Thermal Radiation (DP IB Physics) : Revision Note

Katie M

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Katie M

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Thermal Radiation

  • All bodies (objects), no matter what temperature, emit a spectrum of thermal radiation in the form of electromagnetic waves

    • These electromagnetic waves usually lie in the infrared region of the spectrum

  • Thermal radiation is defined as:

    Heat transfer by means of electromagnetic radiation normally in the infrared region

  • The hotter the object, the more infrared radiation it radiates in a given time

    • This is because atoms and molecules above absolute zero are in constant motion

    • Electric charges within the atoms in a material vibrate causing electromagnetic radiation to be emitted

    • Therefore, the higher the temperature, the greater the thermal motion of the atoms and the greater the rate of emission of radiation

  • Thermal radiation is the only method of thermal energy transfer that does not require matter in order to move or propagate

    • Therefore, thermal radiation is the only way heat can travel through a vacuum

Thermal radiation demonstration on a Leslie cube

Leslie Cube Demonstrating Thermal Radiation, for IB Physics Revision Notes

An image of a hot object, known as a Leslie cube, taken in both Infrared and visible light. The black surface emits more thermal radiation (infrared) than the shiny surface

  • The amount of thermal radiation emitted by an object depends on a number of factors:

    • The surface colour of the object (black = more radiation)

    • The texture of the surface (shiny surfaces = less radiation)

    • The surface area of the object (greater surface area = more area for radiation to be emitted from)

  • Dark, dull objects are better at emitting and absorbing radiation

  • Lightshiny objects are worse at emitting and absorbing radiation

Worked Example

A hot meteorite hits the surface of the Moon.

Identify and discuss the principal means by which the meteorite can dissipate thermal energy once it has landed.

Answer:

Step 1: Identify the types of thermal energy transfer

  • An object can lose energy through conduction, convection or radiation

  • In this case, the hot meteorite will only be able to lose energy via conduction and radiation

Step 2: Explain these choices

  • The meteorite can lose heat energy through conduction because it is in contact with the surface of the Moon

  • The Moon does not have an atmosphere, so convection is not possible

  • Infrared photons emitted by the meteorite are able to travel through a vacuum, so heat loss via radiation is possible

Examiner Tips and Tricks

If a question refers to the colour of something (black, white or shiny) then the answer will likely be about thermal radiation!

If a question involves a vacuum (empty space) remember that radiation is the only mechanism in which heat can be transferred, as conduction and convection require particles to transfer heat.

Black-Body Radiation

  • Black-body radiation is the name given to the thermal radiation emitted by all bodies

    • Black-body radiation can be emitted in the form of infrared light, but also visible light or other wavelengths, depending on the temperature

  • A perfect black body is defined as:

    An object that absorbs all of the radiation incident on it and does not reflect or transmit any

  • Since a good absorber is also a good emitter, a perfect black body would be the best possible emitter too

  • As a result, an object which completely absorbs all radiation will be black

    • This is because the colour black is what is seen when all colours from the visible light spectrum are absorbed

  • The intensity and wavelength distribution of any emitted waves depends on the temperature of the body

  • This can be represented on a black-body radiation curve of intensity against wavelength

    • As the temperature increases, the peak of the curve moves

    • This moves to a lower wavelength and a higher intensity

Black-body radiation curves

black-body-curve-ib-physics-revision-notes

Black body spectrum for objects of different temperatures

  • From the electromagnetic spectrum, waves with a smaller wavelength have higher energy (e.g. UV rays, X-rays)

    • The hotter the object, the greater the amount of infrared radiation it radiates in a given time

  • A higher temperature increases the thermal energy emitted and therefore the wavelength of the radiation emitted at the greatest intensity, λpeak, decreases

    • At room temperature, objects emit thermal radiation in the infrared region of the spectrum (λpeak is in the infrared region)

    • At around 1000°C, an object will emit a significant amount of red visible light (λpeak is in the red region of the visible spectrum)

    • At around 6000°C, an object will mainly emit white or blue visible light (λpeak is in the centre or violet region of the visible spectrum)

    • At even higher temperatures, objects will emit ultraviolet or even X-rays

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Katie M

Author: Katie M

Expertise: Physics Content Creator

Katie has always been passionate about the sciences, and completed a degree in Astrophysics at Sheffield University. She decided that she wanted to inspire other young people, so moved to Bristol to complete a PGCE in Secondary Science. She particularly loves creating fun and absorbing materials to help students achieve their exam potential.

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