Animation of the behaviour of light passing through a double slit, showing the interference pattern produced. When light passes through a double slit, it diffracts and spreads out, and interferes with the light from the adjacent slit. This leads to alternating regions where the interfering waves either cancel each other out or amplify each other, leading to a pattern of dark and light bands. For a given separation of the slits, the width of the bands depends on the wavelength of the light: longer wavelengths produce wider bands, and they are seen to narrow when the colour changes from long wavelength red to short wavelength violet.