Q.Why are the bottoms of cooking utensils blackened and the tops polished?

Every body at a temperature above absolute zero continuously emits thermal radiation, and simultaneously absorbs radiation arriving from its surroundings -- a principle known as Prevost's theory of exchange. A body's temperature rises, falls, or stays constant depending on whether its rate of absorption exceeds, falls short of, or exactly matches its rate of emission; the constant-temperature case is thermal equilibrium with the surroundings. As a body's temperature rises, the wavelength of its dominant emitted radiation shifts from the invisible infrared, through visible red, to white-hot, at progressively higher temperatures.

The rate of heat radiated per unit area per unit time by a surface is called its emissive power, R = Q/(At). Since a perfect blackbody has the maximum possible emissive power at any given temperature, the emissive power of any real surface is naturally compared against it through the coefficient of emission, or emissivity, e = R/RB. A perfect blackbody has e = 1, a perfect reflector has e = 0, and ordinary surfaces fall in between (e.g. e = 0.3 for copper), with rougher surfaces generally having higher emissivity than smooth, polished ones, and emissivity also varying somewhat with temperature and wavelength.