Introduction
- All matters above absolute zero emits and imparts energy to the surrounding
- This transferring of energy and its mode of transfer is known as radiation.
- Radiation is temperature dependent. The relation is as: Higher the temperature, the greater will be the radiation.
- The wave length at which energy is emitted also depends upon the temperature but higher the temperature, short is the wavelength of emission.
- And substances which emit the maximum amount of radiation in all wave length are known as black bodies. Such bodies will absorb completely all the radiation incident upon them.
Some terminologies
a) Wavelength: Wavelength is the shortest distance between two consecutive similar points on a wave train
λ= C/D
Where, λ=Wavelength (nm);
C= velocity of the light,
D= Frequency or vibration/sec
b) Radiation flux density: The amount of solar energy received on a unit surface area per unit of time is called radiation flux density. It is generally expressed in cal/cm2-min.
c) Planck’s Law: This law illustrates that greater is the frequency of wave (i.e. shorter is the wave length), greater will be the energy content of the quantum.
E=h D
Where, E=Energy content
h=Plank’s constant
D=Frequency of radiation
d) Absorptivity (α): It is defined as the ratio of amount of total radiation absorbed by the surface (Ia) to the amount of incident on the surface (I).
Thus, α=Ia/I.
e) Kirchoff’s Law: It states that Absorptivity (α) of an object for radiation of a specific wavelength is equal to its emissivity (e) for the same wave length (λ).
i.e. α (λ)=e(λ)
f) Reflectivity or albedo (r): It is the ratio of the amount of energy reflected from the surface to the total amount incident on the surface; and also referred as albedo.
i.e. r=Ir/I
Radiation reflected in all directions is called ―diffuse reflection and radiation reflected as occurs off a mirror is called ―specular reflection.
According to the nature of surface value of albedo varies. As far;
Fresh snow 0.80
Sand 0.40
Wet earth 0.10
g) Transmissibility (t): It is the ration of amount of radiation transmitted to the amount of radiation incident on the surface
t=It/I
On a given surface, if the total incident radiation is I and Ia is the radiation absorbed by the surface, Ir be the radiation reflected by surface and It is the radiation transmitted by surface
I=Ia+Ir+It
Dividing both sides by I, We get
I/I=Ia/I+Ir/I+It/I
1= α+r+t
h) Stefan Bolzman’s Law: This law states that radiation flux (E) emitted by a radiating body is directly proportional to the fourth power of its temperature i.e.,
E= σT4
Where, E= Radiation flux
σ= Stefan Bolzman’s constant=5.6×10-5 erg cm-2/sec-1 k4 (=8.14 x 10-11 langley min-1 K4)
T=Absolute temperature (0K)
Above law is for the black body, if the body is not black body then
E = σ e T4 …………………….Where, e=emissivity. Value of e lies between 0 to 1. For a perfect black body, it is one
This law illustrates that higher the temperature of the radiating body, the higher will be the amount of the radiation emitted. Since the amount of the radiation is proportional to the fourth power f the temperature, even a small variation in temperature could cause greater changes in radiation emitted.
i) Wein’s law: This law states that the wavelength of maximum intensity of emission from a radiating is inversely proportional to the absolute temperature of the body.
λ α1/T
i.e. Wavelength of maximum intensity of emission=2897 T-1
Wien’s Law tells us that objects of different temperature emit spectra that peak at different wavelengths.
Hotter objects emit most of their radiation at shorter wavelengths; hence they will appear to be bluer, Cooler objects emit most of their radiation at longer wavelengths; hence they will appear to be redder.
Furthermore, at any wavelength, a hotter object radiates more (is more luminous) than a cooler.