Learning outcomes
By the end of this lesson, students should be able to:
- describe thermal radiation as infrared electromagnetic radiation
- state that infrared transfer does not require a medium
- compare emission, absorption and reflection by dull black and shiny light surfaces
- describe experiments comparing emitters and absorbers
- explain how temperature and surface area affect emission rate
11.1 Radiation as a transfer pathway
All objects emit electromagnetic radiation because of their temperature. At everyday temperatures, much of the thermal radiation is infrared. Unlike conduction and convection, infrared radiation does not require particles or a medium, so it can travel through a vacuum. Energy from the Sun reaches Earth mainly by electromagnetic radiation.
An object can emit, absorb and reflect infrared radiation. Its temperature changes according to the balance between energy absorbed and energy emitted. At thermal equilibrium with its surroundings, these rates are equal even though both processes continue.
11.2 Surface colour and texture
Dull black surfaces are good absorbers and good emitters of infrared radiation. Shiny, light-coloured surfaces are poor absorbers and poor emitters but good reflectors. The effect concerns surface finish, not simply the material inside the object.
A good absorber is also a good emitter at the same wavelength and temperature. Students sometimes incorrectly pair black with absorption only and shiny with emission only. Use the complete relationships.
Figure 23. Original KG2UNI diagram.
11.3 Factors affecting emission rate
A hotter surface emits infrared radiation at a greater rate. A larger surface area also emits more energy each second at the same temperature and surface finish. As an object cools closer to room temperature, the net rate of energy loss decreases because the difference between emission and absorption becomes smaller.
The detailed Stefan-Boltzmann equation is not required. Qualitative comparisons are sufficient: hotter, larger and duller-black surfaces are generally better radiators.
11.4 Comparing emitters
A Leslie cube has faces with different finishes but the same internal hot-water temperature. An infrared detector is placed at equal distance from each face. The dull black face gives the largest detector response, while the shiny face gives the smallest. Distance and angle must remain constant.
Alternatively, identical cans with different surface finishes can be filled with equal masses of hot water. The can whose water cools fastest is the better emitter, provided convection and other conditions are controlled.
Figure 24. Original KG2UNI diagram.
11.5 Comparing absorbers
Place identical temperature sensors behind dull black and shiny surfaces at equal distance from an infrared source. The dull black surface warms faster because it absorbs more radiation. Keep area, thickness, starting temperature and distance the same.
An infrared detector must not be confused with an ordinary thermometer. It detects radiation from a surface and estimates temperature without contact. Readings depend on emissivity, distance, field of view and reflected radiation.
Worked examples
Black and shiny cans
If both cans start at the same temperature, the dull black can cools faster because it emits infrared radiation at a greater rate.
Solar absorber
A dull black surface is used in a solar water heater because it absorbs incident radiation effectively.
Infrared thermometer
The device detects infrared radiation from the target. It is useful for hot, moving or inaccessible objects and does not need physical contact.
Practical focus
Investigation
Fill a dull black can and a shiny can with equal masses of hot water at the same starting temperature. Use identical thermometers and record temperature at equal intervals. Plot cooling curves. Keep the cans side by side, away from draughts. The black can should cool more quickly due to greater infrared emission.
Examination guidance
- State that infrared is electromagnetic radiation and can travel through a vacuum.
- Dull black is both a good absorber and a good emitter.
- In radiation experiments, keep detector distance and angle constant.
Check your understanding
- Why can thermal radiation travel through space?
- Which surface is the best infrared emitter?
- How does increasing surface temperature affect emission rate?
- Why is a shiny surface used behind a radiator?
- What must be controlled in a Leslie cube experiment?
Answers
- It is electromagnetic radiation and does not require a medium.
- A dull black surface.
- The emission rate increases.
- It reflects infrared radiation into the room and is a poor absorber.
- Distance, angle, face area and the cube temperature.
