- Two sources of light produce an interference pattern on a screen.
(a) Explain why the two sources need to be coherent for the interference pattern to be observed. [2]
(b) Light of wavelength 720 nm is incident on two narrow slits that are separated by 0.12 mm. An interference pattern is observed on a screen. P₁ and P₂ are the points of destructive interference closest to the central maximum M.
(i) Calculate, in radians, the angular separation ϕ of P₁ and P₂. [2]
(ii) Suggest how energy conservation is consistent with the fact that the energy at P₁ and P₂ is zero. [1]
A beam of light containing all wavelengths in the range [550 nm, 650 nm] is incident normally on a diffraction grating. The grating has 580 lines per mm. A diffraction pattern is observed on a screen as shown.
diffraction‑grating diagram with central maximum M
(c) Outline why the interference at M is constructive. [1]
(d) Determine the largest order of the diffraction pattern in which all the wavelengths in the incident beam are present. [3]