Physics MCQs for NEET — Practice Questions with Answers

(b) A person with dark skin absorbs more heat radiation and feels more heat. It also radiates more heat and feels more cold.

(a) For a black body emissivity = absorptive power.

(b) Highly polished mirror like surfaces are good reflectors, but not good radiators.

(b) Black cloth is a good absorber of heat, therefore ice covered by black cloth melts more as compared to that covered by white cloth.

(c) According to Kirchoff’s law, the ratio of emissive power to absorptive power is same for all bodies is equal to the emissive power of a perfectly black body i.e.,

${\left(\frac{\mathrm{e}}{\mathrm{a}}\right)}_{\mathrm{body}}={\mathrm{E}}_{\mathrm{black} \mathrm{body}}$  for a particular wave length

${\left(\frac{{\mathrm{e}}_{\mathrm{\lambda }}}{{\mathrm{a}}_{\mathrm{\lambda }}}\right)}_{\mathrm{body}}={\left({\mathrm{E}}_{\mathrm{\lambda }}\right)}_{\mathrm{Black} \mathrm{body}}\Rightarrow {\mathrm{e}}_{\mathrm{\lambda }}={\mathrm{a}}_{\mathrm{\lambda }}{\mathrm{E}}_{\mathrm{\lambda }}$

(b) $\mathrm{Absorption} \mathrm{power}=\frac{\mathrm{Heat} \mathrm{absorbed}}{\mathrm{Total} \mathrm{heat} \mathrm{given}}$

(c) 

(a) 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.

(c) When light incident on pin hole, enters into the box and suffers successive reflection at the inner wall. At each reflection some energy is absorbed. Hence the ray once it enters the box can never come out and pin hole acts like a perfect black body.

(c) When blue glass is heated at high temperature, it absorbs all the radiation of higher wavelength except blue. If it 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.