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Practise 23 exam-style questions on Emission and absorption of infrared radiation (physics only) for AQA GCSE Physics. Write your answer as you would in the exam and get it marked in seconds against the mark scheme, point by point, with an AI tutor on hand if you get stuck.
Here are 4 of the questions in this topic, each with a model answer, an explanation of why it earns the marks and the mark scheme an examiner would use.
Which type of surface is the best absorber of infrared radiation?
A matt black surface.
Matt black surfaces absorb almost all the infrared that falls on them, so they are the closest everyday surfaces to a perfect black body. Because good absorbers are good emitters, matt black is also the best emitter.
matt black
allow dull black; do not accept black alone
Parts of some satellites are covered in shiny metallic foil. In space, one side of a satellite may face the Sun while the other side faces cold, empty space.
Suggest how the shiny foil helps to stop the satellite getting too hot or too cold.
The shiny foil reflects most of the radiation from the Sun because it is a poor absorber, so the satellite does not overheat on the sunny side. It is also a poor emitter, so the satellite does not radiate energy away too quickly into cold space and get too cold.
In space there is no air, so radiation is the only way a satellite gains or loses energy. Because poor absorbers are also poor emitters, shiny foil reduces both the energy gained from the Sun and the energy lost to space.
the foil reflects most of the Sun's radiation / is a poor absorber, so the satellite does not absorb too much energy and overheat
the foil is a poor emitter, so the satellite does not lose energy too quickly by radiation and get too cold
A perfect black body at room temperature would look completely black.
Explain why.
A perfect black body absorbs all the light that falls on it and does not reflect or transmit any. At room temperature the radiation it emits is infrared, which our eyes cannot detect. So no visible light reaches our eyes from it, and it looks completely black.
We see an object by the light it reflects or emits. A perfect black body reflects nothing, and at ordinary temperatures it emits only infrared, so it looks black. Heated to a high enough temperature, it would start to glow red and then white.
it absorbs all the (visible) light that falls on it
it does not reflect (or transmit) any light
at room temperature the radiation it emits is infrared, which our eyes cannot detect, so no visible light reaches our eyes from it
allow so no light from it enters our eyes
A thermal imaging camera is pointed at a person standing in front of a window on a winter evening. The person's skin is at 34 °C, their clothes are at 25 °C and the window is at 10 °C.
On the image, the skin appears brightest, the clothes appear less bright and the window appears darkest, but the window can still be seen.
Explain these observations.
The thermal imaging camera detects infrared radiation. All objects emit infrared whatever their temperature, and the hotter an object is, the more infrared it emits each second. The skin, at 34 °C, is the hottest, so it emits the most infrared and appears brightest. The clothes, at 25 °C, emit less and appear less bright. The window, at 10 °C, is the coldest, so it emits the least infrared and appears darkest. However, it still emits some infrared, so the camera can still detect it.
Thermal images are maps of how much infrared each surface emits. Because every object emits some infrared, even cold objects appear on the image; temperature differences show up as differences in brightness.
the camera detects infrared radiation
max 5
all objects emit infrared, whatever their temperature
the hotter an object, the more infrared it emits in a given time
the skin is hottest so it emits the most infrared and appears brightest; the clothes are cooler so they emit less
the window is coldest so emits the least infrared and appears darkest
but the window still emits some infrared, so it can still be detected / seen
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