Question

Thin-Film Interference Consider 3 situations in which light of wavelength 600 nm in air reflects perpendicularly from a thin film of thickness 200 nm and an index of refraction of 1.5. In each situation, the film lies between two media with different indices of refraction as shown. Both source and detector are above the film in air. Situation2 Situation.3 Situation1 n 1.4 n 1.4 n=1.5 n 1.3 10. (1.5 points) In which situation or situations will the film appear the brightest? a) 1 and 2 d) 2 only b) 1 and 3 c) 2 and:3 e) 3 only Relativity Brian and Angie skip physics class to travel through space in identical spaceships. Brian measures his ship to be 300 m long, and measures Angie's spaceship to be 180 m long. 11. (1 point) What length does Angie measure for Brian's ship? a) 600 m b) 360 m d) 180 m c) 300 m e) 120 m 12. (2 points) What does Brian measure for the speed of Angie's ship relative to his?

Answer 1

10] From Fresnel's equations, it is known that when light is incident from lower to higher index medium, the reflected light will have a path difference of half wavelength. When light is incident from higher to lower index medium, the reflected light will have zero path difference.

The film will appear the brightest when constructive interference occurs for light reflecting from upper and lower surfaces.

If we consider situation 1, the reflected light from the upper surface of the layer develops a half wavelength path difference whereas the light from lower surface has zero path difference. The light at the detector will therefore destructively interfere and so the film will not appear bright.

This is not the case for situation 2 and situation 3 because light from both the upper and lower surfaces develop an equal path difference due to the reflection. Therefore, the light at the detector will constructively interfere and make the film appear bright.

[answer - c]