Radiation Questions: Paper – 02

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1. If a = 0.72, r = 0.24, then the value of t is (MHT-CET 2003)

(a) 0.02

(b) 0.04

(c) 0.4

(d) 0.2

Answer: (b)

2. A body cools from 50°C to 46°C in 5 minutes and to 40°C in the next 10 minutes. The surrounding temperature is (MHT-CET-1999)

(a) 30°C

(b) 28°C

(c) 36°C

(d) 32°C

Answer: (a)


According to Wien’s law (PMT MP 88)

(a) λm T = constant

(b) constant

(c) constant

(d) T + λm = constant

Answer: (a)

4. A black body is at 300 K. It emits energy at a rate which is proportional to (AIMS-2002)

(a) (300)

(b) (300)2

(c) (300)4

(d) (300)3

Answer: (c)


Which of the following will radiate heat to large extent? (MNR-92)

(a) Rough surface

(b) Polished surface

(c) Black rough surface

(d) Black polished surface

Answer: (c)


The rate of radiation of black body at 0°C is E watt. The n the rate of radiation of this black body at 273°C will be (MP-PMT 89)

(a) 16 E

(b) 8 E

(c) 4 E

(d) E

Answer: (a)


A black body radiates energy at the rate of E watt m– 2 at a temperature of T Kelvin. When the temperature is reduced to T/2 Kelvin, the radiant energy will become (MPPMT 92, CPMT 88, MNR 93)




(d) None of the above

Answer: (c)


Woolen clothes keep the body warm, because wool (EAMCET 78, AIMS 98)

(a) Is a bad conductor

(b) Increases the temperature of body

(c) Decreases the temperature

(d) All of these

Answer: (a)

9.  The good absorber of heat are (PMT 89)

(a) Non-emitter

(b) Poor-emitter

(c) Good-emitter

(d) Highly polished

Answer: (c)


The maximum energy in the thermal radiation from a hot source occurs at a wavelength of 11 × 105 cm. according to Wien’s law, the temperature of the source (on Kelvin scale) will be n times the temperature of another source on (Kelvin scale) for which the wavelength at a maximum energy is 5.5 × 105 cm. The value of n is (CPMT 91)

(a) 2

(b) 4

(c) 1/2

(d) 1

Answer: (a)


Two sphere of the same material have radii 1 m and 4 m and temperature 4000 K and 2000 K respectively. the energy radiated per second by the first sphere is (I.I.T 88)

(a) Greater then by the second

(b) Less than that by the second

(c) Equal in both cases

(d) The information is incomplete to draw and conclusion

Answer: (c)


A surface at temperature T0 °K receives power P by radiation from a small sphere at temperature T >> T0 and at a distance d. If both T and d are doubled, the power received by surface will becomes approximately (NSEP 89)

(a) P

(b) 2 P

(c) 4 P

(d) 16 P

Answer: (c)


If temperature of a black body increases from 7°C to 287°C, then the rate of energy radiation becomes



(b) 16 times

(c) 4 times

(d) 2 times

Answer: (b)


A bucket full of hot water is kept in a room and it cools from 75°C to 70°C in t1 minutes from 70°C to 65°C in t2 minutes and from 65°C to 60°C in t3 minutes; then (NCERT 80, CBSE 95, MHT-CET 99)

(a) t1 – t2 = t3

(b) t1 < t2 < t3

(c) t1 > t2 > t3

(d) t1 < t2 > t3

Answer: (b)

15.  The velocity with which thermal radiation travels in vacuum is (EAMCET-82)

(a) 3 × 106 m/s

(b) 3 × 107 m/s

(c) 3 × 108 m/s

(d) 3 × 1016 m/s

Answer: (c)


In which process the rate of transfer of heat is maximum? (CMEE 94)

(a) Conduction

(b) Convection

(c) Radiation

(d) In all these heat is transferred with the same velocity

Answer: (c)


A body cools from 60°C to 50°C in 10 minutes. If the room temperature is 25°C and assuming Newton’s law of cooling to hold good, the temperature of the body at the end of the next 10 minutes will be (MP-PMT 98)

(a) 38.5°C

(b) 40°C

(c) 42.85°C

(d) 45°C

Answer: (c)


Two spheres made of same material have radii in the ratio 2:1. if both the spheres are at same temperature, then what is the ratio of heat radiation energy emitted per second by them? (MHT-CET 2004)

(a) 1: 4

(b) 4: 1

(c) 3: 4

(d) 4: 3

Answer: (b)

19. Solar radiation emitted by sun resembles that emitted by a black body at a temperature of 6000 K. maximum intensity is emitted at a wavelength of about 4800 A°. if the sun were cooled down from 6000 K to 3000 K, then the peak intensity would occur at a wavelength of

(PMT MP 91)

(a) 4800 A°

(b) 9600 A°

(c) 2400 A°

(d) 19200 A°

Answer: (b)


A perfectly black body emits radiation at temperature T1 K. If it is to radiate 16 times this power, its temperature T2. will be (BHU 94)

(a) T2 = 16 T1

(b) T2 = 8 T1

(c) T1 = 4 T1

(d) T2 = 2 T1

Answer: (d)


Unit of Stefan’s constant is given by (MHT-CET 2006)

(a) W/ m K2

(b) W/ m2 K2

(c) W2/ m2 K4

(d) W/ mK

Answer: (b)


A black body at hot temperature at 227°C radiates heat at a rate of 5 cal/cm2 s. at a temperature of 727°C the rate of heat radiated per unit area will be

(MHT-CET 2002)

(a) 50 cal/cm2 s

(b) 250 cal/cm2 s

(c) 80 cal/cm2 s

(d) 100 cal/cm2 s

Answer: (c)


According to Kirchhoff’s law of radiation which of the following relation is correct (MHT CET 2006)

(a) Eb = Ea


(c) Eb = E2a

(d) Eb = E – a

Answer: (b)


A person with dark skin as compared to a person with white skin will experience (CPMT-88)

(a) Less heat and less cold

(b) More heat and more cold

(c) More heat and less cold

(d) Less heat and more cold.

Answer: (b)


For which of the following process, the thermal conduction is maximum? (AFMC PUNE-2002)

(a) Combustion

(b) Radiation

(c) Convection

(d) Conduction

Answer: (b)

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