Question

An ideal black body is a:
(a) Lump of charcoal heated to a high temperature
(b) Metal coated with a black dye
(c) Glass surface coated with a coal tar.
(d) Hollow enclosure blackened inside and having a small hole.

Answer 1

Hint: As this question talks about a black body, so we will use this concept only and apply it on the options one by one and find the result. It will be better to discard the options if they contain any shiny particle in them.

Complete answer:
To answer this question we will first understand the concept of black body. A black body is a body which absorbs the radiation up to any wavelengths inside it. The radiation of light if entered in a black body will never be able to come out by reflection as reflection makes it trapped inside itself.

If we see the options one by one then we will find out that a lump of the charcoal has a shiny part in its surface which obviously reflects some amount of light after the light touches its surface. This shiny part is called graphite. When it is heated to a high temperature then its reflectance gets increased.
If a black dye coating is done with a strong thermal emission then only the metal will not reflect light. But due to the presence of some fluorescent minerals, it emits a small amount of light.

Since, a coal tar is light type oil so even after applying its coating on a glass surface it will also emit light.
But a hollow enclosure which is blackened from inside with a small hole in its surface will not emit light at all. This is because of the reflection being done internally inside the box once the light reaches it. The reflection takes place from the inner surfaces of any object like this.

This can be understood from the following diagram as well.

Clearly, from the diagram we can see that the light is entering an enclosure opened by a small hole but there are minute chances of the light to come out from it.

So, the correct answer is “Option D”.

Note:
We need to learn concepts related to black body as:
(1) A black body is just an ideal body which has no property of reflecting any source.
(2) Any black body or any enclosure like this will have complete absorption of any source coming towards it.
(3) We will find a black body without any transference of light or like source because it will never reflect it hence, it will never transmit light or like source.