Engineering Physics

4.3 Planck’s quantum theory of black body radiation

A body that absorbs all wavelengths of radiation at low temperatures and emits all wavelengths of radiation at high temperatures is known as black body. Figure (4.2) shows the graphs plotted between the intensities of emitted light and wavelengths at different temperatures. The area under the plot indicates total radiation (R), the power emitted per unit area. According to Stefan–Boltzmann law, the radiation is proportional to T⁴.

R = σT⁴ _____ (4.19)

 

where σ is the Stefan–Boltzmann constant.

Expression for the radiated energy density per unit wavelength range (R_λ) was derived by Wien based on thermodynamics is:

Figure 4.2 Plots of black body radiation

Figure 4.3 Comparison of the three ...