Short Answer:

Emissivity is defined as the ratio of the radiation emitted by a real surface to the radiation emitted by a perfect blackbody at the same temperature. It shows how effectively a surface emits thermal radiation compared to an ideal emitter. Emissivity is a dimensionless quantity and its value always lies between 0 and 1.

In simple words, emissivity tells how good or poor a surface is in emitting heat energy through radiation. A surface with emissivity 1 is a perfect emitter (blackbody), while one with emissivity near 0 emits very little radiation, like a shiny metal.

Detailed Explanation :

Emissivity

Emissivity is an important property in the study of thermal radiation. It helps to understand how different materials emit heat energy when they are at a certain temperature. Every surface emits some amount of radiation, but not all surfaces emit the same quantity of energy even when they are at the same temperature. This difference arises because of a property called emissivity.

Mathematically, emissivity is defined as:

where,
ε = Emissivity of the surface (dimensionless)
E = Emissive power of the real surface (W/m²)
Eₑ = Emissive power of a perfect blackbody at the same temperature (W/m²)

The emissivity value always lies between 0 and 1:

ε = 1: Perfect blackbody (emits maximum radiation possible)
ε = 0: Perfect reflector (does not emit any radiation)
0 < ε < 1: Real surface (emits less radiation than a blackbody)

Thus, emissivity is a measure of how close a real surface comes to behaving like a perfect blackbody emitter.

Significance of Emissivity

Emissivity determines the ability of a surface to emit thermal energy by radiation. It depends on several factors such as material type, surface texture, temperature, and wavelength of the emitted radiation. In mechanical and thermal engineering, emissivity plays a key role in heat transfer calculations, furnace design, solar collectors, and radiative cooling systems.

A high-emissivity surface emits more heat energy and cools faster, while a low-emissivity surface retains heat because it radiates less energy.

Types of Surfaces Based on Emissivity

Blackbody:
A blackbody has emissivity equal to 1. It absorbs all incident radiation and emits the maximum possible radiation at a given temperature. It is an ideal surface used for theoretical and practical reference in radiation studies.
Gray Body:
A gray body is a surface whose emissivity is less than 1 but remains constant for all wavelengths. Real materials approximately behave as gray bodies for engineering calculations.
Real Body:
A real body has emissivity less than 1 and varies with wavelength and temperature. Examples include metals, paints, and ceramics.

Factors Affecting Emissivity

Surface Material:
Different materials have different emissivity values. For example, polished metals have very low emissivity (around 0.05), while nonmetals and painted surfaces have high emissivity (around 0.9).
Surface Roughness:
Rough surfaces emit more radiation than smooth surfaces because roughness increases the surface area and absorption capability.
Temperature:
Emissivity can change with temperature. For many materials, it increases slightly with temperature.
Wavelength:
Emissivity often depends on the wavelength of emitted radiation. For engineering simplicity, the surface is often assumed to be a gray body where emissivity is constant across all wavelengths.
Surface Coating or Finish:
Coatings can significantly alter emissivity. A dull black coating increases emissivity, while a polished or shiny surface reduces it.

Physical Meaning and Examples

To understand emissivity practically, consider two metal plates—one polished and one painted black—heated to the same temperature. The black plate will emit more heat energy because its emissivity is high (around 0.95), while the polished plate will emit much less because its emissivity is low (around 0.1).

This difference is crucial in engineering designs:

In radiators or solar heaters, high emissivity is desirable to enhance heat emission.
In thermal insulation or spacecraft, low emissivity materials are used to reduce heat loss.

Relation with Absorptivity

According to Kirchhoff’s Law of Radiation, for a surface in thermal equilibrium, the emissivity (ε) of a surface is equal to its absorptivity (α) at the same wavelength and temperature.

This means that a surface that is a good absorber of radiation is also a good emitter. For example, a black surface both absorbs and emits radiation efficiently, while a shiny surface reflects more and emits less.

Practical Applications of Emissivity

Thermal Imaging and Temperature Measurement:
Infrared cameras and pyrometers use emissivity values to accurately measure temperature.
Heat Exchanger Design:
Emissivity affects the rate of radiative heat transfer between components.
Furnace and Boiler Design:
Surfaces with high emissivity coatings are used to maximize heat transfer efficiency.
Spacecraft and Satellites:
Materials with low emissivity coatings are used to control heat radiation in space environments.
Solar Energy Systems:
Absorber plates with high emissivity are used in solar collectors to improve energy absorption and emission characteristics.

Example

If a real surface at 500 K emits radiation at a rate of 2000 W/m², and a blackbody at the same temperature emits 4000 W/m², then:

This means the surface emits only half as much radiation as a perfect blackbody at that temperature.

Conclusion

Emissivity is the ratio of the emissive power of a real surface to that of a blackbody at the same temperature. It represents how efficiently a surface emits thermal radiation. The value of emissivity lies between 0 and 1 and depends on several factors such as surface texture, material, and temperature. High emissivity materials are good radiators and absorbers, while low emissivity materials are effective reflectors. Understanding emissivity is essential in thermal engineering for designing efficient heating, cooling, and radiation control systems.