelength decreases. 

Bending away from the normal, the velocity increases as the ray travels to a less optically dense material and wavelength increases.

Light travels less fast when it travels through materials as opposed to a vacuum as it interacts with particles, and as optical density rises, the more light slows down.

Absolute refractive index is a measure of optical density....

Snell's law for an air-to-material boundary is...

n1sin01=n2sin02

Place a glass box on a piece of paper and draw around it. 

Use a ray box and shine a beam of light into the block, turning off other lights for a clear view. 

Trace the incoming and outgoing beams of light on either side. 

Remove the block and line these up, showing refraction of light as it enters and leaves the block. 

Measure angle i entering the block, and angle r inside the block, and using Snell's Law for an air-to-material boundary calculate its refractive index.