Sciencemadness Discussion Board - spectroscopy and visible light

spectroscopy and visible light

Yttrium2 - 17-3-2015 at 08:18

I remember faintly doing something with spectroscopy, and this girl in our class could see more colors than others

Can anyone explain this

Yttrium2 - 17-3-2015 at 08:33

Another question, When looking up from the ground why is the sky blue?

annaandherdad - 17-3-2015 at 14:25

Quote: Originally posted by Yttrium2

Another question, When looking up from the ground why is the sky blue?

Sunlight is scattered by the air. There is a resonance in the ultraviolet, and blue light is closest to it (in frequency), so it is scattered more strongly than other colors. I think the molecule that does this is O2 not N2. Otherwise called Rayleigh scattering. The formula for the cross section is the Kramers-Heisenberg formula.

careysub - 17-3-2015 at 16:55

Quote: Originally posted by annaandherdad

Quote: Originally posted by Yttrium2

Another question, When looking up from the ground why is the sky blue?

It isn't due to a resonance. It is due to the non-resonance but frequency dependent photon scattering properties of diatomic molecules, which develop dipole moments. The scattering strength varies as the fourth power of frequency (or wavelength). N2 has a larger Rayleigh cross section than O2, and is also more abundant, so it does most of the scattering, but O2 scatters blue light as well.

http://adsabs.harvard.edu/full/1969MNRAS.145..171T

You can demonstrate a similar effect by adding 25 mL of milk to 4 L of water, and shining a strong white light beam though it (a square tank works best for this, bright LED lights work well). The light emerging straight through the tank is reddened, viewing from the side the tank glows blueish.

careysub - 17-3-2015 at 17:02

Quote: Originally posted by Yttrium2

I remember faintly doing something with spectroscopy, and this girl in our class could see more colors than others

Can anyone explain this

Sounds like she is one of the lucky few women who have tetrachromatic vision.

Normal humans are trichromatic, with three color receptor cell types: S, L, and M. The L and M receptors are encoded on X chromosomes.

Since men have only one X chromosome, if they have a defective L or M receptor they are colorblind (two different types, but they can both be bad, with severe colorblindness).

But some women have two different forms of the L receptor and thus have four distinct receptor types and can perceive more colors than normal humans.

The L receptor gene (the OPN1LW gene) has a very high degree of variability, suggesting that variation may be a good thing in the population.

[Edited on 18-3-2015 by careysub]

annaandherdad - 18-3-2015 at 11:31

It isn't due to a resonance. It is due to the non-resonance but frequency dependent photon scattering properties of diatomic molecules, which develop dipole moments. The scattering strength varies as the fourth power of frequency (or wavelength). N2 has a larger Rayleigh cross section than O2, and is also more abundant, so it does most of the scattering, but O2 scatters blue light as well.

http://adsabs.harvard.edu/full/1969MNRAS.145..171T

You can demonstrate a similar effect by adding 25 mL of milk to 4 L of water, and shining a strong white light beam though it (a square tank works best for this, bright LED lights work well). The light emerging straight through the tank is reddened, viewing from the side the tank glows blueish.[/rquote]

Well you are right that Rayleigh scattering applies when the frequency of the incident photon is much less than any resonant frequency, and this gives the omega^4 dependence of the cross section. I didn't mean to say that it was resonant scattering, which has a much, much higher cross section than nonresonant scattering.

However, the actual frequency of blue light is not enormously much smaller than the resonant frequencies, which are in the ultraviolet, so the Rayleigh formula is only rough. And if you take the resonances into account, then two of the powers of omega come from expanding the denominators in 1/(omega - omega_R), where omega is the frequency of the incident photon and omega_R is the resonant frequency. These denominators nominally go to infinity at omega -> omega_R. The infinity is finally made finite in a small frequency range around omega=omega_R by the spontaneous emission. But as omega climbs through the visible toward the UV, these denominators inflate the cross section more and more.

These are my notes on this subject.

http://bohr.physics.berkeley.edu/classes/221/0708/lectures/L...