White light and the prism

Light: not as simple as it seems

The white light from the Sun or a halogen lamp appears uniform, but this is an illusion. It is in fact a combination of light of different colours, each associated with a specific wavelength. White light is said to be polychromatic (multi-coloured), as opposed to monochromatic light (a single colour), such as that from a laser.

The historic prism experiment

In 1666, Isaac Newton passed a narrow beam of white light through a triangular glass prism. At the other end, he did not obtain a white beam but a continuous spectrum of colours ranging from red to violet: this is the spectrum of white light. Newton then demonstrated that by recombining these colours using a second prism, white light was produced: the colours were indeed already present in the original light; the prism merely separated them.

Why do we talk about dispersion?

Dispersion is the name given to the phenomenon whereby a transparent medium deflects the colours that make up light to different degrees. Each colour corresponds to a different wavelength (violet has a shorter wavelength than red), and the glass of the prism does not deflect them in the same way.

Colour	Approx. wavelength
Red	700 nm
Green	550 nm
Violet	400 nm

A common misconception

Do not confuse dispersion with diffraction: dispersion separates colours due to different deflections depending on the wavelength (prism), whilst diffraction is the spreading out of a wave when it encounters an obstacle or a slit (diffraction grating, CD).