The radiation energy emitted per unit time by a body of surface area $A$, emissivity $e$, and absolute temperature $T$ is given by Stefan-Boltzmann Law \begin{align} \mathrm{d}E/\mathrm{d}t=e \sigma A T^4,\nonumber \end{align} where $\sigma=5.67\times10^{-8}\,\mathrm{W/m^2 K^4}$ is Stefan's constant. The emissivity of a body is defined as the ratio of radiation emitted by the body to the radiation emitted by a blackbody at the same temperature. Thus, emissivity of the blackbody is 1 and that of other bodies is between 0 and 1. The Stefan-Boltzmann can be derived from Planck's law by integrating the blackbody spectrum from wavelengths zero to infinity.

A body with area $A$ and emissivity $e=0.6$ is kept inside a spherical black body. Total heat radiated by the body at temperature $T$ is,

- $0.6\sigma eAT^4$
- $0.8\sigma eAT^4$
- $1.0\sigma eAT^4$
- $0.4\sigma eAT^4$

**Solution:**
By Stefan-Boltzmann law, energy radiated per unit time by a body of surface area $A$, emissivity $e$, and absolute temperature $T$ is $e\sigma A T^4$.

Two bodies *A* and *B* have thermal emissivities of $0.01$ and $0.81$, respectively. The outer surface area of the two bodies are the same. The two bodies emit total radiant power at the same rate. The wavelength $\lambda_B$ corresponding to maximum spectral radiance in the radiation from *B* is shifted from the wavelength corresponding to maximum spectral radiance in the radiation from *A* by $1.00\;\mathrm{\mu m}$. If the temperature of *A* is 5802 K,

- the temperature of
*B*is 1934 K. - $\lambda_B=1.5\;\mathrm{\mu m}$.
- the temperature of
*B*is 11604 K. - the temperature of
*B*is 2901 K.

**Solution:**
Let $e_A=0.01$, $e_B=0.81$, and $T_A=5802 \mathrm{K}$. Stefan's law gives radiant power of two bodies as,
\begin{align}
{\mathrm{d}Q_A}/{\mathrm{d}t}=\sigma A e_A {T_A}^{\!4},\nonumber\\ {\mathrm{d}Q_B}/{\mathrm{d}t}=\sigma A e_B {T_B}^{\!4}.\nonumber
\end{align}
Equate ${\mathrm{d}Q_A}/{\mathrm{d}t}={\mathrm{d}Q_B}/{\mathrm{d}t}$ to get,
\begin{align}
T_B=\left({e_A}/{e_B}\right)^{\!1/4}T_A=1934\;\mathrm{K}.\nonumber
\end{align}
Wien's displacement law, $\lambda_m T=b$, gives,
\begin{align}
\lambda_B=(T_A/T_B)\lambda_A=3\lambda_A.
\end{align}
Also, since $\lambda_B > \lambda_A$ and $|\lambda_B-\lambda_A|=1\;\mathrm{\mu m}$, we get,
\begin{align}
\lambda_B-\lambda_A=1\;\mathrm{\mu m}.
\end{align}
Solve above equations to get $\lambda_A=0.5\;\mathrm{\mu m}$ and $\lambda_B=1.5\;\mathrm{\mu m}$. Thus, answer is A and B.

Two spherical bodies $A$ (radius 6 cm) and $B$ (radius 18 cm) are at temperatures $T_1$ and $T_2$, respectively. The maximum intensity in the emission spectrum of $A$ is at 500 nm and in that of $B$ is at 1500 nm. Considering them to be black bodies, what will be the ratio of the rate of total energy radiated by $A$ to that of $B$?

**Solution:**
Stefan-Boltzmann law gives the energy radiated per unit time by a spherical black body of area $A=4\pi r^2$ and temperature $T$ as,
\begin{align}
E=\sigma A T^4=4\pi\sigma r^2 T^4.
\end{align}
The Wien's displacement law relates temperature of the black body to the wavelength at maximum intensity by
\begin{align}
\lambda_m T=b.
\end{align}
Eliminate $T$ from above equations to get,
\begin{align}
E=4\pi\sigma b^4 (r^2/\lambda^4),\nonumber
\end{align}
which gives,
\begin{align}
E_1/E_2=(r_1/r_2)^2\, (\lambda_2/\lambda_1)^4=9.\nonumber
\end{align}

**Question 1:** Which of the following is the correct expression for Stefan's constant? (Here, h is Planck’s constant, c is the speed of light and k is Boltzmann’s constant)

**Question 2:** A blackbody radiates with a total intensity of $I = 5.68\, \mathrm{kW/m^2}$. At what wavelength does the spectral intesity $I(λ)$ peak?

**Question 3:** The figure shows intensity spectrum of a blackbody at two different temperatures $T_1=27$ deg C and $T_2=327$ deg C. If $A_1$ and $A_2$ are the areas under these curves then the ratio $A_1/A_2$ is