# Problems : Thin film interference

1. We wish to coat flat glass (n =1.50) with a transparent material (n= 1.25)so that reflection of light at wavelength 600 nm is eliminated by interference. What minimum thickness can the coating have to do this?
2. The rhinestones in costume jewelry are glass with index of refraction 1.50. To make them more reflective, they are often coated with a layer of silicon monoxide of index of refraction 2.00.What is the minimum coating thickness needed to ensure that light of wavelength 560 nm and of perpendicular incidence will be reflected from the two surfaces of the coating with fully constructive interference?
3. Light of wavelength 624 nm is incident perpendicularly on a soap film (n= 1.33) suspended in air. What are the (a) least and (b) second least thicknesses of the film for which the reflections from the film undergo fully constructive interference?
4. The reflection of perpendicularly incident white light by a soap film in air has an interference maximum at 600 nm and a minimum at 450 nm, with no minimum in between. If n= 1.33 for the film, what is the film thickness, assumed uniform?