(a) We know that a+r+t=1
Where a, r and t are absorbance, reflectance and transmittance respectively of the surface of the body, t is also called emittance (e). Also according to Krichhoff’s law e∝a that is good absorber are good emitters and hence poor reflectors and vice-versa i.e. If r is large (i.e. large reflectively) a is smaller and hence e is smaller i.e. poor emitter.
(b) The thermal conductivity of brass is high i.e. brass is a good conductor of heat. So when a brass tumbler is touched, heat quickly flows from human body to the tumbler. Consequently, the tumbler appears colder. On the other hand, wood is a bad conductor of heat. So heat does not flow from the human body to the wooden tray, thus it appears relatively hotter.
(c) Let T be the temperature of the hot iron in the furnace. Thus according to Stefan’s law, heat radiated per second per unit area (E) is given by E=σT4. When the body is placed in open at temperature T0, then the heat radiated/sec/area (E’) is given by
E′=σ(T4−T04)
Clearly E’<E, so the optical pyrometer gives too low a value for the temperature of a red hot iron piece in open.
(d) Gases are generally insulators. The Earth’s atmosphere acts like an insulating blanket around it and does not allow heat to escape out but reflects it back to the Earth. If this atmosphere is absent, then the Earth would naturally be colder as all its heat would have escaped out.
(e) This is because steam has much higher heat capacity (540 cal g‌–1) than the heat capacity of water (80 cal g‌–1) at the same temperature. Thus heating systems based on circulation of steam are more efficient than those based on circulation of hot water.