Difference Between Black Body and Grey Body

Key Difference – Black Body vs Grey Body
 

The terms black body, white body and grey body are discussed regarding the absorption, emission or reflection of radiation by different substances. A black body is an idealized physical body (object) that can absorb all the incident electromagnetic radiation. A grey body is a body (object) that emits radiation at each wavelength in a constant ratio less than unity to that emitted by a black body at the same temperatures. The key difference between a black body and a grey body is that Black body is an ideal physical body whereas the grey body is a non-ideal physical body.

CONTENTS

1. Overview and Key Difference
2. What is Black Body
3. What is Grey Body
4. Similarities Between Black Body and Grey Body
5. Side by Side Comparison – Black Body vs Grey Body in Tabular Form
6. Summary

What is Black Body?

A black body is an idealized physical body (object) that can absorb all the incident electromagnetic radiation. This absorption occurs regarding the frequency and the angle of incidence of the radiation. This means the absorption of radiation by a black body is independent of the direction from which radiation comes from and the wavelength of the radiation. A black body is a non-reflective body. In contrast, a white body is a physical body (an object) that has a rough surface that reflects all the incident electromagnetic radiation.  The reflection is complete and uniform reflection to all directions.

A black body that is at a constant temperature can emit the absorbed electromagnetic radiation. Hence it is a perfect absorber and perfect emitter. This emitted radiation is known as black body radiation. The black body radiation occurs giving a specific spectrum and specific intensities at a given temperature.

The radiation emitted by the black body obeys the Planck’s law. Planck’s law states that the spectrum of the black body is determined only by the temperature of the black body, not by the shape or the composition of the black body. A black body that is at a constant temperature (at thermal equilibrium) has following characteristic properties.

1. The black body is an ideal emitter – it emits the radiation at every possible frequency that a body at the same temperature can emit
2. The black body is a diffuse emitter – it emits the same frequency at every direction (isotropic radiation).

The realization of a black body is the real world embodiment of the concept of black bodies. In other words, the realization of a black body gives the examples for black bodied occur in the real world (since a black body is an ideal concept). Some examples are given below:

• Stars
• Planets
• Black holes

What is Grey Body?

A grey body is a body (object) that emits radiation at each wavelength in a constant ratio less than unity to that emitted by a black body at the same temperatures. A grey body is a physical body whose absorptivity of a surface does not vary with variation in temperature and wavelength of the incident radiation.

Unlike the black body, a grey body is a non-ideal emitter or an imperfect radiator. That is because a grey body can absorb some of the energy it receives and also it may reflect some of the energy. A grey body emits only a portion of the radiation it absorbs. A grey body is considered as an opaque object.

What is the Similarity Between Black Body and Grey Body?

• Both Black Body and Grey Body are objects that can absorb and emit radiation.

What is the Difference Between Black Body and Grey Body?

Summary – Black Body vs Grey Body

A black body is an ideal physical body that can absorb all the incident radiation and emit radiation to every possible direction isotropically. A grey body is a non-ideal object that can absorb some of the incident radiation and emit only a portion of the radiation. The difference between a black body and a grey body is that Black body is an ideal physical body whereas the grey body is a non-ideal physical body.

Reference:

1.“Black body.” Wikipedia, Wikimedia Foundation, 4 Mar. 2018. Available here
2.“Gray body.” com, Dictionary.com. Available here   
3.jenish patel, Attended djmit “Concept of different bodies.” LinkedIn SlideShare, 7 Sept. 2015. Available here   

Image Courtesy:

1.’Black body’By Darth Kule – Own work, (Public Domain) via Commons Wikimedia