21. When the angle of incidence on a material is 60, the reflected light is completely polarized. The velocity of the refracted ray inside the material is (in ms) (1) 3 x 108 () (2) 108 (3) 3 x 108 (4) 108 22. What is the ratio of de-Broglie wavelengths of proton, deuteron and a-particle if they are accelerated through same potential difference? 1 (1) 1:1:1 (2) 1:1 22 (3) 1:1 22 (4) 1:1:1 23. A non-monochromatic light is used in an experiment on photoelectric effect. The stopping potential (1) is related to the mean wavelength. (2) is related to the shortest wave- length. (3) is related to the longest wave- length. (4) is not related to the wavelength. 24. Two protons are kept at a separation of 10 nm. Let F and F be the nuclear force and the electromagnetic force between them respectively. Choose correct relation between them. (1) F=F (2) F>> F (3) F