1. And the Planck Distribution
HEAT RADIATION
And the Planck Distribution

2. Radiating Heat Standard bodies are black
Measure the radiation given off by a black body at every frequency of the spectrum at a fixed temperature

3. Changing the temperature
Think of a black cooker hot plate.
It does not remain black but glows when heated
Getting Hotter

5. 12000 K
6000 K
Intensity W/m2
3000 K
Wavelength (m)

6. Intensity W/m2
λmax
Wavelength (m)

7. Intensity W/m2
Wavelength (m)

8. The Properties Of The Curves
the curve is flatter for lower temperature
As the temperature increases the wavelength of maximum intensity (λmax) for that temperature increases in promenance
As the temperature increases λmax moves to the left towards higher frequency.
At higher temperatures there is a sharp falling off of radiation at values greater than λmax towards a limiting value in the ultraviolet range which is of very short wavelength but not zero. This is referred to as the ultra violet catastrophe.

9. Wavelength (m) λmaxT = 2.898 x 10-3
In 1893 Wihelm Wein dicovered a simple relationship between λmax and the absolute temperature of a body
λmaxT = x 10-3
Intensity W/m2
Ultraviolet
Catastrophe
Wavelength (m)
As the temperature of the star rises the λmax moves more towards the violet

10. Using Wein’s Law λmaxT = 2.898 x 10-3
The spectrum of star  Andromedae is found to be at maximum intensity at a wavelength of 380 nanometres.
What is the surface temperature of the star.
What colour would you expect this star to be?

11. Wavelength (m) λmaxT = 2.898 x 10-3 Intensity W/m2 T=7626K
λmax = 380 x10-9m
Wavelength (m)
As the star’s maximum wavelength is beyond the violet end of the spectrum most of the visible light produced will be blue. It is a blue star.

12. wavelength of maximum intensity(nm)
Work out the surface temperature of each of the star’s here using Wien's law
Suggest the observed colour of each star
star
wavelength of maximum intensity(nm)
α draconis
430
ε Ursa Minoris
520
 cephei
740