Radiation and its properties

2. :-
DEFINITION OF RADIATION
SPECTRUM OF ELECTROMAGNETIC REDIATIONS.
SURFACE EMISSION PROPERTIES.
ABSORPTIVITY, REFLECTIVITY AND TRANSMISSIVITY
BLACK BODY
OPAQUE BODY
WHITE BODY
GRAY BODY

3. The transfer of energy across a system boundary
by means of an electromagnetic mechanism wich
is caused solely by a temperature difference.
Radiation heat transfer does not require a
medium.
The radiant heat exchange between two bodies
depends on the difference between their
temperature to the Fourth power.
E=σT4 W/m2
E=σAT4 watt

4. The electromagnetic (EM) spectrum is the range of all types of EM radiation. Radiation is energy that
travels and spreads out as it goes – the visible light that comes from a lamp in your house and the radio
waves that come from a radio station are two types of electromagnetic radiation. The other types of EM
radiation that make up the electromagnetic spectrum are microwaves, infrared light, ultraviolet light, X-
rays and gamma-rays.
Radio: Your radio captures radio waves emitted by radio stations, bringing
your favorite tunes. Radio waves are also emitted by stars and gases in
space.
Microwave: Microwave radiation will cook your popcorn in just a few
minutes, but is also used by astronomers to learn about the structure of
nearby galaxies.
Infrared: Night vision goggles pick up the infrared light emitted by our skin
and objects with heat. In space, infrared light helps us map the dust between
stars.
Visible: Our eyes detect visible light. Fireflies, light bulbs, and stars all emit
visible light.
Ultraviolet: Ultraviolet radiation is emitted by the Sun and are the reason
skin tans and burns. "Hot" objects in space emit UV radiation as well.
X-ray: A dentist uses X-rays to image your teeth, and airport security uses
them to see through your bag. Hot gases in the Universe also emit X-rays.
Gamma ray: Doctors use gamma-ray imaging to see inside your body. The
biggest gamma-ray generator of all is the Universe.

5. The rate of emission of radiation by a body depends upon the following factors:-
A. Temperature of surface.
B. The nature of surface, and
C. the wavelength or frequency of radiation.
The parameters which deals with the surface emission propertiesare given below:-
1. Total emissive power:-Total amount of radiation emitted by a body per unit area and time.
Eb=σT4 W/m2
Eb=σAT4 watt σ=Stefan-Boltzmann Constant=5.67x10-8 w/m2K4
2. Monochromatic emissive power:- The rate of energy radiated per unit area of the surface
per unit wavelength.
3. Emission from real surface emissivity:- It can be calculated by.
Eb=ἐσAT4
ἐ=Emissivity of the material=Ability of the surface of a body to radiate heat.

6. Let,
G=Incident radiation
Gr=Reflected radiation
Gt=Transmitted radiation
Ga=Absorbed radiation
By the conservation of energy principle
Ga +Gt +Gr =G
Dividing both sides by G, We get
ABSORBED RADIATIONS

7. Blackbody is an idealized physical body that absorbs all incident
electromagnetic radiation, regardless of frequency or angle of
incidence.
For a black body, τ = 0, α = 1, and ρ = 0.
Planck offers a theoretical model for perfectly black bodies,
which he noted do not exist in nature: besides their opaque interior,
They have interfaces that are perfectly transmitting and non-reflective.

8. When no incident radiarions is
transmitted through the body, It is
called an “Opaque Body”
That is, τ = 0 and α + ρ = 1.
ABSORBED RADIATIONS

9. If all the incident radiation falling
on the body are reflected , It is
called “White Body”.
That is, τ = 1 and α = ρ = 0.
Type of reflection
1. Regular reflection
2. diffuse reflection

10. A grey body is defined as a body
with constant emissivity over all
wavelengths and temperatures.
or
A grey body is one where α, ρ and
τ are constant for all wavelengths.