Higher order spectra from a diffraction grating
We have seen that reinforcement occurs for light diffracted from adjacent slits in a diffraction grating at an angle 8 given by, Reinforcement will also occur when the angle of diffraction is such that the path difference for light from adjacent slits is 2}” 32, and so on. We therefore may obtain diffracted images for any particular wavelength at larger angles given by,
These are called the second and third order images respectively. The actual number of orders possible for any particular colour will depend on the width of the grating space. Sin 8 cannot be greater than I (900 diffraction), so that the maximum number. Two orders of spectra can be seen using the plastic replica described earlier in this chapter. Coarse gratings with a wide spacing give several orders as may readily be
seen if a diffraction experiment is performed using a piece of fine wire gauze.