Changing frequency of EM wave
Started by Chocolatebear89
one way that frequency of an EM wave can change is through the doppler effect.
why does wavelength change but frequency remains constant when changing media?
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This is wrong. The frequency that changes is observed frequency. Whereas the frequency changes by light speed and wv is permanent!
When light is refracted in a medium its speed changes-and its wavelength changes but thats only because the energy that it came with is the same-refraction does not affect the energy of the wave. Since energy of the wave is directly related to frequency, and since the material (causing refraction) does not change energy, the frequency is the same.
So you are correct...
However, OP asked how to change frequency of EM wave....the frequency of the radiation or EM emitted by an object is equal to c/lambda where c is speed of the EM (light).
In general, one can accomplish this by changing the wavelength or the speed of light. But the change has to be made AT THE SOURCE of the EM radiation. a black body radiation emits a specific amount of energy at specific frequencies. Changing the speed of this emitted frequency AFTER it has been emitted (such as in a refraction) will only change its wavelength. However, changing the energy of the source will change the frequency. Also changing the wavelength at the source (electrons jumping to a different orbital for example) will have a different frequency emission.
That is why different colors are emitted by a burning flame etc-the electrons are jumping at different orbitals and releasing energy at different frequencies with different wavelengths since colors are observed at different wavelengths...
However, another poster mentioned that based on doppler effect, frequency can change. However, this change in frequency is different from the change in frequency due to wavelength change.
A doppler effect is a PERCEIVED change in frequency because the source is moving towards or away from the observer. Thus the observer gets a different frequency (relative change in frequency) from that emmitted by the source. But the source frequency is NOT changing. The only way to change the source frequency (absolute change in frequency) would be to change the emitted wavelength or the speed of light...
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I am not sure what you mean. First of all, what is "defraction"? Do you mean refraction? Because when light's speed changes as a result of refraction, only its wavelength changes, not the frequency. You can't assume that because speed changes that frequency must change as well.
You are correct that one cannot assume that a speed change implies frequency change...but I was merely stating that an absolute change in frequency is dependent ONLY on speed of light change and wavelength change. While speed of light may or may not change frequency (as in refraction when it only changes wavelength) the doppler effect has NO absolute change in the frequency of EM.
so we characterize colours of visible light by wavelength, not by frequency, correct?
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depends...i general yes. dispersion is basically refraction and different colors are seen because waves refract at different wavelengths-red light refracts the least (highest wavelength lowest frequency) and violet refracts the most (thats why you see red on top of the rainbow and blue on bottom)
colors are classified by wavelength usually...but that doesn't mean frequency isn't important.
if the electrons of a metal are given certain energies they will have frequencies corresponding to those energies and thus certain colors specific to thsoe frequencies. its just that we measure these colors by their wavelengths.
