Applications of refraction in everyday life

Mirages

Warm air near the ground is less dense and has a slightly different refractive index to the cooler air above it. Light coming from the sky is gradually bent as it passes through these layers of air with varying indices, creating the illusion of a puddle of water on a hot road in summer: this is a mirage.

Optical Lenses

Spectacles, cameras and magnifying glasses use glass or plastic lenses whose curved shape utilises refraction to make light rays converge or diverge. By carefully selecting the curvature and the refractive index of the material, vision can be corrected (short-sightedness, long-sightedness) or an image can be magnified.

Prisms and dispersion

A prism deflects white light and breaks it down into a spectrum of colours (a rainbow). This is because the refractive index of glass depends slightly on the colour (wavelength) of the light: violet is deflected more than red. This phenomenon is called dispersion.

Summary table

Phenomenon	Physical explanation
Broken straw in a glass	Refraction at the water/air interface
Mirage on the road	Gradual refraction in warm air
Optical fibre	Total internal reflection
Rainbow through a prism	Dispersion (refractive index depends on colour)

Common misconception

Do not confuse cause and effect: it is not the colour that causes refraction; rather, it is refraction (which depends on wavelength) that separates the colours. Without this dependence of n on wavelength, there would be no dispersion and therefore no rainbow visible through a prism.