CBSE Class 12 Physics Radiation Solved Examples

CBSE Class 12 Physics Radiation Solved Examples. Please refer to the examination notes which you can use for preparing and revising for exams. These notes will help you to revise the concepts quickly and get good marks.

Ex.1 In a solar spectrum dark lines (fraunhofer lines) are obtained due to
(A) absorption (B) reflection
(C) emission (D) transmission
Sol.(A) Fraunhorfer lines are obtained due to the absorption of certain characteristic radiations.

Hence the correct answer is (A)

Ex.2 Fraunhofer lines are explained by
(A) Provosts theory of heat exchanges (B) Newton's law of cooling (C) Stefan's law (D) Kirchoft's law
Sol.(D) Fraunhofer lines are explained on the basis of Kirchhoff's law.

the correct answer is (D)

Ex.3 The nature of radiation emitted by a black body depends only on -
(A) the shape of a body (B) the nature of a body (C) the temperature of a body (D) the medium
Sol.(C) The nature of radiations emitted by a black body depends only upon the temperature of the body

Hence the correct answer is (C)

Ex.4 Two identical balls of wax are attached on the outer surface of two tin sheets. The inner surface of P is coated with lamp black and that of Q is polished. If a source of heat is placed between P and Q then which ball will melt first
(A) Q ball (B) P ball (C) both simultaneously (D) nothing can be predated
Sol.(B) As the black surfaces are good absorbers of heat. Hence the P will melt first.

Hence the correct answer is (B)

Ex.5 Energy spectrum of radiations emitted by a black body is
(A) line spectrum (B) continuous spectrum (C) bond spectrum (D) line and continuous spectrum

Sol.(B) Energy spectrum of radiation emitted by a black body is continuous

Hence the correct answer is (B)

Ex.6 If the rate of emission of radiation from a body is equal to the rate of absorbing, then the temperature of the body will be -
(A) less than the temperature of the surrounding
(B) greater then the temperature of the surrounding
(C) equal to the temperature of the surrounding
(D) nothing can be said
Sol.(C) The temperature of the body will be equal to the temperature of surrounding

Hence the correct answer is (C)

Ex.7 For the analysis of spectral energy of thermal radiations emitted by a body the prism used made of
(A) quartz (B) crown glass (C) flint glass (D) rock salt
Sol.(D) For the analysis of spectral energy of radiation prism of rock salt is used because it does not absorb thermal radiation. All other absorbs thermal radiations and spectrum is not obtained.

Hence the correct answer is (D)

Ex.8 If the temperature of a body is increased by  50%, then the increase in the amount of radiation emitted by it will be :
(A) 500% (B) 400% (C) 200% (D) 100%
Sol.(B) Q Percentage increase in the amount of radiations emitted

Ex.9 A blackened platinum wire of length 5cm and perimeter 0.02cm is maintained at atemperature of 300K. Then at what rate the wire is losing its energy? (Take s = 57 × 10^–8 units)
(A) 4.62 W (B) 0.462W (C) 46.2W (D) 4620W
Sol.(C) The rate of radiation heat loss is

Ex.10 A blackened sphere of radius 10cm at a temperature 227ºC is placed in a chamber with blackened wall, maintained at 27ºC. Calculate the rate of loss of heat ?
(A) 9.224 cal/s (B) .9224 cal/s (C) 922.4 cal/s (D) 92.24 cal/s
Sol.(D) Rate of heat loss

Ex.11 A blackened metal disc is held normal to the sun's rays, Both of its surfaces are exposed to atmosphere if the distance of earth from
sun is 216 times the radius of sun and the temperature of sun is 6000K, the temperature of the disc in steady state will be –
(A) 80ºC (B) 70ºC (C) 60ºC (D) 50ºC 

Sol.(B) in the steady state the heat received from sun will be equal to heat radiated out. Heat received from sun will be on one side only and it will radiate from both sides.

Ex.12 In the above problem, if the heat capacity (thermal capacity) of the sphere is 1000 cal/ºC then what is the rate of cooling of the sphere ?
(A) 55ºC/min (B) 55ºC/min (C) 550ºC/min (D) 5.5ºC
Sol.(D) The heat capacity of the sphere is ms = 1000 cal/ºC the rate of colling

Ex.13 A slab of stone of area 3600 sq cm. and thickens 10cm is exposed on the lower surface to steam at 100ºC, block of ice is melted in one hour. The thermal conductivity of stone is (in cal/cm-ºC) given (latent heat of ice = 80 cal/gm)
(A) 0.003 (B) 0.03 (C) 0.3 (D) none of these

Ex.14 Behaving like a black body sun emits maximum radiation at wavelength 0.48μm. The mean radius of the sun is 6.96 x 10⁸m. Stefan's constant is 5.67 × 10^–8 wm^–2k^–4 and Wien's constant is 0.293 cm^-k. The loss of mass per second by the emission of adiation from sun is -
(A) 5.32 × 10⁹kg/s (B) 6.24 × 10¹⁰kg/s (C) 8.65 × 10¹² kg/s (D) 2.46 × 10¹⁴kg/s  

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