# What is Radiation [Mode of Heat Transfer]

What is Radiation? It is a method of heat transfer (like Conduction or Convection) by which High Temperature body looses heat to its surroundings. Its a wave having parameters like Amplitude, Frequency or Wavelength (λ).

## What is Radiation – Absorption, Reflection and Transmission

When radiant energy is incident on the surface of any body, part of this energy is absorbed, part is reflected and the remaining part is transmitted by the body.

### Co-efficient of Absorption (a)

Co-efficient of Absorption (a) is defined as the ratio of the amount of the heat absorbed by the body to the total amount of radiant heat upon it. Since it is a ratio, it is a dimensionless quantity and have a value less than 1.0

### Co-efficient of Reflection (r)

Co-efficient of Reflection (r) is defined as the ratio of the amount of the heat reflected by the body to the total amount of radiant heat upon it. Since it is a ratio, it is a dimensionless quantity and have a value less than 1.0

### Co-efficient of Transmission (t)

Co-efficient of Transmission (t) is defined as the ratio of the amount of the heat transmitted by the body to the total amount of radiant heat upon it. Since it is a ratio, it is a dimensionless quantity and have a value less than 1. Athermanous (or Adiathermanous) body is one through which heat radiation cannot be transmitted. Similarly, Diathermanous body is one through which heat radiation can be transmitted.

The various coefficients are related as:
a + r + t = 1

## What is a Black Body

A perfectly black body is one that absorbs all heat radiation incident on it. For this (a = 1, while r = t = 0); co-efficient of emission (emissivity) (e) = 1. There is no substance in nature, which is a perfect black body. The closest are Lamp Black (a = 0.96) and Platinum Black (a = 0.98).
Ferry’s Black Body is a hollow sphere with a small aperture. When radiation enters the aperture, it suffers multiple internal reflection. The ‘small aperture’ acts as perfectly black body; not the sphere.

## What is Radiation – Laws

### Wien’s Displacement Law

The wavelength (λm) for which emissive power of black body is maximum is inversely proportional to the temperature of the black body.
$\lambda_m = \frac{b}{T}$

At a given temperature, the coefficient of absorption of a body is equal to its coefficient of emission (a = e)

The amount of radiant heat emitted by a perfectly black body per unit area per unit time is directly proportional to the fourth power of its absolute temperature.

$Radiant\ Energy\ Q = \sigma e A T^4 t$

$Radiant\ Power = \frac {dQ}{dt} = \sigma e A T^4$

### Newton’s Law of Cooling

Rate of loss of heat by a body is directly proportional to the excess of the temperature of the body over the surrounds, provided that this excess is small.
$Rate\ of\ loss\ of\ heat = \frac {dQ}{dt} = C \left (\theta - \theta_0 \right)$