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Radiation

Heat and matter · level 3, lesson 5 of 6

A question to start

Between the Sun and Earth there are 150 million km of empty space, with no air at all. So how does the Sun's heat reach us?

The idea

Heat can travel as radiation: invisible infrared light. Like ordinary light, it needs nothing to travel through, so it can cross empty space.

Everything gives out some infrared. Hot things give out much more. That is the warmth you feel on your face from a fire or a sunny window, even without touching anything.

black: absorbs,gets hotshiny: reflects,stays cooler
  • Dark, dull surfaces absorb radiation well, and give it out well.
  • Light, shiny surfaces reflect it, and give out little.

At night, the ground gives out its heat as infrared to the cold, dark sky. A cloudy night on Earth stays warmer because clouds send some of it back.

On Mars, the air is so thin that it holds very little of the Sun's heat, and it lets the ground's heat escape to space easily. That is a big reason Mars nights are so cold.

Worked examples

1. A foil blanket. Runners wrap up in shiny foil after a race. The shiny side reflects their body's infrared back at them.

2. Painted houses. In hot countries, many houses are painted white to reflect the sunshine.

3. A clear night. Frost forms on clear nights more than cloudy ones, because the ground radiates heat away freely.

Try it

  1. Can heat travel through empty space? How?
  2. Which absorbs the Sun's heat better, a black car or a silver one?
  3. Why are cloudy nights warmer than clear ones?
  4. Why do Mars nights get so cold?
Answers
  1. Yes, as radiation: infrared light.
  2. The black car.
  3. Clouds send back some of the infrared the ground gives out.
  4. The thin air holds little heat and lets the ground's heat escape to space.

Common mistake

"Radiation only means dangerous rays." The word means anything sent out as rays. Infrared from a warm radiator, or from you, is radiation too, and it is harmless.

Where it leads