Light

How light behaves, and why it needs two descriptions.

Light is electromagnetic radiation. Visible light occupies a narrow band of a much wider spectrum, and it is only special because our eyes evolved to detect the wavelengths the Sun emits most strongly and the atmosphere lets through.

The spectrum

From longest wavelength to shortest: radio, microwave, infrared, visible, ultraviolet, X-ray, gamma. Shorter wavelength means higher frequency and higher energy per photon — which is why ultraviolet damages skin and radio does not.

Reflection and refraction

Reflection obeys a simple rule: the angle of incidence equals the angle of reflection. Refraction is bending as light changes speed entering a new medium, and it is what makes lenses work. A straw in a glass appears bent at the surface for exactly this reason.

Refraction is also wavelength-dependent — shorter wavelengths bend more. That is dispersion, why a prism separates white light into colours, and why rainbows exist. It is also why cheap lenses show coloured fringes.

Colour

An object's colour is the light it fails to absorb. A red apple absorbs most wavelengths and reflects red. Something white reflects nearly everything; something black absorbs nearly everything. Mixing light is additive — red, green and blue give white. Mixing pigment is subtractive — cyan, magenta and yellow approach black. The two systems behave oppositely because one adds emission and the other adds absorption.

Wave and particle

Light diffracts and interferes, which only waves do. It also ejects electrons from metal in discrete quanta, which only particles do. Both are true, and neither picture alone is sufficient — this is wave-particle duality, and it is a description of behaviour rather than a puzzle awaiting resolution.