Radiation and surface properties
Key idea: Thermal radiation transfers energy by electromagnetic waves and can cross a vacuum. Dull black surfaces absorb and emit infrared better than bare shiny metal.
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Guided lessons · coming soon
Guided lesson
Guided steps
- Explain which energy transfer can cross a vacuum.
- Use your position and the cool air to explain the transfer.
- Compare surface finish and wavelength before ranking radiation rates.
- Identify the matched conditions and the differing surface finish.
- Compare the measured rates and name the changed surface property.
- Compare sunlight absorption, then use the separate infrared measurements.
- Compare emitted and absorbed power as the coffee cools.
- Explain how the two radiation rates change as the can warms.
- Compare both exposed areas with mass, finish and temperature matched.
- Explain why curling up reduces the cat’s energy loss.
- Explain why the unequal masses spoil the emitter comparison.
- Find both net transfers and decide which plate warms.
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ContinueStep out of the shade into sunshine and you feel warmer at once. That energy reaches you by radiation, the one transfer process that needs no particles at all. Every object gives out radiation, and how fast it does depends on its surface, its temperature and its size.
Radiation needs no material medium
Conduction needs particles to pass energy along. Convection needs a fluid that can flow. Yet energy from the Sun reaches the Earth across about 150 million kilometres of almost empty space. It travels as electromagnetic radiation.
Radiation is the transfer of energy by electromagnetic waves. It does not need a material medium, so it can cross a vacuum. For objects at everyday temperatures the radiation is mostly infrared, which you cannot see.
Objects at ordinary temperatures emit radiation and absorb some of the radiation that falls on them. The hotter an object is, the more radiation it emits. A vacuum stops conduction and convection, but radiation still crosses it.
| Process | Needs a material medium? | How the energy is carried |
|---|---|---|
| Conduction | yes, mainly solids | vibrating particles pass energy to neighbours; free electrons in metals |
| Convection | yes, a liquid or gas | the fluid flows and carries energy with it |
| Radiation | no | electromagnetic waves, which can cross a vacuum |
Check your understanding 1
You sit a few metres to the side of a campfire and feel your face warm up, although the air between you and the fire is cool. Which process mainly transfers the energy to your face? Rule out the other two.
Show answer
Radiation. Conduction through air is very slow, because air is a poor conductor. Convection carries the hot air upwards from the fire, not sideways towards you. Radiation from the flames and embers travels straight to your face and is absorbed there.
What affects the rate of radiation
Three things affect how fast a surface emits or absorbs radiation: its colour and texture, its temperature and its surface area.
Surface colour and texture
Compare the surface and the radiation involved:
- colour: matched dark surfaces usually absorb more visible sunlight than light ones; visible colour alone does not rank infrared emission;
- finish: in the standard thermal-radiation comparison, a dull black finish is a better infrared absorber and emitter than bare shiny metal. Compare the stated finish: glossy paint and bare polished metal have different infrared properties.
The two extremes are dull black, a good absorber and a good emitter, and shiny silver (polished metal), which reflects most of the radiation falling on it and is a poor absorber and a poor emitter. A good absorber is also a good emitter at the same wavelengths. Compare one property at a time, with everything else the same.
White paint reflects most of the visible light in sunlight, so a white roof heats up less in the Sun. Colour describes visible light only, though: a white-painted surface can still emit infrared well. The U.S. Department of Energy’s cool-roof guide distinguishes reflection of sunlight from emission of thermal radiation; its example black and white roofs have the same thermal emittance despite different solar reflectance.
Worked example 1
Two cans of hot water
Problem
Two identical metal cans hold equal masses of water at 80 °C. One can is painted dull black; the other has a polished, uncoated silver surface. Assume other energy transfers are matched and both surfaces stay hotter than their surroundings. Predict which cools faster by radiation, and explain.
Show full solution
Find what is different
Method
List what is the same and what differs.Reason
A fair comparison changes one factor only.Working
Answer
Same can material and shape, mass of water, starting temperature and surroundings. Only the surface finish differs.
Apply the rule
Method
Compare the two surfaces as emitters.Reason
A dull black surface is a better emitter than a shiny silver one.Working
Answer
The black can emits at the greater rate
Answer
Working
Answer
The dull black can emits radiation at a greater rate, so it loses energy faster and cools faster.
Check your understanding 2
Two white tiles, one glossy and one matt, lie in the same bright sunshine. Measurements show that the matt tile absorbs 40 J each second and the glossy tile absorbs 20 J each second. Their material, mass, starting temperature and other energy transfers are matched. By what factor is the matt tile’s absorption rate greater? Which surface property differs?
Show answer
The matt tile initially heats up faster because its measured absorption rate is 40 ÷ 20 = 2 times as great while the other conditions are matched. The differing property is surface texture or finish; their visible colour is kept the same. Use the measured rates for these tiles.
Check your understanding 3
Two walls of equal area receive the same sunlight. The white wall absorbs 20% of it and the black wall absorbs 80%. Separate measurements show that both absorb infrared equally well. Compare their rates of absorbing sunlight. At the same temperature, can you rank their infrared emission rates from these observations?
Show answer
The black wall absorbs sunlight at 80 ÷ 20 = 4 times the white wall’s rate. Their infrared absorption is equal, so at the same temperature and area their infrared emission rates are equal too: a good absorber at those wavelengths is also a good emitter. Use the absorption evidence at the wavelengths being compared.
Surface temperature
The higher the temperature of a surface, the greater the rate at which it emits radiation.
A body also absorbs radiation from its surroundings, so what matters is the net transfer. In surroundings at one temperature, with no extra radiation source such as sunlight:
- hotter than its surroundings: it emits faster than it absorbs, so it has a net loss and cools;
- colder than its surroundings: it absorbs faster than it emits, so it has a net gain and warms;
- at the same temperature as its surroundings: it emits and absorbs at equal rates, so there is no net transfer. It is in thermal equilibrium.
Worked example 2
Two cups of coffee
Problem
Two identical cups of coffee, at 90 °C and at 50 °C, stand in a room at 25 °C. Which loses energy by radiation at the greater rate? What happens to that rate as the coffee cools?
Show full solution
State the assumptions
Method
Match the finishes and areas.Reason
Only the temperature should differ.Working
Answer
Both cup surfaces are hotter than the room, so both have a net loss of energy by radiation.
Compare absorption
Method
Look at what each cup receives.Reason
Same cups in the same room.Working
Answer
Both absorb radiation from the room at the same rate.
Compare emission
Method
Compare the surface temperatures.Reason
A hotter surface emits at a greater rate.Working
Answer
The 90 °C cup emits at the greater rate and has the greater net loss.
Follow the cooling
Working
Answer
As the coffee cools, its surface temperature falls, so it emits more slowly and cools more slowly. The net loss by radiation stops when the coffee reaches room temperature.
Check your understanding 4
A cold can at 5 °C stands in a room whose surrounding surfaces are at 30 °C. Compare its infrared absorption and emission rates.
Show answer
The colder can absorbs infrared radiation faster than it emits it, so the net transfer by radiation is into the can. As its surface approaches 30 °C, the rates approach equality.
Surface area
The larger the surface area, the greater the rate at which a body emits and absorbs radiation, because there is more surface to emit from and to catch the radiation falling on it.
Worked example 3
Same metal, different shape
Problem
A block of aluminium has a volume of 1000 cm³: a 10 cm cube. An equal mass of aluminium is rolled into a sheet 50 cm by 40 cm by 0.50 cm. Both are painted dull black and are at 200 °C. Which emits radiation at the greater rate?
Show full solution
Area of the cube
Method
Add the six faces.Reason
The rate of emission depends on the surface area.Working
Answer
6 × (10 × 10) = 600 cm²
Area of the sheet
Method
Add the two large faces and the four edges.Reason
Every exposed surface emits.Working
Answer
2 × (50 × 40) + 2 × (50 × 0.50) + 2 × (40 × 0.50) = 4000 + 90 = 4090 cm²
Check the volumes match
Method
Multiply the sheet’s dimensions.Reason
The comparison is fair only for equal amounts of metal.Working
Answer
50 × 40 × 0.50 = 1000 cm³
Compare
Working
Answer
Same metal, mass, finish and temperature, but the sheet has about 7 times the surface area. It emits radiation at a greater rate, so it cools faster.
Heat sinks and air-cooled engines have thin fins to add surface area. This helps transfer energy to the surrounding air by convection as well as by radiation.
Check your understanding 5
On a cold night a cat curls up into a tight ball. How does this help it keep warm?
Show answer
Curling up reduces the cat’s exposed surface area, so it loses energy by radiation to the colder surroundings at a smaller rate. Less exposed area can also reduce transfer to moving air.
Comparing surfaces fairly
To compare two surfaces by how fast they cool, change only the surface. Use objects of the same material, mass, shape and starting temperature, in the same surroundings. Match the lid, supports and air flow as well, to keep conduction and convection comparable. Otherwise a faster cooling time might come from less material to cool, a larger area or a draught, rather than stronger emission.
Check your understanding 6
Decide: A dull black plate cools from 80 °C to 60 °C faster than a shiny plate, but the black plate has half the mass. Does this show that dull black is the better emitter?
Show answer
No. The lighter plate needs to lose less energy to cool by the same 20 °C, so it could cool faster even with the same surface. Repeat with plates of equal mass, material and area before drawing a conclusion.
Check your understanding 7 Emission alone does not decide warming or cooling
At one instant, plate A emits 18 W and absorbs 12 W; plate B emits 7 W and absorbs 12 W. Only radiation transfers energy. Calculate each net rate of internal-energy change, taking a gain as positive. Which plate warms?
Show answer
A has a net 18 − 12 = 6 J/s outward, so it loses internal energy and cools. B has a net 12 − 7 = 5 J/s inward, so it gains internal energy and warms. Both emit radiation, but emission must be compared with absorption to decide the net transfer. As their temperatures approach the surroundings, the emission and absorption rates approach equality. The rates change as the plate temperatures change.
Challenge yourself
A polished metal can and a matt black can contain equal masses of hot water. Design a fair cooling comparison. Would a temperature drop alone measure only radiation?
Check your thinking
Match can material, mass, shape, lids, water mass, starting temperature, supports and surroundings. Record temperature at equal time intervals. Cooling includes conduction, convection and radiation, so temperature alone does not isolate radiation. Keep the other transfer paths comparable when investigating the surface finish.
Common mistakes
- Saying radiation needs air to travel through. It needs no material medium and can cross a vacuum.
- Saying a vacuum stops all energy transfer. It stops conduction and convection; radiation still crosses it.
- Saying only hot objects give out radiation. Every object emits radiation; hotter surfaces emit more.
- Saying black surfaces attract heat. They absorb much of the radiation that falls on them; nothing is attracted.
- Saying a good absorber is a poor emitter. A good absorber is also a good emitter.
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Syllabus and review details
- SEC G2 Science Physics component 2027 · 2027
Physics section, PDF pages 8–23
Last reviewed: