Questions: Atmospheric Optics: Sky Color, Rainbows, Halos, and Mirages
4 questions to test your understanding
Score: 0 / 4
Question 1 Multiple Choice
Why is the Sun reddish at sunset?
AThe Sun gives off more red light in the evening
BSunlight passes through a much longer path of air, and scattering removes much of the blue light before it reaches you
CRed light is reflected off the ground into the sky
DThe Sun is closer to Earth at sunset
Scattering by air molecules increases sharply at shorter wavelengths (about as 1/λ⁴). Near the horizon, sunlight crosses many times more air than at noon, so most of the blue is scattered out of the direct beam and the light that remains looks orange or red.
Question 2 Short Answer
To see a rainbow, where must the Sun be relative to you and the rain?
Think about your answer, then reveal below.
Model answer: The Sun must be behind you, and the rain must be in front of you. The primary bow appears as a circle of about 42° radius centered on the antisolar point, the point directly opposite the Sun.
Light enters each drop, reflects once off the back, and exits toward the observer at about 42° from the direction opposite the Sun. If the Sun is higher than about 42° above the horizon, the primary bow is below the horizon for a ground observer.
Question 3 True / False
In a secondary rainbow, the order of colors is the reverse of the primary rainbow, with red on the inside.
TTrue
FFalse
Answer: True
The secondary bow forms from two internal reflections in each drop, which reverses the color order. It appears at about 51° from the antisolar point and is fainter. The sky between the two bows is darker and is called Alexander's dark band.
Question 4 Multiple Choice
A bright ring of radius about 22° appears around the Sun when thin, high cirrostratus clouds cover the sky. What causes it?
ADiffraction of light by small water droplets
BRefraction of sunlight through six-sided ice crystals
CReflection of sunlight off the ocean
DScattering by air molecules
Hexagonal ice crystals act like prisms with a 60° angle between alternate faces. Light passing through them is bent by a minimum of about 22°, so light piles up at that angle and forms the 22° halo. Diffraction by small droplets makes coronas, which are much smaller rings close to the Sun or Moon.