In front of a lecture hall, a coherent beam of monochromatic light from a helium-neon laser (λ= 632.8 nm) illuminates a double-slit. From there it travels a distance of 20.0 m to a mirror at the back of the hall, and returns the same distance to a screen.
(a) In order that the distance between interference maxima be 10.0 cm, what should be the distance between the two slits?
(b) State what you will see if the lecturer slips a thin sheet of cellophane over one of the slits. The path through teh cellophane contains 2.5 more waves than a path through air of the same geometric thickness.