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Does the frequency of light change when it travels across an interface between two media? What happens to the light wavelength and the light velocity at the interface?

I've gotten different answers some say all three change and some say frequency is constant or some say that the wavelength is constant...

What's the right answer?

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Possible duplicate: physics.stackexchange.com/q/52148/2451 and links therein. Related: physics.stackexchange.com/q/22385/2451 –  Qmechanic Oct 15 '13 at 15:59
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4 Answers

up vote 3 down vote accepted

Frequency is constant. If it were otherwise, you would have more (or fewer) wave crests hitting the interface per unit time than leaving it, which would lead to some sort of pileup that gets worse and worse as time progresses. Another way to think about it is the atoms/electrons/electric field lines/whatever at the interface respond immediately to their neighbors (the wave is essentially continuous), even if those neighbors are in a different medium.

Wavelength can change, and it does so in such a way that $f \lambda = c/n$ at all locations. Here $f$ is the frequency, $\lambda$ is the wavelength, $c$ is the speed of light in vacuum, and $n$ is the index of refraction. As you can see, going from air ($n \approx 1$) to glass (say $n = 1.4$) will result in the wavelength of optical light to decrease by a factor of $1.4$. Also note that in general $n$ can depend on $f$, leading to colors dispersing as in rainbows and chromatic aberration.

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Does the frequency of light change when it travels across an interface between two media? What happens to the light wavelength and the light velocity at the interface?

When an EM wave passes through any medium other than air, it's frequency (number of oscillations per second) still remains the same. Only the distance between individual crests (or troughs) shorten out and thereby a decrease in the wavelength. And hence, it travels less distance at a given period of time (and hence $c_{medium}<c_{vacuum}$). As Chris said, This is probably determined by the refractive index (a relative parameter) of the medium. The color of light still remains the same, since its energy $h\nu$ still hasn't changed. There's another interesting answer for: "Why doesn't color change with wavelength?"

Does light change color on its way through a window?

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Please make a note of it : Frequency of any electro-magnetic wave is a property of the source oscillator . It never changes once EM wave is produced. $ v=\lambda * \nu $ ; $$ \nu = v/\lambda$$ $\nu$ ( frquency is a constant)

$v$ (velocity ) changes as per medium and correspondingly changes $\lambda$.(wavelengh)

$$ v1/v2 = \mu1/\mu2$$ $\mu$ is the refractive index of medium.

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Hi explringnet. I think you're not aware of several of our FAQs like collaborative revision. Users here may revise your post, if it requires a revision. It maybe a TeX correction, a typo or even a spelling correction. Well, I revised your post because it had some grammatical as well as spelling errors. If you don't like others to revise your post, atleast you can keep it error-less ;-) –  Waffle's Crazy Peanut Mar 21 '13 at 7:35
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When a wave travels from medium to another, it's velocity changes based on the refractive index of the medium. It is this change in velocity that causes refraction.

Think of a series of wave-fronts approaching a boundary. If the light is not traveling perpendicular to the boundary, part of this wave-front will hit the boundary before the rest. This part of the wave-front will then have a reduced velocity, and so will distort in relation to the rest of the wave-front. Eventually the rest of the wave-front will also hit the boundary and reduce in velocity. The overall effect of this is that the angle of the wave to the boundary has changed.

View an image of this here.

Note that the frequency and wavelength are constant.

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-1. Wavelength doesn't stay constant. It can't, if the relation $\nu \lambda = c$ is to hold true (it does!) in a medium. –  Kitchi Mar 21 '13 at 9:44
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