Why are sunsets red? [CSE 2013]

Why are sunsets red? [CSE 2013]

• How light waves get scattered depends strongly on the size of the particle compared with the wavelength of the light. Particles that are small compared with the light wavelength scatter blue light more strongly than red light. Because of this, the tiny gas molecules that make up our Earth’s atmosphere (mostly oxygen and nitrogen) scatter the blue portion of sunlight in all directions, creating an effect that we see as a blue sky.
• At sunset, light must travel farther through the atmosphere before it gets to us, so more of it is reflected and scattered and the sun appears dimmer.
• The color of the sun itself appears to change, first to orange and then to red because even more of the short wavelength blues and greens are now scattered and only the longer wavelengths(reds, oranges) are left to be seen.
• Small particles of dust and pollution in the air can contribute to (and sometimes even enhance) these colors, but the primary cause of a blue sky and orange/red sunsets or sunrises is scattering by the gas molecules that make up our atmosphere.
• Large particles of pollution or dust scatter light in a way that changes much less for different colors. The result is that a dusty or polluted sky is usually more GRAYISH WHITE THAN BLUE.
• Similarly, cloud droplets (typically 10 millionths to 100 millionths of a meter) are much larger than visible light waves, so they scatter light without much color variation. This is why light scattered by clouds takes on the same color as the incoming light. For example, clouds will appear white or gray at midday and orange or red at sunrise or sunset.