Explain why a perfect black body absorbs and emits radiation differently from a shiny metal surface.

Eduqas GCSE Physics — 6.5 Black body radiation · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Scientists use the idea of a perfect black body to model how objects absorb and emit electromagnetic radiation. Real objects, such as shiny metal surfaces, behave differently from a perfect black body.

Model answer (4 marks)

A perfect black body absorbs all incident radiation, so no energy is reflected or transmitted. Consequently it is the best possible emitter of radiation at any given temperature, because the absorbed energy is re‑emitted as thermal radiation.

A shiny metal surface, in contrast, reflects most of the incident radiation and only a small fraction is absorbed. Because it absorbs less, it emits far less radiation than a perfect black body at the same temperature, making it a poor emitter.

Examiner tips

Use the terms "absorbs all" and "reflects most" to show contrast
Mention that a black body is the best emitter and a metal is a poor emitter
Keep the answer concise and directly linked to the marks

Common mistakes

Confusing absorption with reflection; saying the metal absorbs all radiation
Forgetting to state that the black body is the best emitter
Using vague phrases like "different" without specifying the physical reasons

Mark scheme (4 marks)

A perfect black body absorbs all incident radiation / no radiation is reflected or transmitted
A shiny metal surface reflects (most) radiation rather than absorbing it
A perfect black body is also the best possible emitter of radiation at any given temperature
The shiny metal surface emits less radiation than a perfect black body at the same temperature / is a poor emitter

Key terms in this question

black body · absorb · emit · radiation