Black Body Radiation:

You have learnt earlier that all heated objects emit electromagnetic radiation and the wavelengths of these radiations depend on the temperature of the object.  For example, at relatively low temperatures, a metal rod emits infrared radiation that we cannot see but feel as heat.  As the temperature is raised to about 600°C a dull red color is visible and at about 2000 °C it glows white. One can visit a blacksmith and observe the changes in the glow of iron as it is heated to higher and higher temperatures.

However, the energy radiated by a hot surface depends on its nature and surface area, besides the temperature.  For a given area and temperature, a black surface radiates more energy per second than a polished surface. At a given temperature the maximum radiation would be given out by a perfectly black surface. The experimentaly observed data on radiant intensity emitted at different wavelengths by a perfect black surface (called black body) at different temperatures could not be explained by classical mechanical principles. This needs a relook at the nature of the radiation.