4.1.2Atomic Structure
The Absorption and Emission of EM Radiation — AQA GCSE Combined Science Trilogy 8464 - Physics Revision Notes & Topical Questions
5 min read 7 parts Lesson 3 of 23
Part 1 of 5Emission and absorption
All objects emit
- All objects emit Every object emits infrared radiation, and the hotter the object, the more radiation it emits each second.
All objects absorb
- All objects absorb Objects also absorb radiation from their surroundings.
Net effect
- Net effect An object hotter than its surroundings emits more than it absorbs, so it cools.
- A cooler object absorbs more than it emits, so it warms.
Steady temperature
- Steady temperature When absorbing and emitting at the same rate, the temperature remains constant.
Part 2 of 5Effect of surface
| Surface | Absorption of infrared | Emission of infrared | Reflection |
|---|---|---|---|
| Matt black | Best absorber | Best emitter | Poor |
| Dull dark | Good | Good | Poor |
| Shiny white or silver | Poor absorber | Poor emitter | Best reflector |
Part 3 of 5Earth's temperature
1Sun's radiation arrives
- Sun's radiation arrives Short-wavelength radiation passes through the atmosphere and warms the surface.
2Surface re-emits
- Surface re-emits Earth emits longer-wavelength infrared radiation back towards space.
3Gases absorb some
- Gases absorb some Greenhouse gases such as carbon dioxide and methane absorb part of this infrared radiation.
4Balance shifts
- Balance shifts More greenhouse gas means more radiation is retained, so the average temperature rises.
Part 4 of 5Radiation and atoms
1
When radiation is absorbed, electrons in the atom can gain energy and move to a higher energy level.
2
If the energy is large enough, an electron leaves the atom entirely and the atom becomes a positive ion.
3
When electrons drop back to lower energy levels, they emit electromagnetic radiation.
4
Higher frequency radiation such as ultraviolet, X-rays and gamma can ionise atoms.
Part 5 of 5Investigating emission (practical idea)
Comparing surfaces
Set-up
- Set-up Use identical containers (Leslie cubes) of hot water, each with a different surface colour or finish.
Measurement
- Measurement Place an infrared detector the same distance from each side and compare the readings.
Control
- Control Keep the water temperature, distance and detector the same;
- the matt black surface gives the highest reading.
Key terms
| Term | Meaning |
|---|---|
| Infrared radiation | Electromagnetic radiation emitted by all objects due to their temperature. |
| Absorber | A surface that takes in radiation, increasing the energy of the object. |
| Emitter | A surface that gives out radiation. |
| Greenhouse gas | A gas in the atmosphere that absorbs infrared radiation from Earth. |
| Ionisation | The removal of an electron from an atom, creating an ion. |
Avoid these mistakes
1
Saying only hot objects emit infrared; all objects do, although cold ones emit much less.
2
Thinking shiny surfaces are the best emitters — they are poor emitters and absorbers but good reflectors.
3
Claiming greenhouse gases block the Sun's radiation from reaching Earth; they absorb the infrared leaving it.
4
Confusing reflection with emission when explaining why a silver surface stays cooler.
Last updated: