6.8 Reflection and Refraction

Law of Reflection

When a wave hits a boundary and bounces back, the angle of incidence equals the angle of reflection, both measured from the normal (a line perpendicular to the surface). This holds for light, sound, and other waves reflecting off smooth surfaces, and explains how mirrors and echoes work.

Refraction and Snell's Law

Refraction is the bending of a wave as it passes into a medium with a different index of refraction (n), because wave speed changes. Snell's Law: n₁sinθ₁ = n₂sinθ₂. Light bends toward the normal entering a denser (higher n) medium, and away from the normal entering a less dense medium.

Total Internal Reflection

When light travels from a higher-n to a lower-n medium at an angle greater than the critical angle, it undergoes total internal reflection — no light refracts out, it all reflects back. The critical angle satisfies sinθc = n₂/n₁. This principle enables fiber optics.

Key terms

Law of Reflection

Angle of incidence = angle of reflection, both measured from the normal.

Refraction

Bending of a wave as it changes speed passing into a medium with a different index of refraction.

Snell's Law

n₁sinθ₁ = n₂sinθ₂, relates angles and indices of refraction at a boundary.

Index of refraction (n)

Ratio of the speed of light in vacuum to speed of light in a medium; higher n means slower light speed.

Total Internal Reflection

Occurs when light in a higher-n medium hits a boundary at an angle greater than the critical angle, reflecting entirely back with no refraction.

Try an AP-style question

A pulse on a string reflects from a fixed end. How does the reflected pulse compare?

1. It is upright
2. It disappears
3. It is inverted
4. It doubles in size