Light: Reflection and Refraction

Light travels in straight lines. When it meets a surface or changes medium, it is reflected or refracted.

Reflection

Reflection is the bouncing back of light when it strikes a surface. The laws of reflection are:

1. The incident ray, the reflected ray and the normal all lie in the same plane.
2. The angle of incidence equals the angle of reflection (i = r).

Reflection from a smooth surface (like a mirror) is called regular reflection; from a rough surface it is diffuse reflection.

Refraction

Refraction is the bending of light as it passes from one medium to another of different density (e.g. air to water).

• Light bends towards the normal when entering a denser medium.
• Light bends away from the normal when entering a less dense medium.

The refractive index (n) of a medium = speed of light in air ÷ speed of light in the medium, or n = sin i / sin r (Snell’s law).

Lenses

• Convex (converging) lenses bring parallel rays to a focus. They can form real or virtual images.
• Concave (diverging) lenses spread rays out. They always form virtual, diminished images.

Summary

Light obeys the laws of reflection (i = r) and is bent by refraction when it changes medium. Convex lenses converge light and can form real images; concave lenses diverge light and form virtual images.