Reflection and refraction

For reflection, angle of incidence equals angle of reflection, measured from the normal perpendicular to the boundary. Refraction is a change of direction associated with a propagation-speed change. Refractive index is n=vc​. At an ideal boundary, n1sinθ1=n2sinθ2. Use degrees consistently and measure both angles from the normal, not from the surface.

Entering a higher-index medium reduces speed and usually bends the ray toward the normal; leaving for a lower index bends it away. A normally incident ray changes speed without changing direction. Frequency stays fixed at the boundary, while wavelength changes according to v=fλ. These rules apply to the stated uniform media and a single boundary; they do not require a claim that all frequencies have identical refractive index.

Watch out: Snell’s law uses angles from the normal. The angle from the surface is its complement.