Why does red light travel faster than blue light in glass or water?

[Pranav Jha]

which light travels faster in water: red or blue? i think the natural frequency of water molecules is more similar to red light than blue. So, the optical density of water for red light should be greater than for blue light contrary to that of a glass prism. I believe red light travels slower in water than blue
But everywhere i have found glass and water treated in the same manner. So, i am not sure any longer.

Please clear my doubt

 

[NobodySpecial]

Which wavelength is bent most by a prism (hint find a pink floyd album)
The higher the refractive index, the more light is bent.

 

[Pranav Jha]

Which wavelength is bent most by a prism (hint find a pink floyd album)
The higher the refractive index, the more light is bent.

i am referring to the comparison between glass and water! i think the two cases should be regarded separately based on the natural frequency of light and glass

 

[Sourabh N]

Get a Pink Floyd anyway!

As NobodySpecial pointed out, higher the refractive index, more the bending. For both glass and water (and I'm guessing, for any medium with positive Refractive index), the order of decreasing/increasing refractive index is same, so both with show similar dispersion pattern.

 

[Pranav Jha]

blue light bends more in prism than red because the frequency of blue light is more similar to that of natural frequency of glass molecules and thus the resulting velocity for blue light is smaller and thus the greater refraction
however, i think, the natural frequency of water molecules more closely resembles that of red light than blue light. So, shouldn't the refraction for red light be greater than blue light in water?

 

[Sourabh N]

Either I'm completely misinterpreting your point here, or check this out - http://en.wikipedia.org/wiki/Visible_spectrum
http://www.madsci.org/posts/archives/1998-02/887203231.Ph.r.html

The frequencies are WAY off.

What do you mean by natural frequency of water anyway? It's so malleable.

 

[K^2]

http://upload.wikimedia.org/wikipedia/en/7/72/Water_absorption_spectrum.png

Note the log scale. There is a lot more absorption in the UV than in IR. This is actually very typical for most materials. It's far easier to excite transition with high energy photon than low energy photon. Hence the fairly universal tendency of transparent media to have higher refraction index in shorter wavelengths.

 

[Pranav Jha]

http://upload.wikimedia.org/wikipedia/en/7/72/Water_absorption_spectrum.png

Note the log scale. There is a lot more absorption in the UV than in IR. This is actually very typical for most materials. It's far easier to excite transition with high energy photon than low energy photon. Hence the fairly universal tendency of transparent media to have higher refraction index in shorter wavelengths.

for glass there is a lot more absorption in the UV than in IR However, for water there is a lot more absorption in the IR than UV. so, red light, which closely resembles IR in terms of frequency is surely slowed down more than blue
so, doesn't that play an important role in refraction of visible light ?

 

[Pranav Jha]

which is more likely to force water molecule to resonate: blue or red?

 

[cjl]

for glass there is a lot more absorption in the UV than in IR However, for water there is a lot more absorption in the IR than UV. so, red light, which closely resembles IR in terms of frequency is surely slowed down more than blue
so, doesn't that play an important role in refraction of visible light ?

Look at the absorption spectrum that K^2 posted - it shows the absorption of water. In water, there is much more absorption in UV than IR, which disagrees with your statement.

 

[Pranav Jha]

okay, i got that fact wrong
so, the absorption in UV is followed by absorption in IR. Keeping that in mind, isn't it confusing that light absorbs red light more than blue light?

 

[K^2]

No. Again, look at the graph. The visible light is near the 1μm mark. In that region, if you move towards the red absorption increases, and as you move towards blue, it decreases. But a bit after you cross into UV, absorption starts to increase rapidly. We do not normally observe that bit because we cannot see far enough into violet, but it has effect on the index of refraction.

 

[Pranav Jha]

so: absorption for red is greater than blue. So, when light is traveling through water, red tends to get absorbed more than blue. Thereby, the speed of red light should be slower in water than blue light contrary to that in glass, right?

 

[K^2]

No. Speed of light doesn't depend on absorption at that specific wavelength. It's the effect of resonances on all possible wavelengths.

So yes, red is absorbed more, but no, it's not slower.

 

[Pranav Jha]

isn't it red absorbed more because it's frequency matches one of the resonant frequencies?

 

[K^2]

No. If it actually matched, you wouldn't be able to see red light under water at all. It's simply closer to one of the absorption peaks in IR. But there are significantly more strong peaks in UV overall, even though they are slightly further away. So they still end up determining index of refraction.

Edit: Here, take a look at this graph. The blue line is index of refraction. Red line is the absorption. If you go far enough into IR, the absorption in IR starts to dominate index of refraction behavior, and you can see it increasing. But in visible spectrum, UV still dominates. Index of refraction increases as you reduce the wavelength.

 

[litup]

People seem to forget that the higher the refractive index, the slower the light is traveling. Red light is bent less than blue, therefore blue goes somewhat slower than red and IR goes faster than UV. The absorption issue is another story. IR will get there first in any race underwater and UV will be the slowest.

 

[Pranav Jha]

so why does red light travel faster than blue light in glass or water? what physical phenomenon is at work in slowing the two lights to different speeds

 

[NobodySpecial]

so why does red light travel faster than blue light in glass or water? what physical phenomenon is at work in slowing the two lights to different speeds

It's complex and not easy to calculate from the material properties.
There are even glasses with negative dispersion where the red light is bent more in some circumstances - this let's you create optical fibres where neither light is slowed more than the other and a pulse is unchanged over very long distances.

If you want to look it up - it's called dispersion - but understanding it involves knowing a lot about real and imaginary group and phase velocities, there isn't a simple answer