Refraction

Light slows in glass and water and speeds back up in air, and the bend at the boundary is a consequence of that speed change rather than a separate effect. The CED states it in that order: refraction is a change in direction, and it is a result of the speed of light changing when light enters a new medium.

Frequency is fixed; wavelength is not. The boundary cannot create or destroy wave cycles, so the same number arrives per second on both sides. With $\lambda = v/f$ and $f$ held constant, a slower medium means a shorter wavelength: $\lambda_{\text{medium}} = \lambda_0 / n$. What you see underwater does not change colour, because colour tracks frequency.

Which way it bends, in the CED's own cases:

• Entering a higher index medium, the ray slows and bends toward the normal.
• Entering a lower index medium, the ray speeds up and bends away from the normal.
• A ray arriving along the normal is not refracted at all, whatever the two indices are.

The arithmetic is Snell's law, $n_1 \sin\theta_1 = n_2 \sin\theta_2$, with both angles measured from the normal. Push the away-from-normal case far enough and the refracted ray runs out of room, which is total internal reflection.

Refraction is also what makes every lens work: bending at the two surfaces is what makes a converging lens converge. Topic 13.3 carries the procedure.