Radiation:

• Medium is not needed
• Heat transfer in the form of electromagnetic wave of speed $3\ast 10^8$m/s
• Fastest Process
• Heat Radiation lies in IR region
• Inverse square law in Heat Radiation i.e $I\propto \frac{1}{R^2}$

Where I=intensity of heat radiation and R =Radius of sphere around the source

Note:

• When heat falls on a surface, a part of it is absorbed, a part of it is reflected and remaining part is transmitted
• Good reflectors are bad absorber and vice versa

Black body:

• Body that can absorb the entire heat radiation incident on it.
• It can also radiate all the wavelengths if heated at high temperature
• No known body is perfectly blackbody.
• A surface coated with lamp black absorbs 96%to 98% of the incident radiation.
• A black body is perfect radiator.
• In ferry`s black body small opening acts as the black body radiator.

emissive power:

• Total energy of all wavelengths radiated per second per unit area of the body.
• Its SI unit is $\frac{W}{m^2}$

emissivity

• Represented by 'e'.
• The ratio of energy radiated per unit area per unit time of a body and perfectly black body at same temperature
• value is 1 for perfectly black body and less than 1 for any other bodies.
• e=$E\epsilon$

stefan-boltzmann law:

• $\frac{P}{A}=\sigma T^4$ J/m${}^2$s, where $\sigma$=$5.67\ast 10^{-8}watt/m^2{K}^4$ is Stefan's Constant
• Total amount of heat energy radiated per second per unit area of a perfectly black body is directly proportional to fourth power of its absolute temperature.
• $E=\sigma T^4$ Where E=Energy radiated per unit time per unit area
• If A is the surface area of the body then

$E=\sigma A{T}^{4}orPower(P)=\sigma T^4$

thermal enclosure

A body of temperature T is placed in an enclosure of Temperature T${}_0$  then,

1. If $T>T_0$, then rate of energy radiated ($P=\sigma (T^4-T_0^4)$)
2. If $T<T_0$, the rate of energy absorbed ($P=\sigma (T_0^4-T^4)$)

prevost's theory of heat exchange

• According to this theory, everybody radiates heat radiation at all temperature
• But when the body is at absolute zero temperature (-273˚C) it will not radiate any heat energy.

Wein's displacement law

• Can be used to determine the temperature of heavenly bodies.
• At maximum intensity of radiation for blackbody.
• $\lambda T=b$ where $b=2.98\ast 10^{-3}$mK is called Wein's constant
• $\therefore \lambda T$ =${\lambda }^{\prime }{T}^{\prime }$

Note:

• If Body absorb radiation = Black
• If body radiate more than surrounding = Bright
• If body radiate same as surrounding = Invisible

Application of Radiation:

1. White clothes are preferred in summer and dark colored clothes in winter. (Reason=White reflect and dark absorb radiations)
2. Polished reflectors are used in electric heaters to reflect maximum heat in the room.
3. Hot water pipes and radiators used in room are painted black because they can radiate maximum amount of heat to the room.

Instruments for detection and measurement of heat radiation:

1. Bolometer
2. Thermopile