Phy100: Blackbody radiation

1. Phy100: Blackbody radiation Goals: To understand radiation spectrum (power versus wavelength); To understand the radiation power (power versus temperature).

2. Radiation Heat can also be transferred by radiating light (i.e. photons) or electromagnetic waves with different wavelengths. For an object with temperature T or a blackbody, Q1: What kind electromagnetic waves are emitted (visible or invisible) ? Q2: How much energy emitted per second per unit area, i.e. radiation power?

3. Basics of waves The distance between two adjacent crests = Wavelength. The number of crests one observes at a given point per second =Frequency Wavelength X Frequency = Speed of waves

4. About radiation: • Radiation as a self-propagating electric and magnetic (EM) wave. • 2) EM waves travel at the speed of light c=3 X 10^8 m/s or 300,000km/s (Boeing Jet cruise speed about 0.3km/s) • Both electric fields and magnetic fields oscillate as a function of time and spatial coordinates. These waves consist crests and troughs.

5. Full range of forms of radiation

6. Q1 Radio frequency electric signals are electromagnetic waves within the frequency range of 3Hz and 3 GHz. The corresponding range of wavelengths (=c/ f ) are • 10cm to 10^8 m; • 1mm to 1m; • 10^{-6}m to 1mm; • 10^{-8}m to 10^{-7}m.

8. Power density: Power per unit area per unit wavelength Total area below a curve= Total power per unit area

9. Conclusions • Radiation power concentrated in an interval of • wavelength. The peak position in the power spectrum • moves to longer wavelengths when T decreases; • The integrated or total radiation power (the area below the power spectrum) decreases as T decreases. Wien’s displacement law Stefan-Boltzmann Law

10. Blackbody radiation at T=310K Estimate the radiation power of a human body