Thermal radiation
Follow energy across a vacuum, compare surface evidence and explain why an object warms or cools.
Core: infrared, no medium and surface comparisons. Supplement / Extended: energy balance, Earth, experiments and emission-rate factors. Separate from Egyptian Baccalaureate / Thanaweya Amma and not Year 3.
Core 2.3.3.1–3 · Supplement 2.3.3.4–9 · PDF pp.200–205 / printed pp.192–197.
Radiation needs no material pathway
Thermal radiation in this syllabus is infrared radiation, an electromagnetic wave. It can carry energy through a vacuum, unlike conduction and convection, which require matter. Ordinary objects emit infrared even when they do not glow visibly.
Worked example
A person, a wall and an ice cube all emit infrared. The Sun’s energy crosses space by electromagnetic radiation; sunlight includes visible light and other wavelengths as well as infrared. Do not describe all solar energy as infrared only.
Think first: Does a vacuum stop every way of transferring thermal energy?
No. It blocks conduction and convection across the empty gap, but radiation can cross it.
Detector reading or fair conclusion?
Can these detector readings compare finishes fairly?
Supplement / Extended. These are illustrative background-corrected signals, not a calibrated material database.
Both faces at80°C; same exposed area; detector10cm away, square to each face, with the same settings.
The digital comparison is unscored. A physical Leslie-cube demonstration needs teacher supervision: hot water and metal can scald. Match detector angle and distance, avoid stray radiation, and allow stable readings.
Constant temperature can hide busy transfers
Predict warming, cooling or steady temperature
Supplement / Extended. An ordinary solid object, with no phase change or other changing energy stores. Input means energy actually absorbed. Output is the total transferred away. Set constant rates over10s; this is energy bookkeeping, not a temperature simulator.
Changing either slider clears the prediction. No temperature value is invented, and no lasting score is stored.
Worked Earth-system bookkeeping example
Use arbitrary model power units, not real climate measurements. If incoming solar power is100units and30units are reflected, absorbed input is70units. Outgoing70units balances it. If reflection increases to35units while output is initially still70units, absorbed input becomes65units and the system initially loses5units of energy per unit time. Its average temperature tends to fall until the balance adjusts. Do not count reflected sunlight as absorbed input.
Greenhouse gases absorb and emit infrared; they are not an opaque lid preventing every outgoing photon. At a new long-term balance, energy still leaves the Earth system. A constant-temperature melting sample is outside the no-phase-change model used above.
Compare temperature rises, not colours alone
| Time / min | Dull black | Polished silver |
|---|---|---|
| 0 | 20 | 20 |
| 1 | 25 | 22 |
| 2 | 29 | 23 |
| 3 | 33 | 24 |
| 4 | 36 | 25 |
Worked interpretation and marking points
- Black rises36−20=16°C; shiny rises25−20=5°C. [1 mark]
- The greater rise supports better absorption by black in the matched setup. [1 mark]
- Match plate mass/material/area and incident radiation, starting temperature, timing and sensor attachment. [1 mark for a valid control]
For a fair experiment, both plates must receive comparable radiation. Equal distance on opposite sides of an uneven heater is not enough by itself. Repeat with positions swapped, use an appropriately uniform source and keep hot plates on heatproof supports. Do not touch or improvise a radiant-heater experiment at home.
Practise, then explain
Four multiple-choice questions, four numerical responses and six written tasks. Basic radiation and surface ideas are Core; experiments, rate balance, Earth and emission-rate factors are Supplement / Extended. Numerical answers are checked automatically; written answers use a self-assessment guide. The suggested marks are for this original practice, not an official Cambridge mark scheme.
Practice stays only while this page is open. Refreshing or leaving clears these responses. Copy useful written work into your notes before leaving.
Which statement explains how energy from the Sun crosses the near-vacuum between the Sun and Earth?
Optional hint
Compare whether each transfer mechanism needs matter.
Source corrections and model limits
The source PDF is preserved. Its hot-cube Q5 is labelled Core, but the emitter-investigation objective2.3.3.7 is Supplement; that investigation is labelled Supplement here. Figures28.1–28.2 informed the matched-geometry and equal-input comparisons. More arrows in a schematic are not a measured radiation ratio.
The PDF groups white surfaces with shiny metal as poor infrared emitters. That is too broad: visible colour is not a complete infrared property. Some white coatings reflect solar radiation while emitting thermal infrared strongly. Keep the standard dull-black versus polished-metal comparison, and use supplied measurements when comparing real coatings. NASA’s thermal-control guidance explains the distinction between solar absorption and infrared emission.
Thermal radiation from everyday-temperature objects is mainly infrared. Very hot objects can also emit visible light; sunlight is not exclusively infrared. The syllabus’s “all objects emit” applies to ordinary objects above absolute zero. Emission is not the same as net energy loss: surroundings also supply radiation.
A processor transfers electrical energy into thermal energy; it does not create energy. Fins can aid convection as well as radiation. Cooling tins can demonstrate overall cooling differences but do not isolate radiation perfectly. The energy-balance activity assumes no phase change or other changing energy stores, so it does not contradict a melting ice sample staying at constant temperature while gaining energy.
Source alignment: supplied Physics-0625 Volume A, PDF pp.200–205 / printed pp.192–197, Figures28.1–28.2; official Cambridge 2026–2028 syllabus, section2.3.3. Original explanations, illustrative data and questions. Source-download access unchanged.