A metal bar is heated in a furnace. As the temperature of the bar increases, the colour of the light it emits changes from dull red, to orange, to yellow, and eventually to white. Explain what these observations tell us about the relationship between temperature and the radiation emitted by the bar.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A metal bar is placed inside a furnace and heated to very high temperatures. At first the bar glows dull red, then orange, then yellow, and finally white-hot.
Model answer (4 marks)
All objects emit electromagnetic radiation; the bar emits because it is hot.
As the temperature rises, the peak wavelength of the emitted radiation moves to shorter wavelengths.
At higher temperatures the bar emits more radiation, i.e. greater intensity or more energy per second.
When the bar appears white it emits across the whole visible spectrum, showing that it has reached a very high temperature.
As the temperature rises, the peak wavelength of the emitted radiation moves to shorter wavelengths.
At higher temperatures the bar emits more radiation, i.e. greater intensity or more energy per second.
When the bar appears white it emits across the whole visible spectrum, showing that it has reached a very high temperature.
Examiner tips
- Use the phrase "peak wavelength" to show understanding of Wien’s law.
- Show the link between temperature and intensity (Stefan‑Boltzmann law).
- Explain that white light means all visible wavelengths are present.
Common mistakes
- Confusing colour change with colour of the bar itself rather than emitted light.
- Failing to mention that the bar emits radiation because it is hot.
- Not recognising that white light indicates a very high temperature.
Mark scheme (4 marks)
- All objects emit electromagnetic radiation / the bar emits radiation because it is hot
- As temperature increases, the peak wavelength of emitted radiation decreases / the radiation shifts towards shorter wavelengths
- At higher temperatures the bar emits more radiation / greater intensity / more energy per second
- White light contains all visible wavelengths / the bar appearing white shows it emits across the whole visible spectrum, consistent with a very high temperature
Key terms in this question
Related
- All Eduqas GCSE Physics revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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