1. Unit 3.4
Thermal Radiation

2. Thermal Radiation
All macroscopic objects emit radiation at all times, regardless of their size, shape, or chemical composition.

3. Thermal Radiation
They radiate mainly because the microscopic charged particles they are made up of are in constantly varying random motion, and whenever charges interact and change their state of motion, electromagnetic radiation is emitted.

4. Thermal Radiation
The temperature of an object is a direct measure of the amount of microscopic motion within it.

5. Thermal Radiation
The hotter the object, that is the higher its temperature, the faster its component particles move, the more violent are their collisions, and the more energy they radiate.

6. Blackbody Spectrum
Intensity is a term often used to specify the amount or strength of radiation at any point in space. Like frequency and wavelength, intensity is a basic property of radiation.

7. Blackbody Spectrum
No natural object emits all its radiation at just one frequency. Instead because particles collide at many different speeds, the energy is generally spread out over a range of frequencies.

8. Blackbody Spectrum
By studying how the intensity of this radiation is distributed across the electromagnetic spectrum, we can learn much about the object’s properties.

9. Blackbody Spectrum
The above figure sketches the distribution of radiation emitted by an object.

10. Blackbody Spectrum
The distribution of radiation curve peaks at a single, well-defined frequency and falls off to lesser values below that frequency.

11. Blackbody Spectrum
Note that the curve is not shaped like a symmetrical bell that declines evenly on either side but it peaks, then falls rapidly.

12. Blackbody Spectrum
This overall shape is characteristic of the thermal radiation emitted by any object, regardless of its size, shape, composition, or temperature.

13. Blackbody Spectrum
This curve is the radiation-distribution curve for a mathematical idealization known as a blackbody – an object that absorbs all radiation falling on it.

14. Blackbody Spectrum
In a steady state, a blackbody must reemit the same amount of energy it absorbs. The blackbody curve shown describes the distribution of that reemitted radiation.

15. Blackbody Spectrum
However, in many cases, the blackbody curve is a good approximation to reality, and the properties of blackbody provide important insights into the behavior of real objects.

16. Radiation Laws
The blackbody curve points towards higher frequencies and greater intensities as an object’s temperature increases. Even so, the shape of the curve remains the same.

17. Radiation Laws
This shifting of radiation’s peak frequency with temperature is familiar to us all: Very hot glowing objects emit visible light and cooler objects produce invisible light.

18. Radiation Laws
From studies of the precise form of the blackbody curve, we obtain a very simple connection between the wavelength at which most radiation is emitted and absolute temperature of the emitting object

19. Radiation Laws
Wavelengths of peak emission α 1 temperature Wein’s law

20. Radiation Laws
Wien’s law tells us that the hotter the object, the bluer is its radiation. The cooler the object, the redder its radiation.

22. Blackbody Curves Comparison of blackbody curves for four cosmic objects. (a) A cool, invisible galactic gas cloud called Rho Ophiuchi. At a temperature of 60 K, it emits mostly low-frequency radio radiation. (b) A dim, young star (shown red in the inset photograph) near the center of the Orion Nebula. The star’s atmosphere, at 600 K, radiates primarily in the infrared. (c) The Sun’s surface, at approximately 6000 K, is brightest in the visible region of the electromagnetic spectrum. (d) Some very bright stars in a cluster called Omega Centauri, as observed by a telescope aboard the space shuttle. At a temperature of 60,000 K, these stars radiate strongly in the ultraviolet.

23. Radiation Laws
It is also a matter of everyday experience that, as the temperature of an object increases, the total amount of energy it radiates increases rapidly.

24. Radiation Laws
Careful experimentation leads to the conclusion that the total amount of energy radiated per unit time is actually proportional to the fourth power of the object’s temperature – this is known as Stefan’s Law.

25. Radiation Laws
From the form of Stefan’s law, we can see that energy emitted by a body rises dramatically as it temperature increases.

26. Astronomical Applications
No known natural terrestrial objects reach temperatures high enough to emit very high frequency radiation.

27. Astronomical Applications
Many extraterrestial objects do emit copious quantities of ultraviolet, X-ray, and even gamma ray radiation.

28. Astronomical Applications
Astronomers often use blackbody curves as thermometers to determine the temperatures of distant objects.

29. Astronomical Applications
For example, an examination of the solar spectrum indicates the temperature of the Sun’s surface.

30. Astronomical Applications
Observations of the radiation from the Sun at many frequencies yield a curve shaped somewhat like that shown in the following slide.

32. Astronomical Applications
The Sun’s curve peaks in the visible part of the electromagnetic spectrum; the Sun also emits a lot of infrared and a little ultraviolet radiation.

33. Astronomical Applications
Using Wein’s law, we find that the temperature of the Sun’s surface is about 6000K (more exact is 5770 K).

34. Astronomical Applications
Other cosmic objects have surfaces very much cooler or hotter than the Sun’s emitting most of their radiation in invisible parts of the spectrum.