wavelength

1
Wiens Displacement Law for black body radiation
Intensity of emitted e/m radiation
peak wavelength
a 1 / Temp
(wavelength of maximum intensity)
6000K
2000K
1500K
1000K
wavelength
VIBGYOR
2
Weins law

• max . T constant (Weins constant)
• Where Weins constant 0.0029 m K
• (metres Kelvin)
• (not milli Kelvins !)
• Use it to estimate the surface temp of a star if
  we approximate the star to being a black body
• E.g. peak wavelength of the sun is 490 nm ?
  effective surface temp of

5920K
3
Stefans law
E ? T4 Total energy radiated by a black body
- in unit time - per unit surface area - is
proportional to the fourth power of the
absolute temperature of the body Where E
is the energy radiated per second per square
metre of surface T is the Temp of the black
body ? is Stephans constant 5.67 x 10-8 W m-2
K-4
4
Estimate the intensity of the radiation emitted
per unit area from a star if its effective
surface Temp is 6000K Estimate the energy emitted
from a star if its peak wavelength is 600nm.
Total power radiated Power output E x
surface area of starSurface area of a star 4
? R 2 Where R is the radius of the
star Conclusion a combination of Weins Law and
Stephans Law can lead to an estimation of the
temp of a star and its power output. This could
allow us to estimate its absolute magnitude and
using a rearrangement of m M 5 log (d/10)
could lead to an estimation of its distance away
from us in parsecs. Of course its power output
depends not only on the temp of the star but also
its size ? radius and therefore surface area
more techniques are required