Coherent scattering among tightly packed molecules enables clear vision.
Evidence
Rayleigh had made a discovery of major significance. He showed that blue, short-wavelength light scatters more strongly from small objects (objects smaller than a wavelength of the light) than does redder, longer wavelength light. As a mnemonic, think of the short wavelength light as a small sports car, which can change directions easily at a sharp intersection. It is difficult for a large truck to make the same turn at the same speed. Air molecules bathed in the light of the sun are tiny compared to the wavelengths of the sun’s rays. Rayleigh thought he had explained the blue sky, for the first time in history. He is credited with the discovery to this day, but actually he explained only why the sky would be blue on another planet with a much lower density atmosphere. His work suffers a fundamental oversight: the air molecules are too tightly packed to ignore coherent effects from neighboring molecules—that is, neighboring molecules will scatter in phase with each other, since they are being perturbed in phase by the incoming light, if the wavelength is long. This coherence actually suppresses the side-scattered blue light from each molecule, and reinforces the forward scattering, with a net effect only of a refractive index change. If it were not for this effect, we’d all be blind, for we rely on this kind of coherence for clear vision through cornea, lens, and vitreous humor, all of which are very lumpy indeed on the molecular scale, and contain far more molecules per cm3 than does air. In fact, when the fluctuations become too great on the scale of visible light wavelengths, we suffer cataracts. It is once again the issue of the size of the roughness compared to the wavelength, as in the reflection issue raised in figure 2.8. — Eric J. Heller, Why You Hear What You Hear- An…, 2635
Categorization
- DEFINE: Explains coherent scattering and why ordinary Rayleigh scattering alone would not produce a blue sky on Earth.
- QUESTION: The threshold criterion for cataracts — roughness on the scale of visible wavelengths — is worth chasing as a precise quantitative relationship.
- XREF: Connects to wave-theory roughness scaling from chapter 2 (figure 2.8 reflection issue). — connects to: Scintillation is a ubiquitous refraction phenomenon across light and radio waves; Air is a seething low-density molecular gas with constant collisions
- corroborated: 2 independent passages support this claim — Eric J. Heller, Why You Hear What You Hear- An…, 2635