Light incident on a very thin film can show interference when parts of the incident beam separate, reflect off the top and bottom of the film, and then recombine. Mathematically, this can be summarized by two equations.

Equation 1 is used for constructive interference and equation 2 is used for destructive interference if either of the following conditions prevail : the index of the film is higher than the indices of the surrounding materials or the index of the film is lower than the indices of the surrounding materials

Equation 1 is used for destructive interference and equation 2 is used for constructive interference if this condition applies: the index of film is lower than the index of the material on one side of the film but higher than the index on the other side of the film.

1. Consider a film (n = 1.40) surrounded by air (n = 1.00). What minimum thickness of this film will yield a strong reflected ray for a wavelength of 680 nm?

2. A thin (.42 mm) film of glass (n = 1.60) is illuminated with white light. What wavelength (and color) is most enhanced in light reflected by the film.

3. An oil slick, n = 1.25 , rests atop a pond (n = 1.33) and is orange-yellow in color (l = 600 nm). What is the minimum thickness of the slick?

4. A non-reflective coating (n = 1.55) is applied to a piece of glass ( n = 1.48). The thickness of the coating is 177.4 mm. What wavelength (and color) is removed from the reflected beam?

This page was last modified by mgosselin on 10/08/2005