Answer to Question 1
ANSWER: At sunrise and sunset, the rays coming directly from the Sun strike the atmosphere at a low angle. They must pass through much more atmosphere than at any other time during the day. (When the Sun is 4 above the horizon, sunlight must pass through an atmosphere more than 12 times thicker than when the Sun is directly overhead.) By the time sunlight has penetrated this large amount of air, most of the shorter waves of visible light have been scattered away by the air molecules. Just about the only waves from a setting Sun that make it on through the atmosphere on a fairly direct path are the yellow, orange, and red.
Answer to Question 2
ANSWER: Daytime: As sunlight enters the atmosphere, the shorter visible wavelengths of violet, blue, and green are scattered more by atmospheric gases than are the longer wavelengths of yellow, orange, and especially red. In fact, violet light is scattered about 16 times more than red light. Consequently, as we view the sky, the scattered waves of violet, blue, and green strike the eye from all directions. Because our eyes are more sensitive to blue light, these waves, viewed together, produce blue light.
Night: There is no sunlight available to be scattered, so our eyes see no color (black).
Tracking Goal-Setting Progress
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Example of simplified note for a nonmedical setting.
Rising sea level threatens coastal communities around the world
For the circuit in Figure 1, assume ideal diodes. The peak inverse voltage is approximately