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5.16: Thermal Radiation

Difficulty Level: At Grade Created by: CK-12
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These three friends are warming their hands over a bonfire. They don’t have to touch the fire to feel its warmth. How is warmth from the fire transferred to their hands? In this article, you’ll find out.

Introducing Thermal Radiation

The bonfire from the opening image has a lot of thermal energy. Thermal energy is the total kinetic energy of moving particles of matter, and the transfer of thermal energy is called heat. Thermal energy from the bonfire is transferred to the friends’ hands by thermal radiation. Thermal radiation is the transfer of thermal energy by waves that can travel through air or even through empty space, as shown below. When the waves of thermal energy reach objects, they transfer the energy to the objects, causing them to warm up. This is how the fire warms the hands of the friends sitting near the bonfire. This is also how the sun’s energy reaches Earth and heats its surface. Without the energy radiated from the sun, Earth would be too cold to support life as we know it.

Thermal radiation is one of three ways that thermal energy can be transferred. The other two ways are conduction and convection, both of which need matter to transfer energy. Radiation is the only way of transferring thermal energy that doesn’t require matter. To learn more about thermal radiation, watch “Radiation” at the URL below.


Sources of Thermal Radiation

You might be surprised to learn that everything radiates thermal energy, not just really hot things such as the sun or a fire. For example, when it’s cold outside, a heated home radiates some of its thermal energy into the outdoor environment. A home that is poorly insulated radiates more energy than a home that is well insulated. Special cameras can be used to detect radiated heat. In the Figure below, you can see an image created by one of these cameras. The areas that are red are the areas where the greatest amount of thermal energy is radiating from the home. Even people radiate thermal energy. In fact, when a room is full of people, it may feel noticeably warmer because of all the thermal energy the people radiate!

Q: Where is thermal radiation radiating from the home in the picture?

A: The greatest amount of thermal energy is radiating from the window on the upper left. A lot of thermal energy is also radiating from the walls of the home.


  • Thermal radiation is the transfer of thermal energy by waves that can travel through air or even through empty space. This is how thermal energy from a fire is transferred to your hands and how thermal energy from the sun is transferred to Earth.
  • Everything radiates thermal energy, even objects that aren’t very warm.


  • thermal radiation: Transfer of thermal energy by waves that can travel through air or across space.


Watch the video “Thermal Radiation” at the following URL. Then answer the questions below.


  1. What does a night vision camera detect?
  2. The kind of light an object gives off depends on its __________.
  3. Thermal radiation occurs when an object gives off __________ light.
  4. Explain why some objects appear brighter than others when viewed through a night vision camera.
  5. In the video, the scientist places her hands inside a black plastic trash bag. You can’t see her hands with your eyes but you can see them with a night vision camera. Explain why.


  1. What is thermal radiation?
  2. If you sit close to a campfire, the fire warms you. Describe how thermal energy is transferred from the fire to you.

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    thermal radiation

    Transfer of thermal energy by waves that can travel through air or across space.

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    Nov 01, 2012
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