(a) (i) Use the Huygens principle to explain how plane parallel wavefronts are diffracted at a single aperture.
(ii) Use the principle of superposition to explain how coherent light emerging from two narrow slits can interfere to produce a pattern on a distant screen.
(b) A laser produces a beam of monochromatic light of wavelength 633 nm, which is incident normally on a diffraction grating whose slits are vertical.
(i) Describe the form of the diffraction pattern that is seen on a distant screen. Comment on the colour of the pattern.
(ii) If the fifth order in the pattern is seen to occur at an angle of 39.3° with respect to the incident beam, how many slits per mm does the grating contain?