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1. (a) Describe 2 conditions for interference. (5 marks) (b) A soap film has an index of refraction n = 1.33. Light of wavelength λ = 500 nm is incident normally on the film. Calculate i. smallest thickness of the film that will give a maximum interference in the reflected light. ii. maximum interference if doubling the thickness of the film calculated in part i. (3 marks) (c) A light source emits visible light of two wavelengths (2=430 nm and 2 = 510 nm). The source is used in a double-slit interference experiment in which L = 1.50 m and d = 0.025 mm. Find the separation distance between the third-order bright fringes. (2.5 marks)

2. (a) For a diffraction grating, explain the advantage of i. many slits, ii. closely spaced slits (5 marks) (b) A diffraction grating has 7,000 lines per centimeter. When the grating is illuminated normally with a monochromatic light, the second order spectral line is found at 62°. Find i. wavelength of the light ii. third order maximum (3 marks) (c) Red laser light from a He-Ne laser (2=632.8 nm) is used to calibrate a diffraction grating. If this light creates a second-order fringe at 53.20 after passing through the grating, and light of an unknown wavelength à creates a first-order fringe at 20.6°, Find A. (2.5 marks)