Lecture 13

Light in the atmosphere.

A good read on this is Chapter 2 of Color and Light in Nature by Lynch and Livingston. The following discussion comes from their book.

Light enters the atmosphere.
Either scattered or absorbed.
Absorbed light is absorbed into some other kind of energy. Absorbtion is not uniform across the spectrum.

Rayleigh's theory of scattering: probability that a photon will be scattered is inversely proportional to the fourth power of the wavelength. So that blue light at 450 nm is 3.2x more likely to be scattered than red which has a wavelength of 600 nm (600/450)^4 = 3.2.

[Luther] Mie's solution to Maxwell's equations: for larger particles (on order of hundreds of nanometers), scattering is not very wavelength dependent and tends to create only small adjustments in the light angle. This effect is seen in haze, the white glow around the sun, etc.

Brightness: more molecules between you and sun = brighter sky. Why? because molecules scatter light outside of the sun position.

Sky is the brightest by the horizon. There's a limit on brightness due to scattering just as snow looks as white as its going to get once it reaches a certain depth. Looks white because there's enough particles to scatter at all wavelengths. Looks bright because it's almost all scattering and almost no absorbtion.

airlight is just skylight but seen against the terrain in the background. Airlight happens when air scatters light at a low altitude.

Low angle effects

Why is the run red as it sets? Because ozone absorbs blue and green. Scattering happens more in blues as well.

[Luther] How much does ozone absorb? I thought it was supposed to be a fairly small amount in the visible spectrum, and that the red was due primarily to Rayleigh scattering.

[Mike] I think you are right here and below. The absorption spectrum for ozone is in Haber's paper. There's not a lot getting absorbed in the greens.

the twilight arch is caused by scattering in the atmosphere. So why isn't it blue? Ozone absorbtion?

[Luther] I think the twilight arch is red because the blue light from the sun is being strongly scattered closer to the sun, while the red light is less scattered and travels farther, far enough to reach around the horizon.

Why does the sun slow down as it sets? because it doesn't. It just looks like it slows down. The sun sets at constant speed, but due to refraction in the atmosphere, it looks like it slows down.

[Luther] In particular, the index of refraction of the air causes light entering the atmosphere at a near-horizontal angle to deflect downward about 2 degrees before reaching the surface. Thus, for the last several degrees of the sun's visible path (and the first several as well) the visible angle to the sun changes more slowly than the geometric angle to the sun.

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