2.3.3

Radiation

Cambridge IGCSE Physics (0625)  · Unit 2: Thermal physics  · 18 flashcards

Radiation is topic 2.3.3 in the Cambridge IGCSE Physics (0625) syllabus , positioned in Unit 2 — Thermal physics , alongside States of matter, Particle model and Gases and the absolute scale of temperature.

This topic is examined in Paper 1 (multiple-choice) and Papers 3/4 (theory), plus Paper 5 or Paper 6 (practical / alternative to practical).

The deck below contains 18 flashcards — covering the precise wording mark schemes reward.

What the Cambridge 0625 syllabus says

Official 2026-2028 spec

These are the exact learning objectives Cambridge sets for this topic. Match the command word (Describe, Explain, State, etc.) in your answer to score full marks.

  1. Know Know that thermal radiation is infrared radiation and that all objects emit this radiation
  2. Know Know that thermal energy transfer by thermal radiation does not require a medium
  3. Describe Describe the effect of surface colour (black or white) and texture (dull or shiny) on the emission, absorption and reflection of infrared radiation
  4. Know Know that for an object to be at a constant temperature it needs to transfer energy away from the object at the same rate that it receives energy Supplement
  5. Know Know what happens to an object if the rate at which it receives energy is less or more than the rate at which it transfers energy away from the object Supplement
  6. Know Know how the temperature of the Earth is affected by factors controlling the balance between incoming radiation and radiation emitted from the Earth's surface Supplement
  7. Describe Describe experiments to distinguish between good and bad emitters of infrared radiation Supplement
  8. Describe Describe experiments to distinguish between good and bad absorbers of infrared radiation Supplement
  9. Describe Describe how the rate of emission of radiation depends on the surface temperature and surface area of an object Supplement
Key Concept Flip

Explain why all objects, regardless of their temperature, emit thermal radiation. What type of electromagnetic wave is thermal radiation?

Answer Flip

All objects emit thermal radiation because their atoms and molecules are in constant motion. This motion causes charged particles within the object to accelerate, which results in the emission of electromagnetic radiation. The hotter the object, the more intense the motion and the more thermal radiation emitted. Thermal radiation is infrared radiation.

Key Concept Flip

A metal block is heated to 50°C and left to cool in a room at 20°C. State two ways in which the thermal radiation emitted by the block changes as it cools.

Answer Flip

1. The amount of thermal radiation emitted decreases.
2. The peak wavelength of the emitted radiation increases (shifts towards longer wavelengths).

Key Concept Flip

Explain why thermal radiation can transfer energy through a vacuum, unlike conduction or convection.

Answer Flip

Thermal radiation involves the emission of electromagnetic waves (primarily infrared) by an object due to its temperature. Electromagnetic waves, unlike particles, do not require a medium to travel and can propagate through the vacuum of space. Conduction requires particles to vibrate and collide, transferring energy. Convection requires particles to move and carry energy.

Key Concept Flip

State why a metal container in a vacuum is more effective at preventing energy transfer by radiation than by conduction or convection.

Answer Flip

Radiation does not require a medium, so it can transfer heat across a vacuum. Conduction and convection cannot efficiently transfer heat across a vacuum because they require a medium (particles).

Key Concept Flip

Two identical metal cubes, one painted matte black and the other shiny white, are placed in a room at 25°C. Both cubes are heated to 80°C. Which cube will cool down faster? Explain why, referring to the properties of the surfaces.

Answer Flip

The matte black cube will cool down faster.

Explanation: Matte black surfaces are good emitters and absorbers of infrared radiation. A shiny white surface is a poor emitter and a good reflector of infrared radiation. Since both cubes are hotter than the surrounding environment, they will lose heat by emitting infrared radiation. The matte black cube will emit more infrared radiation per unit time than the shiny white cube, causing it to cool down faster.

Key Concept Flip

State which surface, matte black or shiny white, is the better absorber of infrared radiation.

Answer Flip

Matte black is the better absorber of infrared radiation.

Key Concept Flip

A metal block is heated by a 120W heater. After some time, the temperature of the block remains constant. Explain why the temperature remains constant even though the heater is still supplying energy.

Answer Flip

The temperature remains constant because the rate at which the block absorbs energy from the heater is equal to the rate at which it emits energy into the surroundings. The block is in thermal equilibrium, meaning there is no net energy transfer.

Key Concept Flip

A car is parked in direct sunlight. Suggest two factors that affect the rate at which the car radiates thermal energy to its surroundings.

Answer Flip

1. The surface temperature of the car. A higher temperature will result in a greater rate of radiation.
2. The surface area of the car. A larger surface area will result in a greater rate of radiation.

Key Concept Flip

A metal plate receives energy from the sun at a rate of 5.0 J/s. It radiates energy away at a rate of 3.0 J/s. Calculate the change in the plate's internal energy after 10 seconds, assuming no other energy transfers occur.

Answer Flip

Change in internal energy = (Rate of energy in - Rate of energy out) x Time
Change in internal energy = (5.0 J/s - 3.0 J/s) x 10 s
Change in internal energy = 2.0 J/s x 10 s
Change in internal energy = 20 J

The internal energy increases because the rate of energy received is greater than the rate of energy emitted.

Key Concept Flip

Describe what happens to the temperature of an object if the rate at which it receives thermal energy is less than the rate at which it radiates thermal energy away from itself.

Answer Flip

The temperature of the object will decrease. Since the object is losing thermal energy at a faster rate than it is gaining thermal energy, its internal energy will decrease. A decrease in internal energy results in a decrease in temperature.

Key Concept Flip

Explain how increasing the concentration of greenhouse gases in the atmosphere affects the Earth's average temperature. Refer to the balance between incoming and outgoing radiation.

Answer Flip

Increased greenhouse gas concentration means more outgoing (infrared) radiation from the Earth's surface is absorbed and re-emitted back towards the Earth. This reduces the amount of heat energy escaping into space. Since less energy leaves than arrives from the sun, the Earth's average temperature increases. This is also known as the greenhouse effect.

Key Concept Flip

State two factors that affect the amount of incoming solar radiation absorbed by the Earth's surface.

Answer Flip

1. Albedo (reflectivity) of the Earth's surface: Higher albedo surfaces (

Example: ice) reflect more radiation.<br>2. Cloud cover: Clouds reflect incoming solar radiation back into space. Thus the higher the cloud cover, the less radiation will reach the surface.
Key Concept Flip

Describe an experiment to compare the rate of infrared radiation emission from a dull black surface and a shiny silver surface, both at the same temperature. Include details of the apparatus, procedure, and how to ensure a fair test.

Answer Flip

Apparatus: Two identical metal cubes (one dull black, one shiny silver), two thermometers, heat source (

Example: Bunsen burner or hot plate), stopwatch.<br><br>Procedure: 1. Heat both cubes to the same temperature (. 80°C) using the heat source. Monitor the temperature with the thermometers. 2. Remove the heat source. 3. Simultaneously record the temperature of each cube every minute for, say, 10 minutes. 4. Plot a graph of temperature against time for both cubes.<br><br>Fair Test: 1. Ensure both cubes have the same surface area and volume. 2. Ensure the thermometers are calibrated and read accurately. 3. Position both cubes the same distance from any drafts. 4. Start recording temperature at the same initial temperature for both cubes.<br><br>The dull black surface will cool down faster than the shiny silver surface, indicating it is a better emitter of infrared radiation.
Key Concept Flip

Two identical metal cubes, one painted matte black and one painted shiny white, are heated to 60°C. They are then placed in a room at 20°C. State which cube will cool down faster and explain why in terms of their emissivity.

Answer Flip

The matte black cube will cool down faster.

Explanation: Matte black surfaces are good emitters of infrared radiation. This means they radiate thermal energy away from the cube more efficiently. Shiny white surfaces are poor emitters of infrared radiation; they reflect more radiation and emit less. Therefore, the black cube will lose heat at a faster rate and cool down quicker.

Key Concept Flip

Describe an experiment using two metal plates, one black and one shiny, to determine which is a better absorber of infrared radiation. Include the apparatus, procedure, and how to compare the results.

Answer Flip

Apparatus: Two identical metal plates (one black, one shiny), two thermometers, heat lamp (infrared source), retort stands.

Procedure:
1. Place the two metal plates at equal distances from the heat lamp, supported by retort stands.
2. Attach a thermometer to the back of each plate to measure its temperature.
3. Ensure both plates are initially at the same temperature.
4. Switch on the heat lamp and record the temperature of each plate every minute for 5 minutes.

Comparison:
The plate that shows a greater increase in temperature over the 5 minutes is the better absorber of infrared radiation. We expect the black plate to show a larger temperature increase than the shiny plate.

Key Concept Flip

State two reasons why the black plate in an experiment to investigate infrared absorption heats up more than a shiny plate when exposed to a heat lamp.

Answer Flip

1. The black plate absorbs more infrared radiation than the shiny plate.
2. The shiny plate reflects more infrared radiation than the black plate.

Key Concept Flip

A metal sphere with a surface area of 0.5 m² is heated to a temperature of 500 K. Another identical sphere with a surface area of 1.0 m² is heated to 250 K. Calculate the ratio of the rate of emission of radiation from the first sphere to the rate of emission from the second sphere.

Answer Flip

The rate of emission of radiation is proportional to surface area (A) and the fourth power of temperature (T⁴). Therefore, Rate ∝ AT⁴.

Rate₁/Rate₂ = (A₁T₁⁴) / (A₂T₂⁴)

Rate₁/Rate₂ = (0.5 * 500⁴) / (1.0 * 250⁴)

Rate₁/Rate₂ = (0.5 * 62500000000) / (1.0 * 3906250000)

Rate₁/Rate₂ = 31250000000 / 3906250000

Rate₁/Rate₂ = 8

Answer: 8

Key Concept Flip

Describe how increasing the surface temperature of an object affects the rate of emission of radiation. Explain why this happens.

Answer Flip

Increasing the surface temperature of an object significantly increases the rate of emission of radiation. This is because the rate of emission is proportional to the fourth power of the absolute temperature (T⁴). Therefore, a small increase in temperature leads to a large increase in the emitted radiation rate. This is due to the Stefan-Boltzmann law.

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2.3.2 Convection 2.3.4 Consequences of thermal energy transfer

More topics in Unit 2 — Thermal physics

Radiation sits alongside these Physics decks in the same syllabus unit. Each uses the same spaced-repetition system, so progress in one informs the next.

2.1.1 States of matter

4 flashcards
2.1.2 Particle model

16 flashcards
2.1.3 Gases and the absolute scale of temperature

6 flashcards
2.2.1 Thermal expansion of solids, liquids and gases

6 flashcards
2.2.2 Specific heat capacity

8 flashcards
2.2.3 Melting, boiling and evaporation

16 flashcards
2.3.1 Conduction

8 flashcards
2.3.2 Convection

4 flashcards
2.3.4 Consequences of thermal energy transfer

4 flashcards

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