RadiationCambridge IGCSE Physics: Revision notes
Section 1
What is thermal radiation?
Thermal radiation is infrared radiation. All objects, no matter their temperature, constantly emit thermal radiation.
Unlike conduction and convection, thermal radiation does not need a medium — it can travel through a vacuum, which is how energy from the Sun reaches the Earth across empty space.
Don't say radiation 'needs air to travel' — that's convection or conduction confusion. Radiation is the only method of the three that works through a vacuum.
Section 2
Surface colour and texture
How well a surface emits, absorbs and reflects infrared radiation depends on its colour and texture:
- Dull, black surfaces are good emitters and good absorbers of infrared radiation
- Shiny, white surfaces are poor emitters and poor absorbers (they reflect radiation instead)
This is why radiators are often painted matt black (to emit heat efficiently) while survival blankets are shiny (to reflect heat back to the body, reducing emission and absorption of unwanted radiation).
A matt black teapot loses heat faster by radiation than a shiny silver one of the same size and temperature.
Section 3
Testing emitters and absorbers experimentally
Experiments can distinguish good and bad emitters and absorbers of infrared radiation:
- To compare emitters: heat identical containers (one dull black, one shiny) to the same temperature and measure which cools faster, or use a detector to compare radiation given off
- To compare absorbers: place identical detectors (e.g. blackened and polished metal plates) the same distance from a heat source and compare temperature rise
In both cases, the dull black surface shows the greater effect, confirming it is the better emitter and absorber.
Always control variables (same starting temperature, same distance from source, same size/material otherwise) so surface finish is the only difference being tested.
Section 4
Rate of emission: temperature and surface area
The rate at which an object emits thermal radiation depends on:
- Its surface temperature — hotter objects radiate energy faster
- Its surface area — a larger surface area radiates energy faster
For an object to stay at a constant temperature, it must transfer energy away (by radiation and other methods) at the same rate it receives energy — otherwise it will heat up or cool down.
Section 5
Radiation balance and the Earth's temperature
The Earth's temperature depends on the balance between:
- Radiation absorbed from the Sun
- Radiation emitted back out from the Earth's surface into space
If absorption and emission are balanced, temperature stays roughly constant. If the balance shifts (for example, more energy is trapped than emitted), the Earth's average temperature changes — this is the basic radiation principle behind climate change.
Think of it like a bath with the tap and plughole both open: if water in (absorption) equals water out (emission), the level (temperature) stays constant.
Must Know
- Thermal radiation is infrared radiation, emitted by all objects, and is the only method of heat transfer that works through a vacuum
- Dull, black surfaces are good emitters and good absorbers; shiny, white surfaces are poor emitters and poor absorbers
- Experiments compare emitters/absorbers by keeping all variables the same except surface colour/texture
- Rate of emission increases with higher surface temperature and larger surface area
- A constant-temperature object emits energy at the same rate it absorbs it
- Earth's temperature depends on the balance between absorbed and emitted radiation
That's the notes covered.
Carry on to the next subtopic.