21. (a) A student is trying to make an accurate measurement of the wavelength of green light from a mercury lamp (= 546 nm). Using a double slit of separation of 0.50 mm, he finds he can see ten clear fringes on a screen at a distance of 0.80 m from the slits. He then tries an alternative experiment using a diffraction grating that has 3000 lines/cm. (i) What will be the width of the ten fringes that he can measure in the first experiment? (ii) What will be the angle of the second-order maximum in the second experiment? (iii) Suggest which experiment you think will give the more accurate measurement of wavelength (D). [2] [2] [1] (b) A physics student went to buy polaroid sunglasses. The shopkeeper gave him two similar-looking sunglasses. In what way he can differentiate between polaroid sunglasses and non-polaroid sunglasses? [1] (c) At what angle of incidence will the light reflected from water (= 1.3) be completely polarized? [2]