Determination of Wavelength of Different Colours

When white light is used with a diffraction grating instead of monochromatic light, the diffraction pattern consists of a white central (zero-order) maximum, with continuous, coloured spectra spread out on both sides of it for each successive diffraction order. The central maximum stays white because every colour meets there with zero path difference and hence interferes constructively together. As the angle $\theta$ increases away from the centre, the path difference $(a+b)\sin\theta$ passes through the maxima condition for different orders, one colour (and wavelength) at a time, from violet through to red, producing a full visible spectrum in each order on either side of the central maximum. By measuring the angle at which each individual colo …