AP Physics 2 : Wave Interference and Diffraction

Study concepts, example questions & explanations for AP Physics 2

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Example Questions

Example Question #1 : Wave Interference And Diffraction

Wave interference is __________ when the amplitudes of the waves add together to make a larger wave overall, while it is __________ when the amplitudes of the waves work against one another to make a smaller wave overall.

Possible Answers:

destructive . . . constructive

diffusive . . . destructive

refractive . . . constructive

constructive . . . destructive

refractive . . . detractive

Correct answer:

constructive . . . destructive

Explanation:

When waves add together and make a larger wave, think of them as constructing a big wave. On the other hand, waves that make a smaller wave are said to be destructive because they are "destructing" each other and making a smaller wave, or no wave at all.

Example Question #2 : Wave Interference And Diffraction

When light __________ interferes, the result is __________ light overall, and when light __________ interferes, the result is __________ light overall.

Possible Answers:

None of these

additionally . . . brighter . . . subtractively . . . less bright

destructively . . . brighter . . . contsructively . . . less bright

constructively . . . brighter . . . destructively . . . less bright

subtractively . . . brighter . . . additionally . . . less bright

Correct answer:

constructively . . . brighter . . . destructively . . . less bright

Explanation:

Because light has properties of waves, it can have interference. When two crests of the wave meet, this is known as constructive interference, and results in the two crests adding, meaning the light gets brighter. When a crest and a trough meet, this is known as destructive interference, and results in the two partially or completely cancelling each other out, making the observed light less bright.

Example Question #3 : Wave Interference And Diffraction

Suppose that monochromatic light is passed through a sheet of glass from air. As it travels through the glass, it is refracted. Which of the following parameters of the light does not remain the same?

Possible Answers:

Period

All of these parameters stay the same

Energy

Frequency

Wavelength

Correct answer:

Wavelength

Explanation:

This question is describing a scenario in which a ray of monochromatic light is being refracted by passing from air into glass. We're then asked to determine which parameter of the light does not change.

First of all, let's review what refraction is. Refraction is an event that happens whenever light passes from one medium into another medium. In doing so, the ray bends. This bending of light is due to the fact that the speed of light changes depending on the medium in which it is traveling.

As the light crosses from the air and into the glass, its speed is slowed down. This is because glass is denser than air. Also, since the speed of light is a function of both wavelength and frequency, then one or both of these variables must change. The frequency remains unchanged while the wavelength becomes altered. One way to think about this is by applying conservation of energy. If the frequency changed, then the energy of the light would also change. However, the energy of the light remains unchanged during refraction. Thus, only the wavelength and the speed change.

Since frequency remains the same, we also know that the period will stay the same, since period is just the inverse of frequency. Also, as was mentioned above, energy will not change because energy is a function of frequency.

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