Explanation
The velocity of heat radiation in vacuum is equal to that of light.
Radiation is the fastest mode of heat transfer.
A thermopile is a sensitive instrument, used for detection of heat radiation and measurement of their intensity.
The polished surface reflects all the radiation.
Infrared radiations are detected by pyrometer.
Good absorbers are always good emitters of heat.
According to Kirchhoff’s law in spectroscopy if a substance emits certain wavelengths at high temperature, it absorbs the same wavelength at comparatively lower temperature.
A person with dark skin absorbs more heat radiation and feels more heat. It also radiates more heat and feels more cold.
Highly polished mirror like surfaces are good reflectors, but not good radiators.
Because Planck’s law explains the distribution of energy correctly at low temperature as well as at high temperature.
When blue glass is heated at high temperature, it absorbs all the radiations of higher wavelength except blue. lilt is taken inside a dark room, it emits all the radiation of higher wavelength, hence it looks brighter red as compared to the red piece.
The black spot on heating absorbs radiations and so emits them in the dark room while the polished shining part reflects radiation and absorbs nothing and so does not emit radiations and becomes invisible in the dark.
As for a black body rate of absorption of heat is more. Hence, thermometer A shows faster rise in temperature but finally both will acquire the atmospheric temperature.
According to Kirchhoff’s law, a good emitter is also a good absorber.
Black and rough surfaces arc good absorber that’s why they emit well [Kirchhoff’s law).
Initially black body absorbs all the radiant energy incident on it. So it is the darkest one. Black body radiates maximum energy if all other condition are same. So, when the temperature of the black body becomes equal to the temperature of furnace it will be brightest of all.
Open window behaves like a perfectly black body.
At low temperature short wavelength radiation is emitted. As the temperature rises colour of emitted radiations arc in the following order.
Red → Yellow → Blue → White
(at highest temperature)
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