Photons

[monkeyvokes]

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So, the frequency stays the same (just as with a standing wave entering a new medium http://i.imgur.com/WnTmWKd.png ). The wavelength also doesn't change, but the energy does because some photons are lost? Is that what this is saying?

 

[sps27]

So, the frequency stays the same (just as with a standing wave entering a new medium http://i.imgur.com/WnTmWKd.png ). The wavelength also doesn't change, but the energy does because some photons are lost? Is that what this is saying?

What this is saying is, as per my understanding, the energy of a light wave is split between reflected and refracted waves but the sum of the energies of those two waves is the same as the original wave. There are no photons lost. Remember that E = h*Nu i.e., h = planks constant and Nu is frequency, so energy depends on frequency which remains the same for both reflected and refracted waves but the intensity which has to do with the no of photons, gets split up into reflected and refracted waves.

Also I think for the reflected wave there is no change in frequency and wavelength as the medium remains the same but for the refracted wave the frequency remains the same, but 'wavelength', does indeed change because of change in refractive indexes. n1w1 = n2w2 (w1, w2 means wavelength 1 and wavelength 2) from one medium to next. And because the wavelength changes, the speed of the light wave also changes ex., light travels slowly in denser (more refractive index) medium.

The statement on twice the energy is a bit confusing for me too, but I understand what they are alluding to is the fact that energies get split up into 2 waves but the sum remains the same.

Correct me if I am wrong....🙂

 

[monkeyvokes]

So when light enters a different medium, the change in velocity is inversely proportional to the change in wavelength. When, for instance, looking at objects that are under water, do they appear to slightly be a different color?

 

[sps27]

So when light enters a different medium, the change in velocity is inversely proportional to the change in wavelength. When, for instance, looking at objects that are under water, do they appear to slightly be a different color?

Lets just take monochromatic light i.e., light of a single wavelength because the term light has several wavelengths in it with different frequencies and it gets confusing. For ex., red light which has wavelength around 700nm. So when red light travels from air to glass, glass being a denser medium, it will bend and change speed. And this phenomenon we call refraction. So how much will it bend and what should the velocity be, we will try to establish from equations.

1) refractive index n = c/v where c=velocity of light in vacuum and v=velocity of light in medium. The velocity of light in a denser medium can never be more than velocity of light in vacuum, so the c/v will always be greater than 1 i.e., n will be greater than 1 for any medium.
2) More the value of n, means denser the medium or lesser is the speed of red light in that medium.
3) v = frequency * wavelength tell us that if frequency remains same and if v decreases, wavelength must also decrease. So for red light traveling into a denser medium, we have established that speed decreases. So therefore it must follow that wavelength decreases as well for red light. Which means speed and wavelength are directly propotional, not inversely.

So now for the color aspect, visible light itself is a combination of so many different wavelengths (colors) and when this light crosses a medium, all these different wavelengths will change their speed and wavelength by different amounts (each color will have its own refractive index in that medium) and bend by different amounts (higher wavelength gives less bending, lesser wavelength gives more bending) giving rise to 'dispersion' which we see as different colors so things appear colored under water.

If you have access to TBR physics part 2, look at Example on the bottom of page 244. That will help explain things better than I have.

Hopefully I have not confused the heck out of you and myself included. If I have stated anything wrong, do let me know.

 

[monkeyvokes]

good answer, ok,
so n = c/v, lets say water has an n value of 1.3 and air is 1. light goes into water, then the velocity is .80 of what it was in air. So, then the wavelength is .80 of what it was in air, correct?
So if red light (700nm) enters water, then it becomes green light (530nm)?
something doesnt seem right..?

 

[sps27]

good answer, ok,
so n = c/v, lets say water has an n value of 1.3 and air is 1. light goes into water, then the velocity is .80 of what it was in air. So, then the wavelength is .80 of what it was in air, correct?
So if red light (700nm) enters water, then it becomes green light (530nm)?
something doesnt seem right..?

Color is frequency and wavelength not absorbed. But is frequency changing when light crosses mediums....nay not so, so then color will not change.........

 

[monkeyvokes]

Color is frequency and wavelength not absorbed. But is frequency changing when light crosses mediums....nay not so, so then color will not change.........

Wow interesting, didn't realize. thank you