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**Phy100: Blackbody radiation**

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

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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?

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**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

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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.

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**Full range of forms of radiation**

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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.

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**Total area below a curve= Total power per unit area**

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

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**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

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**Blackbody radiation at T=310K**

Estimate the radiation power of a human body

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