Energy - Infrared Radiation

This GCSE Physics quiz asks questions about infrared radiation. Infrared (sometimes written infra-red or infra red) radiation was discovered by the famous astronomer, William Herschel. As happens quite often in scientific discoveries, it came from noticing something unexpected, asking questions and then carrying out more tests in order to try to find answers. In this case, he had been observing the Sun.

Looking at the Sun directly is dangerous to the eye, especially through a telescope. He had been using different dark glass filters to protect his eyes during observations and noticed that some let through different amounts of light and others let through different amounts of heat. So he took this further and measured the temperature increase in different parts of a visible spectrum using thermometers.

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He found the greatest increase in the red part of the spectrum but as the Sun moved across the sky, he noticed that the temperature became even higher when the thermometer was just beyond the end of the red part of his spectrum. Further experimentation showed the heating decreased further away from the spectrum and that there was no heating effect beyond the violet.

We now know infrared is part of the electromagnetic spectrum and that all objects emit (give out) and absorb this thermal radiation. The hotter an object is, the more infrared it emits. But temperature is not the only factor, the type of surface affects how much heat radiation is emitted or absorbed. Dark matt surfaces are better at doing both, whilst light-coloured highly-polished surfaces absorb and emit the least. Houses in hot countries are often painted white, to reflect the Sun's infrared radiation which helps to keep them cooler. Solar panels for heating water have matt black surfaces in order to absorb as much infrared radiation as possible. They are so efficient that they still work on cloudy days - but obviously not as well as on sunny days.

Infrared radiation is one of three ways in which heat can travel. Since infrared is an electromagnetic wave, it does not require a material medium through which to travel (conduction needs a solid medium and convection requires a liquid or a gaseous medium) and can travel through solids, liquids, gases and a vacuum. Since space is a vacuum, it is the only way that heat from the Sun can reach the Earth. Provided the medium is transparent to infrared radiation wavelengths, it can pass through.

Infrared is incredibly useful - it is used in night vision goggles, police helicopter cameras (to find suspects hiding in gardens, woods and at night), security lights have sensors that switch them on by detecting the heat from people who are near to them and of course it is used for TV remote controls. For astronomers, it is great as it can pass through interstellar dust and gas clouds, allowing them to see through to objects hidden on the other side.

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  1. Which of the following emits the most infrared radiation?
    It is the hottest on the list therefore gives off the most infrared
  2. How fast does infrared radiation travel at?
    Infrared radiation is a form of electromagnetic radiation and so travels at the same speed as light
  3. When was infrared radiation discovered?
    William Herschel also discovered the planet Uranus
  4. How can we sense infrared radiation, even though we cannot see it?
    Our skin contains sensors that can detect heat radiation
  5. Why is infrared radiation good for investigating areas of space where large gas clouds have formed?
    Scientists use telescopes with special detectors to see the infrared radiation from space
  6. What kinds of bodies emit infrared radiation?
    Absolute zero is a little more than 273 degrees below zero Celsius. It is the starting point for the Kelvin temperature scale
  7. Why do dark objects get hot in direct sunlight even though a lot of the sun's infrared radiation does not get through the atmosphere?
    Water vapour is a strong absorber of infrared radiation which is why the temperature suddenly drops when the Sun is hidden by a cloud and also why infrared detecting telescopes are at high altitudes or in space
  8. What is the wavelength band of infrared waves?
    Just slightly longer than the red end of the visible spectrum
  9. Does infrared radiation have a longer or shorter wavelength than visible light?
    Infrared waves have a wavelength band ranging from 750nm - 1mm, whereas visible light has a wavelength band ranging from 350nm to 750nm. The wavelengths of light that our eyes can see is only very small in comparison to the size of the electromagnetic spectrum
  10. What is infrared radiation?
    Infrared radiation is given off by all objects that have a temperature above absolute zero. If an object is hotter than your body, you can detect infrared radiation using your skin ~ your hands and cheeks are particularly sensitive

Author: Martin Moore

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