TBR interference question

[cloak25]

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A thin layer of plastic (n=1.46) coats the surface of a glass plate (n=1.52). When monochromatic light is shined normally onto the plastic from the air above, constructive interference occurs. Destructive interference will occur when the coated glass is immersed in which of the following liquids?

I. water (n=1.33)
II. Carbon Tetrachloride (n=1.46)
III. Benzene (n=1.50)

A. I
B. I and II
C. II and III
D. III

Answer: D. explanation was quite long and I just didn't fully understand it. Can someone provide a 5th grade level explanation please? thanks 😳

 

[MedPR]

When two waves that are 180deg out of phase interfere, they cancel each other out. This is termed destructive interference.

Draw out the refraction/reflection diagrams through the media in each of the two scenarios (air and benzene) and you will see that there is destructive interference when the plastic+glass is immersed in something with a higher refractive index than the plastic.

 

[MedPR]

Actually, I just read this last night. The section right before the question does a good job of explaining it. Try reading that and see if it helps.

 

[cloak25]

thanks! ill give it another read.

 

[SaintJude]

can you still type out the explanation, cloaks25? (please)...cause the rest of us are sinking...at least i am

 

[ivtekk]

Here's the explanation:

"Since only the medium above the plastic changes (and not the thickness of the plastic itself), a change from constructive to destructive interference must relate to phase flips. Phase flips of the light occur when the light bounces off of a medium whose refractive index exceeds that of the medium through which the light passes. In the original constructive set-up, light bounces off of an air-plastic interface and a plastic-glass interface. For both reflections, the refractive index of the reflecting medium is greater than that of the medium through which the light passes. For both reflections, the phase of the light will flip by 180 degrees. To get destructive interference, we would need to change the experiment such that one of these two phase flips does not occur. If this is achieved, the single-phase flip would lead to destructive interference (i.e. 180 degrees out-of-phase) between the two final rays. Since the plastic-glass interface is fixed, we must get rid of the phase flip at the air-plastic interface. To do so, we select a liquid whose refractive index is greater than that of the reflecting plastic. Benzene is the only liquid with this property. Water will not change the constructive interference because it is less optically dense than the plastic (i.e. a phase flip will occur at the water-plastic interface). Carbon tetrachloride will simply lead to reflection at the plastic-glass interface, because the have they same refractive index."