Physics · Ch 6 — Optics
Scattering of Sunlight
Scattering of Sunlight
When sunlight enters the atmosphere, atmospheric particles change the direction of the light -- a process called scattering. If the scattering particles are much smaller than the light's wavelength (, true for air molecules), the process is called Rayleigh's scattering, whose intensity is inversely proportional to the fourth power of wavelength, . Violet light, having the shortest wavelength, is scattered the most; since the human eye is more sensitive to blue than to violet, the daytime sky appears blue. At sunrise and sunset, sunlight travels a much longer path through the atmosphere, so nearly all the strongly-scattered blue light is scattered away before reaching the observer, leaving mostly the weakly-scattered red light to get through directly -- the reason for the reddish sky at these times. If the scattering particles are much larger than the wavelength (, as with dust and water droplets in clouds), the scattering becomes essentially wavelength-independent, so all colours scatter equally and clouds appear white (rain clouds appear dark instead, because their water droplets, condensed onto dust particles, become opaque and absorb rather than scatter light). With no atmosphere at all, there is no scattering, which is why astronauts see a black sky even in daylight. A rainbow is a natural example of dispersion by water droplets, each acting as a …
What this figure shows. Two panels compare how sunlight reaches an observer at different times of day. In panel (a), around midday, sunlight travels a comparatively short path through the atmosphere before reaching the observer, so only a modest amount of the shorter (blue/violet) wavelengths get scattered out of the direct beam and towards the observer's eye from all directions, giving a blue sky. In panel (b), at sunrise, sunlight grazes in almost horizontally and must travel a much longer path through the atmosphere to reach the observer, so nearly all the strongly-scattered blue and violet light is scattered away long before arriving, leaving mostly the weakly-scattered red and orange light to reach th …
Worked out. A rainbow forms when sunlight enters countless suspended water droplets in the air, each one dispersing the sunlight into its constituent colours much like a tiny glass prism. A primary rainbow forms from light that undergoes exactly one total internal reflection inside each droplet before emerging back towards the observer, with red appearing at the outer edge and violet at the inner edge across a viewing angle of about 40 to 42 degrees. A fainter secondary rainbow, with its colour order reversed (violet outside, red inside) and appearing outside the primary bow, forms from light undergoing two internal reflections inside each droplet, spanning a viewing angle of about 52 to 54 degrees; in …