**Q1. The thermal diffusivities for solids are generally**

(a) less than those for gases

(b) jess than those for liquids

**(c) more than those for liquids and gases**

(d) more or less same as for liquids and gases

(e) zerci.

**Q2. Thermal diffusivity of a substance is**

(a) directly proportional to thermal con¬ductivity

(b) inversely proportional to density of substance

(c) inversely proportional to specific heat

**(d) all of the above**

(e) none of the above.

**Q3. The ratio of the emissive power and absorptive power of all bodies is the same and is equal to the emissive power of a perfectly black body. This statement is known as**

**(a) Krichoff’s law**

(b) Stefan’s law

(c) Wien’ law

(d) Planck’s law

(e) Black body law.

**Q4. According to Stefan’s law, the total radiation from a black body per second per unit area is proportional to**

(a) absolute temperature

(b) T2

(c) T5

**(d) t**

(e) l/T.

**Q5. According to Wien’s law, the wavelength corresponding to maximum energy is proportion to**

**(a) absolute temperature (T)**

(b) I2

(c) f

(d) t

(e) 1/r.

**Q6. Depending on the radiating properties, a body will be white when**

(a) p = 0, x = 0 and a = 1

**(b) p=l,T = 0anda = 0**

(c) p = 0, x = 1 and a = 0

(d) x = 0, a + p = 1

(e) a = 0, x + p = 1.

where a = absorptivity, p = reflectivity, x = transmissivity

**Q7. Depending on the radiating properties, a body will be black when**

**(a) p = 0, x = 0 and a = 1**

(b) p= l,T = 0anda = 0

(c) p = 0, x = 1 and a = 0

(d) x = 0, a + p = 0

(e) a = 0,x + p= 1.

where a = absorptivity, p == reflectivity, X = transmissivity.

**Q8. Depending on the radiating properties, a body will be opaque when**

(a) p = 0, x = 0 and a = 1

(b) p=l,x = 0anda = 0

(c) p = 0, x = 1 and a = 0

**(d) x – 0, a + p = 1**

(e) a=0,x + p= 1.

where a = absorptivity, p = reflectivity, X = transmissivity

**Q9. The total emissivity power is .defined as the total amount of radiation emitted by a black body per unit**

(a) temperature

(b) thickness

(c) area

**(d) time**

(e) area and time.

**Q10. The ratio of the energy absorbed by the body to total energy falling on it is called**

**(a) absorptive power**

(b) emissive power

(c) absorptivity

(d) emissivity

(e) none of the above.