Heat travels through vacuum by 
(1) Conduction                 

(2) Convection

(3) Radiation                     

(4) Both (a) and (b)

Taking into account the radiation that a human body emits which of the following statements is true?

The earth radiates in the infra-red region of the spectrum. The spectrum is correctly given by

(1) Wien’s law                               

(2) Rayleigh  law

(3) Planck’s law of radiation             

(4) Stefan’s law of radiation

Pick out the statement which is not true .

(1) IR radiations are used for long distance photography

(2) IR radiations arise due to inner electron transitions in atoms

(3) IR radiations are detected by using a bolometer

(4) Sun is the natural source of IR radiation

A hot and a cold body are kept in vacuum separated from each other. Which of the following cause decrease in temperature of the hot body ?

(1) Radiation

(2) Convection

(3) Conduction

(4) Temperature remains unchanged

For a perfectly black body, its absorptive power is

(1) 1                 

(2) 0.5

(3) 0                 

(4) Infinity

Certain substance emits only the wavelengths ${\mathrm{\lambda }}_1, {\mathrm{\lambda }}_2, {\mathrm{\lambda }}_3$ and ${\mathrm{\lambda }}_4$ when it is at a high temperature. When this substance is at a colder temperature, it will absorb only the following wavelengths ?
(a) ${\mathrm{\lambda }}_1$                      (b) ${\mathrm{\lambda }}_2$
(c) ${\mathrm{\lambda }}_1$ and ${\mathrm{\lambda }}_2$          (d)  ${\mathrm{\lambda }}_1, {\mathrm{\lambda }}_2, {\mathrm{\lambda }}_3$ and ${\mathrm{\lambda }}_4$

As compared to the person with white skin, the person with black skin will experience 
(1) Less heat and more cold           

(2) More heat and more cold

(3) More heat and less cold             

(4) Less heat and less cold

Relation between emissivity e and absorptive power a is (for black body)
(1) e = a                     

(2) $\mathrm{e}=\frac{1}{\mathrm{a}}$

(3) $\mathrm{e}={\mathrm{a}}^2$                    

(4) $\mathrm{a}={\mathrm{e}}^2$

Which of the following statements is wrong ?

(1) Rough surfaces are better radiators than smooth surface

(2) Highly polished mirror like surfaces are very good radiators

(3) Black surfaces are better absorbers than white ones

(4) Black surfaces are better radiators than white