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.
refractive . . . detractive
destructive . . . constructive
constructive . . . destructive
refractive . . . constructive
diffusive . . . destructive
constructive . . . destructive
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 #1 : Wave Interference And Diffraction
When light __________ interferes, the result is __________ light overall, and when light __________ interferes, the result is __________ light overall.
subtractively . . . brighter . . . additionally . . . less bright
destructively . . . brighter . . . contsructively . . . less bright
None of these
additionally . . . brighter . . . subtractively . . . less bright
constructively . . . brighter . . . destructively . . . less bright
constructively . . . brighter . . . destructively . . . less bright
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 #1 : 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?
Period
Energy
Wavelength
Frequency
All of these parameters stay the same
Wavelength
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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