Question

27) A piece of glass has a th g on it. A beam of light is shining perpendicular on the film. If the wavelength of light incident on the film is 560 nm and the indices of refraction of gasoline and glass are ively, what is the minimum nonzero thickness of the film to see a bright reflection? A) 300 nm B) 500 nm C) 100 nm D) 400 nnm E) 200 nm

Answer 1

At the first interface, the incident light is in air, which has a lesser index of refraction then the thin film, so a phase shift occurs . At the second interface, the incident light is in the thin film, which has a lesser index of refraction than that of glass, so a phase shift occurs . Because both reflections cause the same phase shift, they tend to put the waves in phase . The condition for constructive interference is, t = m lambda_0/2 n where m = 0, 1, 2, 3, ......... the condition for destructive interference is, t = (m + 1/ 20 lambda_0 /2n where m = 0, 1, 2, 3 .......... Here lambda_ 0 is the wavelength of light in vacuum, and n is the index of refraction of the thin film the minimum thickness