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Absorption lines

Waves, sound and light · level 8, lesson 3 of 6

A question to start

Spread sunlight out finely enough and the rainbow is crossed by hundreds of thin dark lines. Something is taking those exact wavelengths out of the light. What?

The idea

An atom can absorb a photon too, but only one with exactly the energy of one of its jumps up. So if white light passes through a cooler gas, the gas takes out its own wavelengths, and dark lines appear in the spectrum where the gas would have shone. This is an absorption spectrum.

  • The Sun's dark lines come from the cooler gases in its outer layers. They show hydrogen, sodium, iron, calcium and many more.
  • A planet's air does the same. Sunlight reflected from Mars has passed through its air twice, and comes back with the absorption bands of carbon dioxide in the infrared.

Molecules absorb in bands of many close lines, because they can also spin and vibrate. Carbon dioxide's strong infrared bands are the reason it is a greenhouse gas.

hot source: continuoushot gas: emission linesthrough cool gas: absorption lines
Carbon dioxide in the air Mars Earth
Share by volume 95.1% about 0.04%
Air above each square metre about 170 kg about 10,300 kg
Carbon dioxide above each square metre about 165 kg about 6.7 kg

So a beam of sunlight crossing Mars's air passes through about 25 times as much carbon dioxide as one crossing Earth's, and its infrared bands are deep.

Worked examples

1. Same lines. Hydrogen absorbs at 656.3 nm for the same reason it emits there: the same jump between \(n = 2\) and \(n = 3\), going up instead of down.

2. Twice through. Sunlight reflected from Mars's surface crosses the air on the way down and again on the way up, doubling the absorption.

3. 25 times. \(165/6.7 \approx 25\).

Try it

  1. What makes the dark lines in the Sun's spectrum?
  2. Why does a gas absorb exactly the wavelengths it would emit?
  3. Why do molecules give bands rather than single lines?
  4. Why are carbon dioxide's infrared bands so strong in light from Mars?
Answers
  1. Cooler gases in the Sun's outer layers absorbing their own wavelengths.
  2. Both come from the same jumps between energy levels, up or down.
  3. They can also spin and vibrate, giving many close energy levels.
  4. There is about 25 times as much carbon dioxide above each square metre, and the light crosses it twice.

Common mistake

"Dark lines mean the colours were never there." They were there in the hot source, and were absorbed on the way. The darkness marks the gas in between.

Where it leads