Explain that all bodies (objects) emit radiation and that the intensity and wavelength distribution of any emission depends on the temperature of the body.
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GCSE Physics Revision
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GCSE Physics revision
Perfect black bodies and radiation
Black body radiation (physics only)
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Apply the scientific explanation of perfect black bodies and radiation to a relevant example.
Analyse a new situation involving perfect black bodies and radiation and explain the scientific reasoning.
Revision summary
Key knowledge
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Absorbers
- What is Infrared Radiation & Heat Transfer?
- Objects that are hotter than their surroundings emit infrared radiation as a method of heat transfer.
- All objects both emit and absorb infrared radiation simultaneously, and the balance between the two determines whether temperature changes.
Thermal Equilibrium
- Thermal equilibrium occurs when the rate of absorption of infrared radiation equals the rate of emission, keeping temperature constant.
- If absorption exceeds emission, the kinetic energy of particles increases and temperature rises.
- If emission exceeds absorption, the kinetic energy of particles decreases and temperature falls.
Black Body Radiation & Global Warming
- Greenhouse gases reduce the amount of infrared radiation emitted by the Earth into space, meaning absorption exceeds emission and global temperature rises over time.
- This is a real-world application of the principle that an imbalance between absorption and emission causes a change in temperature.
- What is a Black Body?
- A black body is a hypothetical object that is a perfect absorber and perfect emitter of radiation, absorbing 100% of all radiation that hits it.
- A black body re-emits 100% of the radiation it absorbs rather than reflecting any of it.
- No real object is a perfect black body, but the concept is used in physics to make useful approximations and calculations about temperature.
- Vanta black is a real material that absorbs 99.96% of all radiation, making it one of the closest real-world approximations to a black body.
Black as the Best Emitter & Absorber
- Black surfaces are the best absorbers of radiation, which is why wearing black clothing on a hot day causes you to feel warmer.
- Black surfaces are also the best emitters of radiation, meaning they radiate heat away most effectively compared to lighter colours.
Black Body Radiation Curves
- Black bodies emit radiation across a range of wavelengths, but at different intensities — this is shown on a black body radiation curve with wavelength on the x-axis and intensity on the y-axis.
- The peak of the curve shifts to a shorter (lower) wavelength as the temperature of the black body increases.
- A shorter wavelength corresponds to a higher-energy part of the electromagnetic spectrum, towards the blue or violet end of the visible spectrum.
Linking Wavelength to Temperature
- A hotter object emits peak radiation at a shorter wavelength, whilst a cooler object emits peak radiation at a longer wavelength.
- This explains why a blue Bunsen burner flame is hotter than a red one — blue light has a shorter wavelength than red light.
- This relationship between peak wavelength and temperature allows scientists to estimate the temperature of distant objects, such as stars, from their colour.