2.3.3 Radiation Revision Notes
1. Overview
Thermal radiation is the transfer of energy by infrared waves. Unlike conduction and convection, radiation does not require a medium (matter) to travel through, making it the only way heat can reach Earth from the Sun through the vacuum of space.
Key Definitions
- Thermal Radiation: Infrared radiation (electromagnetic waves) emitted by the surface of an object due to its temperature.
- Infrared (IR): A type of electromagnetic radiation with longer wavelengths than visible light.
- Emitter: An object that sends out (emits) radiation.
- Absorber: An object that takes in (absorbs) radiation.
- Reflector: An object that bounces radiation off its surface.
- Vacuum: A space entirely devoid of matter.
Core Content
Nature of Thermal Radiation
- Thermal radiation is Infrared radiation.
- All objects, regardless of their temperature, emit some thermal radiation.
- The hotter an object is, the more infrared radiation it radiates per second.
Transfer without a Medium
- Radiation does not require particles. It can travel through a vacuum (empty space) at the speed of light.
- This is how energy from the Sun reaches the Earth.
Effect of Surface Colour and Texture The ability of a surface to emit, absorb, or reflect radiation depends on its appearance:
| Surface | Absorption | Emission | Reflection |
|---|---|---|---|
| Black | Excellent | Excellent | Poor |
| Dull / Matt | Good | Good | Poor |
| White | Poor | Poor | Good |
| Shiny / Silvery | Very Poor | Very Poor | Excellent |
Two metal cans, one painted matt black and one shiny silver, placed equal distances from a radiant heater. A thermometer inside each shows the black can heating up faster.
Extended Content (Extended Curriculum Only)
Constant Temperature and Energy Balance
- For an object to remain at a constant temperature, it must transfer energy away (emit) at the same rate it receives energy (absorbs).
- Heating Up: If the rate of absorption > rate of emission, the object’s temperature increases.
- Cooling Down: If the rate of absorption < rate of emission, the object’s temperature decreases.
Earth’s Temperature Balance The Earth’s temperature is determined by the balance between:
- Incoming Radiation: Short-wavelength radiation from the Sun.
- Outgoing Radiation: Long-wavelength infrared radiation emitted by the Earth’s surface back into space.
- Factors such as atmosphere composition (greenhouse gases) and surface cover (ice reflects more, dark oceans absorb more) affect this balance.
Experiments: Distinguishing Emitters and Absorbers
- To test Emission (Leslie Cube): Fill a hollow metal cube (with different surface sides: shiny silver, matt black, etc.) with boiling water. Use an infrared detector to measure the radiation from each side at the same distance. The matt black side will show the highest reading.
- To test Absorption: Place two metal plates (one matt black, one shiny silver) at equal distances from a heater. Attach a thermometer or a small coin with wax to the back of each. The wax on the matt black plate will melt first because it absorbs radiation faster.
Factors Affecting Rate of Emission
- Surface Temperature: The higher the temperature of an object relative to its surroundings, the greater the rate at which it emits radiation.
- Surface Area: The larger the surface area, the greater the rate of emission (more "space" for the waves to leave the object).
Key Equations
Note: In IGCSE Physics, radiation is largely descriptive. However, understand the relationship: $\text{Power (Rate of emission)} \propto \text{Surface Area} \times \text{Temperature}^4$ (You do not usually need to calculate $T^4$, but you must know that a small increase in temperature leads to a massive increase in the rate of radiation.)
Common Mistakes to Avoid
- ❌ Wrong: Two objects at the same temperature must emit radiation at the same rate.
- ✓ Right: Even at the same temperature, a matt black object will emit radiation faster than a shiny silver one.
- ❌ Wrong: Radiation only works in a vacuum.
- ✓ Right: Radiation travels through vacuums, but it also travels through transparent solids, liquids, and gases (like air).
- ❌ Wrong: Using "Shiny Black" as a good absorber.
- ✓ Right: Any degree of shininess increases reflection. A "matt black" surface is the best absorber/emitter.
- ❌ Wrong: Thinking black surfaces "hold onto" heat better.
- ✓ Right: If a surface is a good absorber, it is always a good emitter. Black objects cool down faster than silver ones if they start at the same high temperature.
Exam Tips
- Context Matters: If a question asks why a house is painted white in a hot country, focus on it being a poor absorber (to keep it cool). If it asks why a cooling fin on a computer is black, focus on it being a good emitter (to get rid of heat).
- Vacuum Keyword: If the question mentions "vacuum" or "space," the answer is almost certainly radiation, as conduction and convection require particles.
- Two-Way Street: Always remember that if a surface is "good" at taking heat in (absorbing), it is also "good" at letting heat out (emitting).