2.3.3 Radiation
- Syllabus
- 0625–2026–2027
- Topic
- 2.3.3
- Level
- —
Learning objectives
2.3.3.1Thermal radiation is infrared• Know: thermal radiation is infrared radiation and that all objects emit this radiation2.3.3.2Thermal energy transfer by thermal• Know: thermal energy transfer by thermal radiation does not require a medium2.3.3.3Effect of surface colour (black or• Describe the effect of surface colour (black or white) and texture (dull or shiny) on the emission, absorption and reflection of infrared radiation2.3.3.4For an object to be at a constant• Know: for an object to be at a constant temperature it needs to transfer energy away from the object at the same rate that it receives energy2.3.3.5What happens to an object if the rate• Know what happens to an object if the rate at which it receives energy is less or more than the rate at which it transfers energy away from the object2.3.3.6How the temperature of the Earth is• Know how the temperature of the Earth is affected by factors controlling the balance between incoming radiation and radiation emitted from the Earth’s surface2.3.3.7Experiments to distinguish between• Describe experiments to distinguish between good and bad emitters of infrared radiation2.3.3.8Experiments to distinguish between• Describe experiments to distinguish between good and bad absorbers of infrared radiation2.3.3.9Rate of emission of radiation depends• Describe how the rate of emission of radiation depends on the surface temperature and surface area of an object
Thermal radiation is infrared radiation
Thermal radiation is infrared radiation, a region of the electromagnetic spectrum. Every object emits infrared radiation.
| Claim | Meaning |
|---|---|
| type of radiation | infrared electromagnetic radiation |
| emitters | all objects, not only visibly glowing objects |
| transfer | an object can emit infrared that another object absorbs |
Visible light may also be emitted by a very hot object, but the thermal-radiation region named in this syllabus is infrared.
Thermal radiation can cross a vacuum
Infrared radiation does not require a material medium. As electromagnetic radiation, it can transfer thermal energy through a vacuum.
| Process | Needs matter between source and receiver? | Can cross a vacuum? |
|---|---|---|
| conduction | yes | no |
| convection | yes, a moving fluid | no |
| radiation | no | yes |
Energy from the Sun and stars reaches Earth across space by radiation. The receiving surface warms when it absorbs that radiation.
A vacuum does not block radiation. It prevents conduction and convection between separated objects because those processes require matter.
Use surface colour and texture to predict radiation
Surface colour and texture affect how strongly infrared radiation is emitted, absorbed and reflected.
| Surface feature | Absorption | Emission | Reflection |
|---|---|---|---|
| black | better | better | poorer |
| white | poorer | poorer | better |
| dull | better | better | poorer |
| shiny | poorer | poorer | better |
For the strongest contrast, dull black surfaces are good absorbers and emitters, while shiny white or polished silver surfaces are good reflectors and poor absorbers and emitters. Compare otherwise identical surfaces at the same temperature and area.
Do not describe a surface as a ‘conductor of radiation’. Conduction is a different transfer process; surfaces absorb, emit or reflect infrared radiation.
Constant temperature requires balanced transfer rates
For an object in a steady state with no change of state, temperature remains constant when the rate at which it receives energy equals the rate at which it transfers energy away.
| Energy transfer in | Energy transfer out | Temperature result |
|---|---|---|
| equal rates | equal rates | constant |
Energy can continue to enter and leave the object. Constant temperature means no net change in its internal energy because the two rates balance.
Thermal balance is not the same as zero energy transfer. A heater may supply energy continuously while the object transfers energy away at the same rate. During melting or boiling, temperature can also remain constant while internal energy changes; that phase-change case is treated separately.
Use energy-rate imbalance to predict temperature change
When no change of state occurs, temperature changes if the rate of energy transfer into an object differs from the rate of transfer out.
| Comparison of rates | Internal energy | Temperature |
|---|---|---|
| input rate greater than output rate | increases | rises |
| input rate equal to output rate | constant | remains constant |
| input rate less than output rate | decreases | falls |
As a hot object cools, its temperature difference from the surroundings becomes smaller, so its cooling rate usually decreases. It approaches the surroundings' temperature as the rates move towards balance.
Compare rates, not just total amounts already transferred. With no phase change, the sign of input rate minus output rate determines whether temperature is rising or falling at that moment.
Earth's temperature depends on radiation balance
Earth's average surface temperature is controlled by the balance between incoming radiation absorbed from the Sun and outgoing infrared radiation emitted towards space.
| Planetary balance | Temperature tendency |
|---|---|
| absorbed incoming rate greater than outgoing rate | average temperature rises |
| absorbed incoming rate equal to outgoing rate | average temperature is steady |
| absorbed incoming rate less than outgoing rate | average temperature falls |
An increase in greenhouse gases can reduce the rate at which outgoing infrared escapes to space. While absorbed incoming radiation exceeds outgoing radiation, the surface warms until a new balance may be reached.
Incoming sunlight and outgoing infrared are different parts of Earth's energy budget. A temperature rise indicates a rate imbalance, not that Earth has stopped emitting radiation.
Compare infrared emitters with a fair experiment
A good emitter produces a larger infrared detector response than a poor emitter when surface temperature, area and distance are the same.
| Stage | What to do |
|---|---|
| set up | use equal-area dull black and shiny or white surfaces on the same hot container, or identical containers with equal hot-water volumes and temperatures |
| measure | place the same infrared detector or matched black-bulb thermometers at equal perpendicular distances; record readings at equal times |
| result | the dull black surface gives the larger detector response or faster thermometer rise |
| conclusion | dull black is the better emitter; shiny or white is the poorer emitter |
Control surface area, surface temperature, detector distance and angle, water volume and initial temperature, container dimensions and observation time.
A faster fall in a hot container's temperature can also indicate stronger emission, but only when other energy-transfer paths and all comparison variables are controlled.
Compare infrared absorbers with a fair experiment
A good absorber produces a faster temperature rise than a poor absorber when both receive the same infrared radiation.
| Stage | What to do |
|---|---|
| set up | use identical dull black and shiny or white cans containing equal volumes of water at the same initial temperature |
| expose | place both cans at equal distance and orientation from the same radiant heater; switch it on for the same time |
| measure | record both water temperatures at equal time intervals |
| result and conclusion | the water in the dull black can warms faster, so dull black is the better absorber |
Control can material and dimensions, exposed area, water mass, initial temperature, heater power, distance, angle and heating time.
This compares absorption of incoming radiation. Do not infer the result from thermal conductivity of the can, and do not change both surface and geometry at once.
Temperature and surface area control emission rate
For otherwise comparable objects, infrared emission increases with surface temperature and with emitting surface area.
| Change while other factors stay fixed | Rate of infrared emission |
|---|---|
| higher surface temperature | greater |
| lower surface temperature | smaller |
| larger emitting surface area | greater total rate |
| smaller emitting surface area | smaller total rate |
Compare one factor at a time: use the same surface finish when testing temperature or area. Surface colour and texture also affect emission, as covered separately.
A larger area increases the total emitted power; it does not mean each square centimetre emits more when temperature and surface finish are unchanged.