2 An oil film ($n = 1.5$) floats on the surface of a bowl of water. The
3 film is illuminated by a white light placed directly above the bowl.
4 Red light at $\lambda = 650\U{nm}$ is the most strongly reflected
6 \Part{a} How thick is the oil?
7 \Part{b} What color is most strongly transmitted? % warning, students don't like ultraviolet as a color.
8 \end{problem} % Based on Problem 27.15
12 Constructive intereference on reflection follows Equation 27.11
14 2nt = \p({m+\frac{1}{2}})\lambda \qquad m = 0,1,2,\ldots
16 Where the $+\frac{1}{2}$ is due to the $180\dg$ phase change from a
17 reflection off the interface in the direction of increasing index of
18 refraction (for the path that bounces off the air-oil interface).
20 The strength of the reflected color decreases as $m$ increases, due to
21 some light being absorbed by the oil and decoherence from scattering.
22 Since red light is the most strongly reflected, $m = 0$, and the
23 thickness of the film is
25 t = \frac{\lambda}{4n} = \ans{108\U{nm}}
29 Constructive interference on transmission occurs when
31 2nt = m\lambda \qquad m = 1,2,3,\ldots
34 Because of absorbtion, the strongest transmission will be for $m=1$, so the
35 most strongly transmitted color is
37 \lambda_t = 2nt = \ans{325\U{nm}}
39 which is in the ultraviolet.