CBSE Class 12 Physics Radiation Theory and Examples

CBSE Class 12 Physics Radiation Theory and 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.

1. HEAT TRANSFER
The process of transmission of heat is by way of
(i) conduction (in solids)
(ii) convection (in liquids and gases)
(ii) radiation (through vacuum in the form of electromagnetic waves

(i) Conduction - The process in which the material takes an active part by molecular action and energy is passed from one particle to another is called conduction. It is predominat in solids.

(ii) Convection - The transfer of energy by actual motion of particles of medium from one place to another is called conection. It is predominant in fluids (liquids and gases)

(ii) Radiation : Quickest way of transmission of heat is known as radiation. In this mode of energy transmission, heat is transferred from one place to another without affecting the intervenning medium.

2. NATURE OF RADIATION
(i) Radiation is electromagnetic wave (0 to ∞ wavelength) and travels with velocity of light
(ii) Radiation travels in straight line
(iii) Radiation can travel through vacuum
(iv) Radiation obeys the laws of reflection, refraction , interference, diffraction, polarization etc (all optical phenomena)
(v) Radiation does not affects the medium through which it travel.
(vi) For point source , radiation energy obeys inverse square law i..e
(a) For point source intensity at a point is inversely proportional to the square of the distance of point from the source I α1/r²
(b) For line source , intensity at a point is inversely proportional to the distance of the

(iii) r = → reflection coefficient or reflecting power a =Qa/Q → absorption coefficient or absorptive power t =Qt/Q → transmission coefficient or transmitting power
(iv) If a = 0, t = 0, then r = 1. Such a surface is a total or perfect reflector.
(v) If r = 0, t = 0, then a = 1, Such a surface is a total absorber and is called a black body
(vi) If r = 0, a = 0, then t = 1. The surface is then perfectly transparent and the substance is called diathermanous.

5. DEFINITIONS
(i) Reflecting power or refracting coefficient(r) : Ratio of amount of radiation reflected by the surface to the amount of heat incident over it

(ii) Transmitting power or transmitting coefficient (t) : Ratio of amount of radiation transmitted by the surface to the amount of heat incident over it

(iii) Absorption power or absorption coefficient (a) : Ratio of amount of radiation absorbed by the surface to the amount of heat incident over it

Note :– (a) A radiation incident over a surface is either refracted , transmitted or absorbed , so a + t + r = 1
(b) The reflecting , refracting and transmitting powers of a surface depends on nature of surface as well as on the wavelength of radiation falling on them

(iv) Emissive power (e) : Emissive power is defined as the amount of heat radiated by unit area of the surface in one second at a particular temperature. It's unit is Joule/m² or watt-sec/m²

(v) Spectral emissive power (e_λ) : Amount of heat radiated by unit area of the body in one second in unit spectral region at a given wavelength. It's unit is watt/m²-AºC^–1. Its value depends on temperature,λ (wavelength and nature of surface)

(vi) Total emissive power (e) : Emissive power obtained for total range of spectrum is related to total emissive power as follows e = e_λd_λ

(vii)Spectral absorptive power : Like emissive power , the absorptive power of the surface also depends on the wavelength of radiation incident on it. It is defined as the ratio of radiation absorbed by the surface in unit spectral region at a given wavelength to the amount of radiation incident over it.

Note :– Absorption power and spectral absorptive power for perfect black body is taken as one

(viii)Emissivity or relative emissive power (E): Ratio of emissive power of a body at a given temperature to the emissive power of a black body at the same temperature. It has no units.

6. IDEAL BLACK BODY
(i) Total absorber is said to be ideal black body

(ii) aλ = 1

(iii) For lamp black and platinum black aλ = 1 , therefore both can be assumed to be ideal black body

(iv) Ferry's black body is practical example of black body

(v) For a black body , a = 1 ,t = 0 and r = 0

(vi) When black body is heated , it radiates all the radiations absorbed by it.

(vii) The perfect black body is only an ideal conception The body which completely absrobs the radiation of all wave lengths incident on it and there is no reflection and transmission is called a perfectly black body. Black body emits radiations of all wavelengths on heating and it is a good emitter. The nature of radiations emitted by the block body depends only on the temperatrue. It does not depend on its nature, size, mass etc.

7. LAWS OF RADIATION
(A) Prevost’s theory of heat exchange

(i) Mutual exchange of heat between any object and it's surroundings occurs according to their temperatures

(ii) This process is continuous

(iii) A body at high temperature emits more radiations and absorbs less. Therefore it experiences cooling. While the body at low temperature absorbs more heat and experiences heating

(iv) At absolute zero temperature the radiated energy is zero 

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