🏠 White houses in hot countries
The aim is to keep energy out. White walls are poor absorbers and good reflectors. They reflect most of the Sun's radiation, so the houses stay cooler inside.
Energy Transfers » Thermal Radiation
What you'll learn this session
Study time: 30 minutes
Edexcel spec: 4.6, 4.8
- How thermal energy is transferred by radiation, even through empty space
- Why every object emits and absorbs infrared all the time
- How the temperature of an object changes the radiation it emits
- How the colour and texture of a surface change emission, absorption and reflection
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Unlock This CourseWhat thermal radiation is
Sit beside a bonfire on a cold evening. Your face feels hot, yet your back stays cold. The warm air above the fire is rising, not moving sideways towards you, so this is not convection. Air is a poor conductor, so it is not conduction either. The energy reaches your face in a third way: thermal radiation.
Thermal radiation is energy carried by infrared waves. Remember, infrared is part of the electromagnetic spectrum, so it travels in straight lines at the speed of light.
Key terms:
- Thermal radiation: the transfer of thermal energy by infrared waves.
- Emitter: an object that gives out radiation.
- Absorber: an object that takes in radiation. The energy it takes in goes to its thermal store.
- Reflector: a surface that sends radiation back instead of taking it in.
Radiation needs no particles
Conduction and convection both need particles. Radiation does not. Infrared is an electromagnetic wave, so it needs no particles and can cross a vacuum, where there are no particles at all.
The best evidence is the Sun. Between the Sun and the Earth there is empty space. No particles means no conduction and no convection. Yet you can feel the Sun warm your skin. All of that energy arrived by radiation.
| Conduction | Convection | Radiation | |
|---|---|---|---|
| How the energy travels | Passed from particle to particle | Carried by a moving fluid | Carried by infrared waves |
| Needs particles? | Yes | Yes | No |
| Works through a vacuum? | No | No | Yes |
Radiation also passes through transparent materials such as air and glass. That is why you can feel the Sun through a closed window.
Everything emits and absorbs
Thermal radiation is not only for very hot things. All objects emit infrared and all objects absorb infrared, all the time. You, your chair, a mug of tea and an ice cube are all doing both right now.
When an object emits radiation, energy leaves its thermal store. When it absorbs radiation, energy is added to its thermal store. What happens to its temperature depends on which is bigger.
| The object... | Result |
|---|---|
| emits more radiation each second than it absorbs | It cools down |
| absorbs more radiation each second than it emits | It warms up |
| emits and absorbs at the same rate | Its temperature stays constant |
Radiation and temperature
The first thing that decides how much radiation an object emits is its temperature.
Learn this
The hotter an object is, the more radiation it emits each second.
Take a loaf of bread out of a hot oven and put it on the worktop. The loaf is much hotter than the kitchen around it, so it emits far more radiation each second than it absorbs from the room. It cools down. As it cools, it emits less and less radiation each second, so it cools more slowly. In the end it reaches the temperature of the room. It is still emitting infrared, but now it absorbs at the same rate, so its temperature stays constant.
The opposite happens to an ice lolly taken out of the freezer. It is colder than the room, so it absorbs more radiation each second than it emits, and it warms up.
Radiation and surface
The second thing that matters is the surface of the object: its colour, and whether it is dull or shiny. Learn the two ends of the scale.
| Surface | Emitting | Absorbing | Reflecting |
|---|---|---|---|
| Dull black | Best emitter | Best absorber | Poor reflector |
| Shiny white or shiny silver | Worst emitter | Worst absorber | Good reflector |
Notice the pattern. A surface that is good at absorbing is also good at emitting. A surface that is poor at absorbing is also poor at emitting. A shiny surface is a poor absorber because it reflects most of the radiation that falls on it.
Other surfaces sit in between. A dull surface is a better emitter and absorber than a shiny one of the same colour, and a dark surface is better than a light one.
Worked example
A black car and a silver car are parked side by side in sunshine. Explain why the black car becomes hotter.
Step 1: Radiation (infrared) from the Sun falls on both cars equally.
Step 2: The black surface is a good absorber, so it absorbs most of the radiation that falls on it.
Step 3: The silver surface is a poor absorber and a good reflector, so it reflects most of the radiation away.
Step 4: More energy is transferred to the thermal store of the black car each second, so its temperature rises more.
Using the right surface
Ask two questions. Is the aim to keep energy in, keep energy out, or collect it? Is the surface emitting or absorbing?
🌞 Black solar heating panels
The aim is to collect energy. Water flows through pipes in a panel on a roof. The panel is dull black, the best absorber, so it absorbs as much of the Sun's radiation as possible and the water gets hot.
🏃 Shiny foil blankets
The aim is to keep energy in. A runner wrapped in a foil blanket after a race stays warm. The shiny foil is a poor emitter, so little radiation leaves its outer surface. It also reflects the infrared emitted by the runner's body back towards the body.
🍵 Shiny teapots
The aim is to keep energy in. A shiny silver teapot is a poor emitter. It emits less radiation each second than a dull, dark teapot at the same temperature, so the tea stays hot for longer.
Common mistakes
- Saying only hot objects emit radiation. All objects emit infrared. Hotter ones emit more each second.
- Saying radiation needs air to travel through. It needs no particles and crosses a vacuum.
- Writing 'black attracts heat'. Nothing is attracted. A black surface absorbs more of the radiation that falls on it.
- Saying a shiny surface is a good emitter because it looks bright. It looks bright because it reflects. Shiny surfaces are the worst emitters.
- Mixing up emit and absorb. Decide first whether the object is giving out energy (emitting) or taking it in (absorbing), then use the right word.
- Writing 'white reflects cold' or 'keeps the cold in'. Always write about infrared radiation being reflected, absorbed or emitted.
Exam-style question
A large metal tank stores fuel outdoors in a hot, sunny country. The outside of the tank is painted shiny white.
(a) Name the type of wave that transfers thermal energy by radiation. [1 mark]
(b) Explain why energy from the Sun cannot reach the Earth by conduction or convection, but can reach it by radiation. [2 marks]
(c) Explain why the fuel stays cooler in a shiny white tank than it would in a dull black tank. [3 marks]
(d) At sunset the tank is warmer than its surroundings, so it cools down during the night. State what happens to the amount of radiation the tank emits each second as it cools. [1 mark]
Model answer
(a) Infrared (1).
(b) Space is a vacuum, so there are no particles, and conduction and convection both need particles (1). Radiation is an electromagnetic wave (infrared) and does not need particles, so it can travel through a vacuum (1).
(c) A shiny white surface is a poor absorber of radiation / a dull black surface is a good absorber (1). The white surface reflects most of the Sun's radiation (1). So less energy is transferred to the tank and the fuel each second, and the temperature rises less (1).
(d) It decreases (1).
Exam tip
In part (c) the tank is taking energy in from the Sun, so write about absorbing and reflecting, not emitting. With 3 marks, give three linked points: poor absorber, reflects most of the radiation, so less energy is transferred to the fuel.