State Stefan's law and Wien's displacement law. Draw graphs showing the distribution of energy in the spectrum of a black body. Explain what quantity is plotted against the wavelength . By considering how this energy distribution varies with tempaerature expalin the colour changes which occur when a piece of iron is heated from cold to near the melting point.

Name the law which helps us to explain the distribution of energy in the spectrum of a black body for (i) short wavelength only (ii) long wavelength only.

State Stefan's law. Sketch graphs showing the distribution of energy in the spectrum of black body radiation at three temperatures, and indicate which curve corresponds to the highest temperature.

(a) Describe the principle of (i) a thermopile and (ii) a bolometer and show how these devices are used. (b) Define the term black body. What is meant by black body radiation ? As the temeprature of a black body rises, what changes take place in (i) the total energy radiated from it and (ii) the energy distribution amongst the wavelengths radiated ?

The radiations emitted by the sun are analyzed and the spectral energy distribution curve is plotted. as shown. The radius of the sun is R , the earth is situated a distance 'd' from the sun. Treat the sun as perfectly black body which radiates power at constant rate fill till its store of hydrogen gets exhausted. (Stefan's constant = sigma , Wien's constant =b , speed of light =c ) Assume the sun produces energy only by nuclear fusion of hydrogen nuclei and the fraction of nuclear mass of hydrogen that can be converted to energy is eta . How long will the sun keep on emittied the energy if the mass of hydrogen at present in Sun's core is m?