MHT-CET Physics Crash Course “Perfectly Black Body” Class 12

“Perfectly Black Body” MHT-CET Class 12 Physics Crash Course | Chapter Kinetic theory of Gases and Radiation

In this session G.S. Khairnar discussed the theory and MCQ’s of “Perfectly Black Body”. The session will be useful for all the aspirants of MHT-CET 2022. The session will be in Marathi and English.

Perfectly Black Body:-

Hello everyone in previous article we have learnt that when heat is incident on the substance it can get absorb, can get reflected or transmitted by the substance. In general all the dark colours are considered as good absorbers whereas light colours are considered as good reflectors. For certain purposes we need to store heat, so that it should completely get absorbed from its source, such sources are termed as black body.

Let’s learn about Black Body in details.….!

Perfectly Black Body:

A body which absorbs all the heat radiations incident on its surface is called as perfectly black body.

In practice the perfectly black body cannot be realized. But the lamp black or the platinum black can be considered as a perfectly black body as they absorb more than 98% of the incident radiant energy. Sir Ferry discovered that a sphere coated with lamp black can be used to absorb maximum radiation incident on it, known as Ferry’s black body.

Ferry’s black body:

Construction

It consists of double walled hollow sphere with small opening called aperture as shown in fig. The outer side of sphere is nickel polished and inner wall is coated with lamp black. There is a conical projection exactly at diametrically opposite to the aperture. As shown in fig. below.

Working

When the radiation is incident on the hole of this sphere, due to the conical projection the radiation that enters through hole suffers the multiple reflections. During each reflection more than 98% of the incident radiant heat gets absorbed, hence after few reflections all the heat is absorbed in the sphere. In this way sphere acts as a perfectly black body. But it should be noticed that, the cross-sectional area of the hole is the only effective area of this Ferry’s black body.

Some important points….!

1. The coefficient of absorption is one i.e. a = 1, while ‘t’ and ‘r’ are zero.
2. The emissive power of the perfectly black body at the given temperature is greater than any other ordinary body.

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