Reflection and Refraction of Light

Reflection and Refraction

At the boundary between two media, light is partly reflected and partly refracted.

• Law of reflection: angle of incidence = angle of reflection. Angles are measured from the normal.
• Law of refraction: sinα/sinβ = n₂/n₁. Here α is the angle of incidence, β is the angle of refraction, n₁ is the refractive index of the first medium, and n₂ is the refractive index of the second medium.
• Refractive index: n = c/v, where c = 3·10⁸ m/s.

Substance	n
Water	≈ 1,33
Glass	≈ 1,5
Diamond	≈ 2,42

The angle is measured from the normal, not from the surface.

Example: in a medium with n = 1,2, the speed of light is v = 3·10⁸/1,2 = 2,5·10⁸ m/s.

Total Internal Reflection

When light passes from a denser medium to a less dense medium, it is totally reflected if the angle of incidence is greater than the critical angle.

• Critical angle: sinα₀ = 1/n.
• Optical fibers are based on this phenomenon.

1. Light travels in the denser medium
2. The angle is greater than the critical angle α₀
3. The ray does not leave the medium and is completely reflected

Example: if n = √2, then sinα₀ = 1/√2, so α₀ = 45°.