5.3.1Light and the Electromagnetic Spectrum
Thermal Radiation
5 min read 5 parts Lesson 9 of 11 Unit quiz
Section 1 of 5All Objects Emit Radiation
Emission
- Every object, however cold, emits electromagnetic waves because of the energy of its particles.
Infrared
- At everyday temperatures most thermal radiation is in the infrared region of the electromagnetic spectrum.
No medium needed
- Thermal radiation can travel through a vacuum, which is how energy reaches us from the Sun.
Detection
- Infrared cameras detect the radiation emitted by warm objects and display it as a thermal image.
Hotter means more
- The hotter an object is, the more radiation it emits every second.
Section 2 of 5Radiation and Temperature
1
At room temperature an object emits mainly infrared radiation, which we cannot see.
2
Heated to around 1000 °C an object glows dull red because it now emits visible light as well.
3
At higher temperatures, such as the surface of the Sun, the emission is strongest in the visible range and the object looks white.
4
Very hot objects also emit ultraviolet radiation, and the hottest can emit X-rays.
5
So as temperature rises, the amount of radiation increases and the wavelength of the strongest emission becomes shorter.
Section 3 of 5The Radiation Curve
The Radiation Curve
Intensity
- The graph shows how much power is emitted at each wavelength, and a taller curve means more radiation.
Peak position
- The peak is the wavelength at which most energy is emitted by the object.
Higher temperature
- The curve rises higher and the peak moves towards shorter wavelengths, which is to the left on the graph.
Lower temperature
- The curve is lower and the peak lies at longer wavelengths, deeper into the infrared.
| Temperature | Main radiation emitted |
|---|---|
| Room temperature | Infrared |
| About 1000 °C | Infrared and red light |
| Several thousand °C | Visible light, mainly white, and ultraviolet |
Section 4 of 5Absorption, Emission and Equilibrium (Higher Tier)
Absorbing
- An object that absorbs radiation faster than it emits it gains energy and its temperature rises.
Emitting
- An object that emits radiation faster than it absorbs it loses energy and its temperature falls.
Equilibrium
- When absorption and emission happen at the same rate the temperature stays constant.
Surfaces
- Dark matt surfaces are good absorbers and good emitters, while light shiny surfaces are poor at both.
Section 5 of 5Exam Checklist & Tips
5.3.1 – Key Points for the Exam
- Thermal radiation Electromagnetic waves, mainly infrared, emitted by all objects because of their temperature.
- Infrared Electromagnetic radiation with wavelengths longer than visible red light.
- Emitter A surface or object that gives out radiation.
- Absorber A surface or object that takes in radiation and transfers its energy to internal energy.
- Thermal equilibrium A state where an object absorbs and emits radiation at the same rate, so its temperature is steady.
Common Mistakes
- Believing that only hot objects emit radiation; every object emits some.
- Saying that cold objects emit cold rays; they simply emit less radiation than warmer objects.
- Thinking that thermal radiation needs particles to travel through; it can cross a vacuum.
- Describing the shift of the curve as towards longer wavelengths when an object gets hotter;
- it moves to shorter wavelengths.