Dispersion and Scattering of Light: Prisms, Rainbows and Sky Colour
This domain looks at how a prism separates white light and why the sky, sunsets and clouds have the colours they do.
Dispersion
White light is a mixture of wavelengths. The refractive index of glass is greater for violet than for red, so violet bends most and red least when light passes through a prism. The emerging band, VIBGYOR, is the spectrum. Newton showed that a second, inverted prism recombines the colours into white light, proving the prism does not add colour. A rainbow forms by refraction, dispersion and total internal reflection inside water droplets.
Scattering
Rayleigh scattering by molecules is proportional to 1/λ⁴. Blue light (short wavelength) is scattered far more than red, so the sky looks blue; at sunset the long path through the atmosphere removes the blue and the sun appears red. Large particles such as cloud droplets scatter all wavelengths nearly equally, which makes clouds white. Red is used for danger signals because it is scattered least and travels farthest through fog. The Tyndall effect - scattering by colloidal particles - makes a beam of light visible.
Worked example
Compare blue (450 nm) and red (700 nm): the scattering ratio is (700/450)⁴ ≈ 5.9. Blue is scattered about six times as strongly as red by the same molecules, which is why the scattered light from the sky is blue rather than red.
Traps that cost marks
- Saying the prism colours the light.
- Claiming red deviates most in a prism.
- Explaining the blue sky by reflection from the sea.
- Forgetting that clouds scatter all colours, not only blue.