Thermal radiation · Grade 7
In Grade 7 you get fluent with: radiation: emitting and absorbing infrared; the best and worst absorbers and emitters.
All objects give out infrared radiation, and the hotter they are, the more they emit. Infrared is a wave, so it needs no particles and can cross the vacuum of space: that is how the Sun warms the Earth. Matt black surfaces are the best absorbers and emitters; shiny, light surfaces are the worst, because they reflect infrared instead.
Key facts
- Radiation: all objects emit infrared; hotter objects emit more. It needs no particles.
- Matt black surfaces are the best absorbers and emitters; shiny silver surfaces are the worst (best reflectors).
Worked examples
Example 1
Which object emits infrared radiation?
Answer: every object, more so the hotter it is
Example 2
Which way of transferring energy can work through the vacuum of space?
Answer: radiation
Example 3
How does the Sun’s energy reach the Earth?
Answer: by infrared (and other) radiation through space
Example 4
Which surface is the best absorber of infrared radiation?
Answer: matt black
Example 5
Houses in hot countries are often painted white. Why does this keep them cooler inside?
Answer: white surfaces reflect most of the Sun’s infrared radiation, so the walls absorb less energy and warm up less
Example 6
Two cans hold the same amount of water at 80°C. One is painted matt black and the other is shiny silver. Which cools faster, and why?
Answer: the matt black can: dark, matt surfaces are the best emitters of infrared radiation
Example 7
Why are the fins on the back of a fridge painted black?
Answer: black surfaces are good emitters, so they lose heat quickly