Q. A double-slit experiment is set up using red light (l = 714 nm). A first order bright fringe is seen at a given location on a screen. What wavelength of visible light (between 380 nm and 750 nm) would produce a dark fringe at the identical location on the screen?

Q. A ray of light enters the long side of a 45-90-45prism and undergoes two total internal reflections, as indicated in the figure . The result is a reversal in the ray's direction of propagation.

Find the minimum value of the prism's index of refraction, for these internal reflections to be total.