Thin films: A puddle of water has a thin film of gasoline floating 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 water are 1.40 and 1.33, respectively, what is the minimum thickness of the film to see a bright reflection?

Question

Quizplus · Accepted Answer

The minimum thickness of the film to see a bright reflection is when the optical path length difference between the light reflected from the top and bottom surfaces of the film is an integer multiple of the wavelength of light. For a bright reflection, the optical path length difference should be an odd multiple of half the wavelength. Therefore, the minimum thickness of the film is:t = (m + 1/2) * lambda / (2 * n)where m is an integer, lambda is the wavelength of light, and n is the index of refraction of the film.For m = 0, the minimum thickness is:t = (1/2) * lambda / nSubstituting the given values, we get:t = (1/2) * 560 nm / 1.40 = 200 nmTherefore, the minimum thickness of the film to see a bright reflection is 200 nm.

Thin Films: a Puddle of Water Has a Thin Film