IGCSE Physics 0625 · Topic 2.3.3

IGCSE Physics Radiation Questions and Worked Solutions

50 topic-wise practice questions on Radiation (Unit 2: Thermal Physics), written to the Cambridge IGCSE Physics 0625 syllabus for 2026–2028, each with a step-by-step worked solution.

50 questions19 Core21 Extended10 Challenge124 marksTextbook §2.3 · pp. 115–126

Questions tagged Core are for all candidates (Papers 1 & 3). Questions tagged Extended are Supplement content (Papers 2 & 4). Questions 41–50 are harder Challenge questions. Try each question first, then tap Show solution to check your working.

Topic guide: Radiation

Thermal radiation is infrared electromagnetic radiation. It needs no medium, so it can travel through a vacuum. The surface colour and texture of an object affect how well it emits and absorbs radiation.

Key formulas

rate in = rate out → constant temperature

energy balance

Extended

Prerequisite knowledge

Electromagnetic spectrum (3.3)
Energy transfers (1.7.1)

Exam tips & common mistakes

'Black' alone is not enough — say 'dull/matt black'
Shiny surfaces are poor emitters AND poor absorbers
In energy-balance questions, compare the RATE in with the RATE out

Key facts from the syllabus (2.3.3)

Thermal radiation is infrared radiation; all objects emit it
It needs no medium and travels through a vacuum (from the Sun to the Earth)
Dull black surfaces are the best emitters and absorbers; shiny (white/silver) surfaces are the worst emitters and absorbers and the best reflectors
Extended an object at constant temperature transfers energy away at the same rate it receives it
Extended the Earth's temperature depends on the balance between incoming radiation and radiation emitted; greenhouse gases reduce the rate of loss
Extended the rate of emission depends on surface temperature and surface area
Extended experiments with a Leslie cube compare emission; heater with wax-and-pin plates compares absorption

Textbook: §2.3 Transfer of thermal energy, pp. 115–126

Core[1 mark]

State the type of electromagnetic radiation that is thermal radiation.

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Method: Infrared.

Infrared radiation

Core[1 mark]

State which objects emit infrared radiation.

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Method: All objects.

All objects

Core[2 marks]

Explain why energy from the Sun can reach the Earth through space.

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Method: Radiation needs no medium.

Thermal radiation (electromagnetic waves) does not need a medium, so it travels through the vacuum of space

Core[2 marks]

State which surface is the best emitter of infrared radiation: dull black, shiny black, dull white or shiny white.

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Method: Dull black.

Dull black

Core[2 marks]

State which surface is the best reflector of infrared radiation.

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Method: Shiny/light surfaces reflect.

Shiny white (or shiny silver)

Core[2 marks]

Explain why houses in hot countries are often painted white.

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Method: White reflects infrared.

White surfaces are poor absorbers/good reflectors, keeping the house cooler

Core[2 marks]

Explain why the cooling fins on the back of a refrigerator are painted dull black.

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Method: Good emitter.

Dull black is a good emitter of infrared radiation, so energy is removed faster

Core[3 marks]

Two identical containers of very hot water, one dull black and one shiny white, are left to cool. State which cools faster over the first 5 minutes and explain.

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Method: Dull black is a better emitter.

The dull black container: dull black surfaces are better emitters of infrared radiation

Core[2 marks]

Explain why emergency blankets for marathon runners are shiny.

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Method: Shiny surfaces are poor emitters.

Shiny surfaces are poor emitters (and reflect infrared back), reducing energy loss from the body

Core[2 marks]

Explain why solar water panels are painted dull black.

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Method: Good absorber.

Dull black is a good absorber of infrared radiation

Core[2 marks]

State one difference between thermal radiation and conduction.

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Method: Medium.

Radiation needs no medium; conduction needs a material

Core[2 marks]

State which surface is the best absorber of infrared radiation.

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Method: Dull black.

Dull (matt) black

Core[2 marks]

State which surface is the poorest emitter of infrared radiation.

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Method: Shiny silver/white.

Shiny white or silver

Core[2 marks]

Explain why the inside of a vacuum flask is silvered.

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Method: Reflects IR.

Shiny surfaces are poor emitters and reflect infrared back into the liquid

Core[2 marks]

Explain why black clothes feel hotter in sunshine than white clothes.

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Method: Absorption.

Black is a better absorber of infrared radiation

Core[2 marks]

State why a thermal imaging camera can see people in the dark.

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Method: IR emission.

People emit infrared radiation, which the camera detects

Core[2 marks]

Explain why kettles are often made with shiny surfaces.

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Method: Poor emitter.

Shiny surfaces emit less infrared, so less energy is lost

Core[2 marks]

Explain why astronauts' suits are often shiny white or silver.

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Method: Reflect.

They reflect infrared radiation from the Sun, preventing overheating

Core[3 marks]

Two cans, one shiny and one matt black, contain cold water and are placed in sunlight. State which warms faster and explain.

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Method: Absorber.

The matt black can, because matt black is a better absorber of infrared

Extended[2 marks]

State the condition for an object to remain at a constant temperature.

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Method: Energy in = energy out.

It must transfer energy away at the same rate that it receives energy

Extended[2 marks]

State what happens to an object's temperature if it receives energy faster than it transfers energy away.

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Method: Temperature rises.

Its temperature increases

Extended[2 marks]

After 30 minutes, the water in a container stays at a constant temperature. Explain in terms of energy transfers.

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Method: Balance.

The rate of energy received from the surroundings equals the rate of energy lost

Extended[3 marks]

Explain how greenhouse gases affect the temperature of the Earth.

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Method: Less IR escapes.

They absorb infrared radiation emitted from the Earth's surface and re-emit some back, reducing the rate at which energy is lost, so the Earth's temperature rises

Extended[3 marks]

Describe an experiment using a Leslie cube to compare the emission of infrared from different surfaces.

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Method: Hot water inside; IR detector at equal distance.

Fill a Leslie cube with hot water; place an infrared detector the same distance from each face; the dull black face gives the highest reading

Extended[3 marks]

Describe an experiment to compare the absorption of infrared by dull black and shiny surfaces.

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Method: Equal distance from heater.

Place a dull black plate and a shiny plate at equal distances from a radiant heater, with wax and pins on the back; the pin on the dull black plate falls first

Extended[2 marks]

State two factors that affect the rate at which an object emits infrared radiation.

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Method: Temperature and surface area.

Surface temperature and surface area (also colour/texture)

Extended[2 marks]

Explain why a large radiator emits more energy per second than a small one at the same temperature.

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Method: Larger surface area.

It has a larger surface area to emit radiation

Extended[2 marks]

Two identical objects are at 40 °C and 80 °C. State which emits infrared at the greater rate.

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Method: Higher temperature.

The object at 80 °C

Extended[2 marks]

Explain why the Earth's surface temperature would fall if it emitted energy faster than it received energy from the Sun.

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Method: Net loss.

More energy leaves than arrives, so the energy stored decreases and temperature falls

Extended[2 marks]

An object is at a constant temperature. Compare the rate at which it absorbs and emits infrared.

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Method: Equal.

The rates are equal

Extended[3 marks]

Explain what happens to the temperature of a planet if its atmosphere starts absorbing more infrared emitted from its surface.

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Method: Less escapes.

Less energy escapes, so the rate of loss falls below the rate received and the temperature rises until a new balance is reached

Extended[2 marks]

State the effect of increasing surface temperature on the rate of infrared emission.

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Method: Increases.

The rate of emission increases

Extended[2 marks]

State the effect of increasing surface area on the rate of infrared emission.

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Method: Increases.

The rate of emission increases

Extended[3 marks]

A Leslie cube has four faces: matt black, shiny black, matt white and shiny silver. Predict the order of infrared readings from highest to lowest.

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Method: Matt black best.

Matt black, shiny black, matt white, shiny silver

Extended[3 marks]

Describe how to make the Leslie cube experiment a fair test.

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Method: Control variables.

Same distance from detector to each face; same detector; wait for readings to settle; same water temperature

Extended[2 marks]

Explain why a cup of tea eventually reaches room temperature and stops cooling.

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Method: Balance.

It then receives energy from the surroundings at the same rate it loses energy

Extended[2 marks]

Explain why the Earth's temperature is fairly constant over a year.

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Method: Energy balance.

It absorbs energy from the Sun at about the same rate it emits energy into space

Extended[3 marks]

Explain how increasing CO₂ in the atmosphere could lead to global warming.

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Method: Greenhouse effect.

CO₂ absorbs infrared emitted by the Earth and re-emits some back towards the surface, reducing the rate of energy loss, so the average temperature rises

Extended[2 marks]

Explain why a cloudy night is usually warmer than a clear night.

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Method: Clouds reflect/absorb IR.

Clouds absorb and re-emit infrared back to the ground, reducing energy loss

Extended[2 marks]

Explain why radiator fins on electronic equipment are often painted black.

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Method: Good emitter.

Black surfaces emit infrared well, removing energy faster

Challenge questions

Harder, multi-step questions at the top end of the exam. Attempt these once you are confident with the questions above.

ChallengeCore[4 marks]

Describe an experiment to compare the rate of cooling of a matt black can and a shiny silver can, stating variables to control and the expected result.

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Method: Fair test.

Fill both with equal volumes of water at the same starting temperature, use lids and thermometers, record temperature every minute; same surroundings; the matt black can cools faster

ChallengeCore[3 marks]

Explain why the cooling fins on a motorbike engine are dull black.

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Method: Good emitter.

Dull black surfaces emit infrared well, so energy is removed faster

ChallengeCore[3 marks]

Explain why the Sun's energy reaches the Earth only by radiation.

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Method: Vacuum.

Space is a vacuum with no particles, so conduction and convection cannot occur

ChallengeExtended[4 marks]

Explain why the temperature of the Earth stays roughly constant, and what happens if the amount of greenhouse gas increases.

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Method: Energy balance.

It absorbs energy from the Sun at the same rate it radiates energy to space; more greenhouse gas absorbs more infrared and re-emits some back, so the rate of loss falls and the temperature rises until a new balance is reached

ChallengeExtended[4 marks]

A metal plate is placed in sunlight. Its temperature rises, then stays constant. Explain this in terms of rates of absorption and emission.

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Method: Equilibrium.

At first it absorbs faster than it emits; as it gets hotter it emits more; when rates are equal the temperature is constant

ChallengeExtended[4 marks]

Two identical radiators at 60 °C: one painted matt black, one shiny silver. Explain which heats a room better by radiation and why the difference may be small in practice.

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Method: Emission and convection.

The matt black radiator emits more infrared; but most energy from radiators is transferred by convection, so the difference is small

ChallengeExtended[3 marks]

Explain why polar bears have black skin under white fur.

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Method: Absorb IR.

Black skin absorbs infrared well; the fur traps air to reduce losses

ChallengeExtended[4 marks]

State two factors affecting the rate of infrared emission from a surface and use them to explain why a large, hot radiator emits more than a small, warm one.

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Method: Temperature and area.

Surface temperature and surface area: the large hot radiator has both larger area and higher temperature

ChallengeExtended[4 marks]

A Leslie cube with faces matt black, shiny black, matt white and shiny silver is filled with hot water. Predict the order of detector readings and explain.

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Method: Colour and texture.

Matt black > shiny black > matt white > shiny silver: dark and matt surfaces are better emitters than light and shiny ones

ChallengeExtended[4 marks]

Explain why a cloudless desert gets very cold at night even though it is very hot during the day.

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Method: Radiation loss.

With no clouds, infrared from the ground escapes to space at night; the dry air absorbs little and re-emits little back

All IGCSE Physics topics ← 2.3.2 Convection 2.3.4 Consequences of Thermal Energy Transfer →

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