Kirchoff's Law

Q.

Which one of the following statements is not true about thermal radiations?

1. All bodies emit thermal radiations at all temperatures

2. Thermal radiations are electromagnetic waves

3. Thermal radiations travel in free space with a velocity of

4. Thermal radiations are not reflected from a mirror

Q. In which of the following modes is the transfer of heat the fastest?

1. All of the above
2. Radiation
3. Convection
4. Conduction

Q. Power radiated by a black body at temperature $T_1$ is $P$ and it radiates maximum energy at a wavelength ${\lambda }_1$. If the temperature of the black body is changed from $T_1$ to $T_2$, it radiates maximum energy at a wavelength $\frac{{\lambda }_1}{2}$. The power radiated at $T_2$ is:

1. $2P$
2. $4P$
3. $8P$
4. $16P$

Q.

The graph shown in the adjacent diagram, represents the variation of temperature (T) of two bodies, x and y having same surface area, with time (t) due to the emission of radiation. Find the correct relation between the emissivity ($e$) and absorptivity ($a$) power of the two bodies. (IIT JEE 2003)

1. e_x>e_y, a_y> a_x

2. e_x>e_y, a_x> a_y

3. e_y>e_x, a_y> a_x

4. e_y>e_x, a_x> a_y