Relation between angle of diffraction and wavelength
Fig. 26.17 shows the usual arrangement for a diffraction grating experiment. A parallel beam of light is incident on the grating from an illuminated slit S placed at the
principal focus of a converging lens L!. The first order diffracted beam from the grating enters a second lens L2 which brings it to a focus in the principal focal plane. Here we get an image of the slit S which may be formed either on a white screen or else viewed through an eye lens. If we are using onochromatic (= one colour) light, then for first order diffraction, the distances between successive wavefronts leaving the grating in the same phase will be equal to the wavelength, A.