Absorptive Power

The absorptive power of a body (or a surface) is defined as the ratio of energy absorbed over time (or over a certain time) to the radiant energy incident on it at the same time. It is a measurement of how much heat a thing absorbs. When heat is applied to the body’s surface, some of it is absorbed and the rest is reflected.

Absorptive Power:

When heat is applied to the surface of an object, some of it is absorbed and the rest is reflected. Because it absorbs the radiant energy of all wavelengths incident on it, a black body’s absorptive power is one. A good radiator is an excellent absorber. The absorbed radiation is usually transformed to thermal energy, which raises the temperature of the object. For example, a black body absorbs all incident radiation and has an absorptive power of 1. As a result, the body with greater absorptive power also has greater emissive power. The amount of heat absorbed by the body is determined by the body’s nature.

At a certain temperature, a body’s absorptive power is specified, and wavelength is defined as the ratio of heat energy absorbed to heat energy incident on it over a wavelength range. This thermal energy is used by scientists to generate electricity. The most efficient method of converting low-grade heat into electricity is absorption. There are a variety of other applications. A minimal quantity of heat is used to operate the continuous-cycle absorption cooling unit. Gas, electricity, or kerosene are used to provide heat.

The wavelength of Radiations:

We have a range of wavelengths and frequencies of different waves in the magnetic spectrum, such as gamma rays, x-rays, ultraviolet, visible rays, radar waves, microwaves, television waves, and radio waves, and these visible rays consist or comprise of seven colours: violet, indigo, blue, green, yellow, orange, and red, which also differ in their frequencies and thus the wavelength of the colours changes accordingly.

The frequency of these several waves has reduced in the same order as previously described. Because we already know that frequency is inversely related to wavelength and that wavelength increases in the same order as frequency, radio waves have the longest wavelength. Red has the longest wavelength in the visible spectrum because it has the lowest frequency in the VIBGYOR.

Because radio waves have the lowest frequency, they have the longest wavelength.

Note:

We know that the relation between the frequency and wavelength is inversely proportional or the wavelength is inversely proportional to the frequency. That is

Where is the wavelength

Is the speed of light

Frequency

Emissive Power, Absorptive Power and Emissivity:

1) Monochromatic Emittance or Spectral emissive power : For a given surface it is defined as the radiant energy emitted per sec per unit area of the surface within a unit wavelength around i.e. lying between to .

Spectral emissive power

2) Total Emittance (or) Total Emissive Power : It is defined as the total amount of thermal energy emitted per unit time, per unit area of the body for all possible wavelengths.

3) Monochromatic absorptance or spectral absorptive power : It is defined as the ratio of the amount of the energy absorbed in a certain time to the total heat energy incident upon it in the same time, both in the unit wavelength interval. It has no dimensions and has no units.

4) Total absorptance (or) Total absorpting power : It is defined as the total amount of thermal energy absorbed per unit time, per unit area of the body for all possible wavelengths.

5) Emissivity : Emissivity of a body at a given temperature is defined as the ratio of the total emissive power of the body to the total emissive power of a perfect black body at that temperature, i.e.

(i) For a perfectly black body,

(ii) For a highly polished body,

(iii) For practical bodies emissivity lies between zero and one 

Conclusion:

The absorptive power of a body (or a surface) is defined as the ratio of energy absorbed over time (or over a certain time) to the radiant energy incident on it at the same time.

As a result, absorptive power, a, equals the amount of energy absorbed divided by the amount of energy incident.

Because it absorbs radiant energy of all wavelengths incident on it, a black body’s absorptive power is one. A good radiator is an excellent absorber. As a result, the body with the most absorptive power also has the highest emissive power.