1. What is black body?

If a body absorbs all incident radiation, regardless of wavelength and direction, then it is said to be black body.

For a prescribed temperature and wavelength, no surface can emit more energy than black body.

                                      

2. State Planck’s Distribution law.

It states that the relationship between the monochromatic emissive power of a black body and wavelength of a radiation at a particular temperature.

 

3. Wien’s Displacement law.

It states that the relationship between temperature and wavelength corresponding to the maximum spectral emissive power of the black body at that temperature.

 

4. State Stefan-Boltzmann law.

 

It states that the emissive power of a black body is proportional to the fourth power of absolute temperature.

                             Eb ∞ T4

                             Eb = σ T4

            Where

            Eb - Emissive power in W/m2

            σ – Stefan Boltzmann-constant=5.67*10-8 W/m2K4

T – Temperature in K

 

5. Define emissivity.

It is defined as the ability of the surface of a body to radiate heat.

It is also defined as the ratio of emissive power of a body to the emissive power of a black body at equal temperature.

 

6. What is meant by gray body?

If a body absorbs a definite percentage of incident radiation irrespective of their wavelength, the body is known as gray body.

The emissive power of a gray body is always less than that of black body.

 

7. State Kirchhoff’s law of radiation.

It states that the ratio of total emissive power to the absorptivity is constant for all surfaces which are in thermal equilibrium with the surroundings.

            E1/α1 = E2/α2 = E3/α3

 

8. Define intensity of radiation.

It is defined as the rate of energy leaving a space in a given direction per unit solid angle per unit area of the emitting surface normal to the mean direction in space.

                   In = Eb / π

 

9. State Lambert’s cosine law.

It states that the total emissive power from a radiating plane surface in any direction proportional to the cosine of the angle of emission.

                        Eb ∞ cos θ

 

10. What is the purpose of radiation shields?

Radiation shields are constructed from low emissivity (high reflectivity) materials.

It is used to reduce the net radiation transfer between two surfaces.