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# Absorptive & Emissive Power

Radiation transfers heat from one position to another without heating the intervening medium.

Radiation transfers heat from one position to another without heating the intervening medium. The term ‘radiation’ refers to electromagnetic waves. These waves, which carry energy, are formed by the superposition of perpendicular electric and magnetic fields. In general, the term radiation refers to the phenomenon of waves that tend to radiate from a source. This article will discuss the difference between the emission and absorption spectra. These two topics are among the most crucial in Physics.

## What is absorptive power?

The absorptive power of a body or the surface is described as the ratio of energy absorbed in a given instant of time to the radiant power incident on it at the same instant. It is a measurement of the amount of heat absorbed by an element. When heat is incident on the body’s surface, some are absorbed, and some are reflected.

Mathematically, we can say that,

absorptive power = quantity of energy absorbed / quantity of energy incident

When heat is incident on the object’s surface, some of it is absorbed while the rest is reflected. A black body has an absorptive power of one because it consumes the radiant energy of all wavelength incidents on it. A good absorber also serves as a good radiator. Typically, absorbed radiation is transformed into heat, raising the object’s temperature. For example, a black body absorbs all incident radiation and has an absorptive power of one. As a result, the body with a higher absorptive power also has a higher emissive power. The amount of heat taken into the body is determined by its composition.

A body’s absorptive power is defined at a given temperature, and its wavelength is the ratio of heat energy absorbed to heat energy incident on the wavelength range. Scientists use this thermal energy to generate electricity. The most efficient method of transforming low-grade energy into electricity is absorption. There are other applications as well. A small amount of heat is used to power the absorption constant cooling unit. Either electricity, kerosene or gas provides this heat.

## What is emissive power?

The quantity of electricity radiated per unit time per unit object’s surface for a given wavelength is defined as a body’s emissive power at a given temperature. At any given temperature, it is the power of thermal energy radiated in all directions per unit time of each unit area of a surface.

The emissive power is defined as the ratio of energy radiated and emitted per unit area per sec to the amount of heat energy emitted per unit area per second by a perfectly black body at the same temperature. When a body’s temperature rises above absolute zero, it emits heat. When this emitted heat travels in the form of an electromagnetic wave, it is referred to as thermal radiation. At any given temperature, this energy is the power of heat transfer radiation emitted in all directions per time unit per unit area of a surface.

Let us assume that ‘Q’ is the quantity of emitted radiant energy, ‘A’ denoted surface area, ‘t’ is the time the body radiates. Now, the emissive power is denoted by:-

E = Q/At

## What is the difference between Absorption Spectra and  Emission Spectra?

The primary distinction between emission and absorption spectra is that an emission spectrum contains different coloured lines, whereas an absorption spectrum contains dark-colored lines. The table below presents more differences between the absorption and emission spectra.

 Emission Spectra Absorption Spectra Emission Spectra is generated when an atom releases energy Absorption Spectra is generated when an atom absorbs energy Included in the spectrum are coloured lines Make up dark lines or gaps in the spectrum It is useful in determining the composition of a given matter It can be used to determine an object’s ability to retain heat and its level of absorption. The type of photons emitted can be used to determine the elemental composition of a substance because each element emits an amount of energy and has a different emission level. The wavelengths of light absorbed aid in determining the number of substances present in the sample.

### Conclusion

In the above article we read about the definition, formula, and examples of the absorptive and emissive power. Radiation is one of the most crucial topics of the physics chapter. Radiation is described as heat transfer from one place to another without heating the intervening medium. The term ‘radiation’ here refers to electromagnetic waves. These waves, which carry energy, are formed by the superposition of perpendicular electric and magnetic fields. In general, the term radiation refers to the phenomenon of waves that tend to radiate from a source.