Light in the Atmosphere: Quick Revision

A brief note on Light in the Atmosphere. Read it before a test. Try each question at the end before you open its answer.

The big ideas

• The air is denser near the ground, so starlight bends on the way down: stars look higher, and they twinkle as the air moves. Planets, small discs, shine steadily.
• We see the Sun about $2\ \text{min}$ before true sunrise and $2\ \text{min}$ after true sunset.
• Scattering by very small particles goes as $\dfrac{1}{\lambda^4}$: blue far more than red. Large drops scatter all colours alike: white.
• The Tyndall effect: particles in a colloid or a mist make a beam's path visible.
• The sky is blue (scattered blue from everywhere); the low Sun is red (blue scattered out of a long path).

The facts at a glance

A worked example

How many times more is blue light ($440\ \text{nm}$) scattered than red light ($690\ \text{nm}$) by the air?

1. The ratio of wavelengths. $\dfrac{690\ \text{nm}}{440\ \text{nm}} = 1.568$.
2. The fourth power. $1.568^4 = 6.05$.
3. So blue is scattered about six times as much.

Practice with new numbers

A program to try

The program finds how many times more each colour is scattered than red light.

Mistakes to avoid

• 'Stars twinkle because they flicker.' Their light is steady; our air makes it flicker.
• Using $\dfrac{1}{\lambda}$ instead of $\dfrac{1}{\lambda^4}$. The fourth power matters.
• 'Clouds are white because they reflect the ground.' Their large drops scatter all colours.
• 'The setting Sun changes colour.' The Sun is the same; the path through the air is longer.

Check yourself

1. Why do we see the Sun before it has truly risen?

2. How many times more is light of $430\ \text{nm}$ scattered than light of $650\ \text{nm}$?

3. Which colour suits a fog lamp best: blue or yellow? Why?

4. The Sun is $8^\circ$ high. About how many times as much air does its light cross as from overhead?