Dispersion of light

Rainbow

A rainbow isn’t really a “thing” and it doesn’t exist in a particular “place.” It is an optical phenomenon that appears when sunlight and atmospheric conditions are just right—and the viewer’s position is just right to see it.

A rainbow requires water droplets to be floating in the air. That’s why we see them right after it rains. The Sun must be behind you and the clouds cleared away from the Sun for the rainbow to appear.

Why is a rainbow a bow—or arc?

A full rainbow is actually a complete circle, but from the ground, we see only part of it. From an airplane, in the right conditions, one can see an entire circular rainbow.

Atmospheric Refraction

Why sun looks red while rising and setting?

The light from the Sun travels through Earth’s atmosphere it undergoes scattering before it reaches us. The extent of scattering is not uniform for all colors.

Light of shorter wavelengths, such as violet, blue, green, and yellow undergoes greater scattering than those of longer wavelengths, such as orange and red.

Because of the spherical geometry of the earth, the sunlight travels a longer distance in the thick of the earth’s atmosphere during sunset and sunrise when the Sun is at the horizons than when the Sun is at the zenith (midday).

Thus, there is more probability for shorter wavelength light to get more scattered than for the longer wavelength light.

At sunrise, the sun is near the horizon; hence, sunlight has to travel a longer distance through the atmosphere to reach us. During this journey, most of the blue color present in sunlight, which is of a shorter wavelength, gets scattered from out line of sight. Thus, the red color present in sunlight, which has a longer wavelength and is scattered the least, reaches our eyes. As we see red light everywhere above, the sun and the surrounding sky appear red at sunrise. 

Scattering of Light

Why the sky is blue?