### Introduction

• Radiation is the energy emitted by matter in the form of electromagnetic waves.
• Here we will look into thermal radiation (10-7 m to 10-4 m) since we are dealing with heat transfer.
• In radiation the internal energy of an object decreases.

### Rate of emission of radiation by a body depends on-

• Surface temperature
• Surface Nature
• Wavelength or frequency of radiation

### Important Definitions

#### Total Emissive Power (E)

• Total amount of radiation (all wavelength range) emitted by a body per unit area and time.
• A/C to Stefan-Boltzman, for a black body emissive power is proportional to absolute temperature to the fourth power.

Eb = σAT4 W/m2

σ = Stefan-Boltzman Constant (5.67*10-8 W/m2K4)

#### Monochromatic Emissive Power (Eλ)

• Rate of energy radiated per unit area of the surface per unit wavelength.
• At any given temperature the amount of radiation emitted per unit wavelength varies at different wavelengths. So, Monochromatic Emissive Power (Eλ) of the surface is used.

#### Emissivity (ε)

• Ability of the body surface to radiate heat.
• It is also defined as the ratio of emissive power of any body to emissive power of a black body of equal temperature.
• Its value varies for different substances ranging from 0 (white body) to 1 (black body).
• Emissivity may vary with temperature and wavelength.

• Total incident radiation on a surface from all directions per unit time per unit area of the surface.

• It refers to all of the radiant energy leaving a surface per unit area of the surface.

J = ε + ρG

ρ = reflectivity

#### Reflectivity (ρ)

• Fraction of incident radiation reflected.
• For white body :- ρ = 1, α = 0, τ = 0

#### Absorptivity (α)

• Fraction of incident radiation absorbed.
• For Black Body α = 1, ρ = 1, τ = 0

#### Transmittivity (τ)

• τ = 0 for black, white and opaque body (α + ρ = 1).
• Fraction of incident radiation transmitted.