Thin Film Interference Quiz

1.

Different colours are visible when looking perpendicularly on a thin film of soap because

A.

The film reflects some of the light
B.

The film is thin enough so that the refracted ray is close to the reflected ray
C.

The reflected ray changes phase by λ/2
D.

All of the above
E.

None of the Above

Correct Answer
D. All of the above

Explanation
When looking perpendicularly on a thin film of soap, different colors are visible because all of the given statements are true. The film reflects some of the light, causing certain wavelengths to be reflected and others to be transmitted. The film's thinness allows for the refracted ray to be close to the reflected ray, resulting in interference patterns that create different colors. Additionally, the reflected ray changes phase by λ/2, further contributing to the color variations observed. Therefore, all of the above statements explain why different colors are visible when observing a thin film of soap perpendicularly.

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2.

If you look perpendicularly at the thin glass film and move your head away from the glass film (staying perpendicular to the film), you will notice

A.

The reflected light becomes brighter and brighter
B.

The reflected light alternates between dark and bright spots
C.

The reflected light becomes darker and darker
D.

There is no reflected light
E.

None of the Above

Correct Answer
B. The reflected light alternates between dark and bright spots

Explanation
This is the same as moving your head up/down on the screen in a single or double slit experiment.

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3.

A lens (n=1.52) is coated witha film of magnesium fluoride (n=1.25). What should be the least thickness of the film in order to minimize reflected light with a wavelength of 550 nm?

A.

6.0 X 10^-7 m
B.

1.8 X 10^-6 m
C.

1.1 X 10^-7 m
D.

4.8 X 10^-6 m
E.

None of the Above

Correct Answer
C. 1.1 X 10^-7 m

Explanation
A similar question is solved in the sample problems section of this unit on the website.

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4.

When light travels from a fast medium to a slower medium, the refracted ray

A.

Changes phase by λ/2
B.

Changes phase by λ/3
C.

Changes phase by 3λ/2
D.

Changes phase by λ/4
E.

None of the Above

Correct Answer
B. Changes phase by λ/3

Explanation
The reflected ray changes phase by λ/2, but the refracted ray travels without change, although it does lose some energy.

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5.

When a thin glass pane is viewed perpendicularly in white light, only bright white and dark spots are visible.

A.

True
B.

False

Correct Answer
B. False

Explanation
Since different colours in white light have different wavelengths the different colours change phase by different amounts and therefore, there are many colours visible at different distances and not only white.

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6.

A soap film with a thickness of 93 nm has a refractive index of 1.35. When viewed perpendicularly in white light, what wavelength of light is strongly reflected?

A.

600nm
B.

483nm
C.

500nm
D.

650nm
E.

None of the Above

Correct Answer
C. 500nm

Explanation
Solution to a similar question is given on the thin film interference sample problems section on the website.

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7.

Interference doesn't occur in thick films because the refracted ray slows down too much.

A.

True
B.

False

Correct Answer
B. False

Explanation
No, it doesn't occur in thick films because the refracted ray is redirected too much and therefore isn't close to the reflected ray of light.

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8.

The optical path of light in a thickness of glass is larger than the optical path in the same thickness of air.

A.

True
B.

False

Correct Answer
A. True

Explanation
The optical path of light is determined by the refractive index of the medium it passes through and the distance it travels. In general, the refractive index of glass is higher than that of air, meaning that light travels slower in glass than in air. As a result, the optical path of light in a given thickness of glass will be longer than in the same thickness of air. Therefore, the statement is true.

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9.

Interference in thin films follows the same mathematical model as Single Slit Diffraction for constructive interference.

A.

True
B.

False

Correct Answer
B. False

Explanation
Completely different model. Explained on the Thin Films Interference page on the website.

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10.

Thin film interference occurs because the film polarizes the reflected light.

A.

True
B.

False

Correct Answer
B. False

Explanation
The farther apart the slits, the narrower the diffraction pattern because the K max becomes smaller and therefore less diffraction occurs.

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