Optics: Lens, Prism, Rainbow
The geometry of light
Summary
When light passes through glass or a drop of water it is refracted: it changes direction. This simple fact makes lenses, prisms and rainbows possible. The principle behind your glasses, your camera and even your own sight is this law of refraction.
In depth
Geometric optics models light as straight lines (rays) and calculates their changes of direction with Snell's law of refraction: n₁·sin(θ₁) = n₂·sin(θ₂). This single equation explains how a lens focuses an image, why a prism splits white light into colours, and why a rainbow appears at exactly 42°. The thin lens equation 1/dₒ + 1/dᵢ = 1/f is the mathematics of the camera, the eye, the telescope and the microscope. Dispersion (n depending on wavelength) is the basis of Newton's discovery with a prism in 1666: white light is really a mixture of all the colours.
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