It refers to the relative intensities of different wavelengths i.e. the proportion of red to blue in the radiation.
Relative intensity is defined as the energy radiated per second per unit surface area of the body in a unit wavelength interval.
The quality of radiation depends on the temperature of the body. Suppose an iron bar is thrust into a fire. At first, it will look black though it has become hot. This is because the radiation given out by the bar is entirely in the infra-red region. As the temperature increases, iron bar becomes red hot. Now the bar gives out more radiation than before. A larger portion of it is in the visible region than before. As the temperature rises, the bar .appears orange, then yellow and finally white. Thus, the nature of the radiation given out by the bar and the relative amounts of the radiations of different wave lengths depend on the temperature of the radiator.
Qn. Explain why the deepest part (centre) of a fire looks brightest (white)
Soln.
At the centre of a fire, temperature is highest. Since the quality of radiation (relative intensity of different wavelengths) depends on temperature, the relative intensity is therefore maximum at the deepest part of the fire. This maximum relative intensity corresponds to all the different wavelengths being emitted and since these wavelengths are in the visible region, a combination of all these colors results into white.
