Course: Separate Physics only | Tier: Foundation + Higher (Earth’s temperature is HT) | Linked practical: Required Practical 10: Infrared Radiation | See also: Electromagnetic Waves
Notes
Emitting and absorbing infrared
All bodies (objects), whatever their temperature, emit and absorb infrared radiation. The hotter an object is, the more infrared it emits each second.
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- A perfect black body is an object that absorbs all of the radiation incident on it. It does not reflect or transmit any radiation.
- A good absorber is also a good emitter, so a perfect black body is the best possible emitter.
- As an object gets hotter, the intensity of the radiation it emits increases and the peak wavelength gets shorter.
- An object at a constant temperature absorbs radiation at the same rate as it emits it.
- If it absorbs faster than it emits, its temperature rises. If it emits faster than it absorbs, its temperature falls.
The temperature of the Earth (HT)
The Earth’s temperature depends on the balance between the rate at which it absorbs radiation (mostly from the Sun) and the rate at which it emits radiation into space. Some radiation is reflected straight back into space by clouds, ice and snow. Some of the infrared emitted by the Earth’s surface is absorbed by gases in the atmosphere, which keeps the Earth warmer. If the rates of absorption and emission change, the Earth’s temperature changes.
Required Practical 10: Infrared Radiation
Aim: to investigate how the amount of infrared radiation emitted by a surface depends on the nature of that surface.
- Place a Leslie cube on a heatproof mat. Its four faces are matt black, shiny black, matt white and shiny silver.
- Fill the cube with very hot water and put the lid on.
- Place an infrared detector a fixed distance (e.g. 10 cm) from one face and record the reading.
- Repeat for each face, keeping the distance the same.
- Compare the readings.
✏️ Copy into your book: practical key points
- Result: matt black emits the most infrared; shiny silver emits the least.
- Independent variable: type of surface.
- Dependent variable: infrared detector reading.
- Control variables: distance from the detector, temperature of the water (all faces are at the same temperature because they’re on the same cube).
- Safety: take care with very hot water; place the cube on a heatproof mat.
Questions
Q1 (F) Which objects emit infrared radiation?
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All objects, whatever their temperature.
Q2 (F) What is a perfect black body?
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An object that absorbs all the radiation incident on it, reflecting and transmitting none.
Q3 (F) Which face of a Leslie cube emits the most infrared radiation, and which emits the least?
Show answer
Matt black emits the most; shiny silver emits the least.
Q4 (F) In the Leslie cube practical, why must the detector be the same distance from each face?
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The reading depends on distance as well as the surface, so it must be controlled to make it a fair test.
Q5 (F/H) What happens to the radiation emitted by an object as it gets hotter?
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It emits more radiation each second (higher intensity), and the peak wavelength gets shorter.
Q6 (F/H) One star looks red and another looks blue. Which is hotter? Explain.
Show answer
The blue star. Hotter objects have a shorter peak wavelength, and blue light has a shorter wavelength than red light.
Q7 (H) A cup of tea is placed in a room and eventually stops cooling. Explain why its temperature stays constant even though it is still emitting radiation.
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It has reached room temperature, so it is now absorbing radiation from its surroundings at the same rate as it emits radiation. There is no net energy transfer, so its temperature stays constant.
Q8 (H) Explain how the temperature of the Earth depends on radiation.
Show answer
The Earth absorbs radiation from the Sun and emits infrared radiation into space. Some incoming radiation is reflected by clouds, ice and snow, and some emitted infrared is absorbed by gases in the atmosphere. If the Earth absorbs radiation at the same rate as it emits it, its temperature stays constant. If it absorbs more than it emits, its temperature rises.
