A beam of monochromatic light with wavelength L is incident upon two slits S1 and S2 a distance d apart, as shown in Fig. 15.11. Derive an expression, in terms of L, L , d , and m for the distance ym from the central point to the mth bright fringe of the interference pattern on a screen a distance L away when L >> ym . Two narrow slits separated by 2 mm are illuminated with a helium-neon laser of wavelength 612 nm. Calculate the spacing of the fringes observed on a screen 4 m.