Radiation - I

A man was visiting a temple on a sunny day.

He was walking up the stairs of temple barefooted where Sun rays were falling.

It was very hot and so, he was feeling uncomfortable to walk on the heated stairs of the temple.

There were two types of stones on the stairs. One was black marble while the other one was white marble.

It was a bit more comfortable to walk on the white stone than the black one.

The person was unknown to the reasoning behind it. He thought for some time and then entered inside the temple.

The reason is simple, some bodies like black marble can absorb the maximum amount of radiation falling on it and so, are hotter.

while some bodies like white marble are good reflector of heat and so, are less hot when radiation fall on them.

So let's understand the radiation.

The radiation is the emission or transmission of energy in the form of electromagnetic waves through space or medium.

The radiation happens in the form of wave particles. It can be completely absorbed by some bodies or can be completely reflected by some bodies.

Now let's understand electromagnetic (EM) waves.

EM waves are the waves of the electromagnetic field, propagating through space.

These waves carry electromagnetic radiant energy and can travel through vacuum also.

The velocity of these waves can be given by,

Most of the radiation falling from the Sun is in the the visible region lying between the red and violet regions.

The region left of the visible radiation is infrared while that to its right is the ultraviolet.

On the right side of visible light, frequency increases and lights of x-rays and gamma rays are found having high penetration power.

On the left side of visible light, wavelength increases and microwaves are found in this range of wavelength.

The complete band of electromagnetic radiation around the visible region appears as,

Now, we are going to learn the different properties of radiation.

Let's understand the absorption: a property of radiation.

Suppose the radiation of radiant energy is falling on a surface then it will absorb amount and will be reflected.

So the absorptive power for the surface can be defined as the fraction of energy absorbed to the total incident energy.

If a body absorbs all the radiation falling on it then such a body is called perfect black body

A black body absorbs all frequencies and wavelength. It has zero reflectivity and 100 % absorptivity.

Let's now understand the emission: a property of radiation

Emissivity is the energy radiated by a body to the energy radiated by a perfect black body.

Every body having temperature greater than emits radiation.

Consider two bodies in which one is perfectly black while the other one is any normal body.

Consider the temperature of surrounding as .

Suppose a radiation of energy falls on both bodies.

They start to emit radiation so as to remain in thermal equilibrium with the surroundings.

Since the black body has absorptivity of , it will emit the same amount of radiation as that falling on it.

But the normal body will absorb a part of the radiation, i.e. and radiates energy

The ratio of energy radiated by a body to the energy radiated by a perfect black body at the same temperature is defined as emissivity of the body.

This emissivity is equal to the absorptivity of the body in thermal equilibrium.

So we can say that good absorber are also good emitters and the amount of heat radiated by a black body will always be greater than the normal body.

Revision

EM waves are the waves of the electromagnetic field, propagating through space.

The radiation is the emission or transmission of energy in the form of electromagnetic waves through vacuum or a medium.

The absorptive power for the surface can be defined as the fraction of energy absorbed to the total incident energy.

The emissivity of a body is the ratio of the total emissive power of a body to the total emissive power of a perfectly black body.

The End