Thin Film Interference Quiz

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

True

False

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.

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.

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

The reflected light becomes brighter and brighter

The reflected light alternates between dark and bright spots

The reflected light becomes darker and darker

There is no reflected light

None of the Above

When looking perpendicularly at a thin glass film, the interference of light waves reflected from the top and bottom surfaces of the film causes the reflected light to alternate between dark and bright spots. This phenomenon is due to constructive and destructive interference as the optical path length changes. As you move your head, the angle of incidence changes slightly, altering the path difference and causing the alternating pattern of light intensity.

Explanation

When looking perpendicularly at a thin glass film, the interference of light waves reflected from the top and bottom surfaces of the film causes the reflected light to alternate between dark and bright spots. This phenomenon is due to constructive and destructive interference as the optical path length changes. As you move your head, the angle of incidence changes slightly, altering the path difference and causing the alternating pattern of light intensity.

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

The film reflects some of the light

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

The reflected ray changes phase by λ/2

All of the above

None of the Above

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.

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.

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

True

False

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.

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.

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

True

False

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

Explanation

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

6. 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?

6.0 X 10^-7 m

1.8 X 10^-6 m

1.1 X 10^-7 m

4.8 X 10^-6 m

None of the Above

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

Explanation

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

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

True

False

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.

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.

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

True

False

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

Explanation

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

9. 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?

600nm

483nm

500nm

650nm

None of the Above

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

Explanation

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

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

Changes phase by λ/2

Changes phase by λ/3

Changes phase by 3λ/2

Changes phase by λ/4

None of the Above

When light travels from a fast medium to a slower medium, it bends towards the normal, and there is no phase change due to the refraction itself. Phase change of λ/2 typically occurs when light reflects off a boundary where it moves from a medium with lower refractive index to a medium with higher refractive index, not during refraction. Therefore, "None of the above" is the correct answer.

Explanation

When light travels from a fast medium to a slower medium, it bends towards the normal, and there is no phase change due to the refraction itself. Phase change of λ/2 typically occurs when light reflects off a boundary where it moves from a medium with lower refractive index to a medium with higher refractive index, not during refraction. Therefore, "None of the above" is the correct answer.