Question

We consider the radiation emitted by the human body, which of the following statements is true?
(A) The radiation is emitted only during the day.
(B) The radiation is emitted during the summers and absorbed during the winters.
(C) The radiation emitted lies in the ultraviolet region and hence is not visible.
(D) The radiation emitted is in the infrared region.

Answer 1

Hint To answer this question, we need to examine the phenomena by which the radiation is emitted by a body. Then, we have to use the relation between the wavelength and the temperature to find out the region of electromagnetic spectrum corresponding to the emitted radiation.
The formula which is used in solving the above question is given by
$\Rightarrow \lambda = \dfrac{b}{T}$, here $\lambda $ is the wavelength of the radiation which is emitted by a body, which is at the absolute temperature of $T$.

Complete step by step answer
All the bodies in the universe emit the radiations. The reason is that the atoms inside the body keep on vibrating. So, they undergo acceleration. Therefore, the charges present inside it are also accelerated. We know that when a charge is accelerated, then it produces the electromagnetic radiation. Thus, the atomic vibrations result in the emission of the radiations from the bodies. Now, the frequency of the emitted radiations is proportional to the frequency of the atomic vibrations. So the larger the frequency of the emitted radiations, larger is the frequency of the emitted radiations, and hence shorter is the wavelength. Therefore the wavelength of the emitted radiations is inversely proportional to the temperature of the body. This relation is given by
$\Rightarrow \lambda = \dfrac{b}{T}$ (1)
Now, we have seen that each and every body in the universe emits radiation. So, the time and season are not the factors to consider this emission. Therefore, the options A and B are incorrect.
Now, we know that the temperature of the human body is
$\Rightarrow T = {37^ \circ }C = 310K$
Substituting in (1) we get
$\Rightarrow \lambda = \dfrac{b}{{310}}$
We know that $b = 2898\mu mK$. So we get
$\Rightarrow \lambda = \dfrac{{2898}}{{310}}$
On solving we get
$\Rightarrow \lambda = 9.35\mu m$
From the electromagnetic spectrum, we know that this wavelength corresponds to the infrared region. So the infrared radiations are emitted by the human body.
Hence, the correct answer is option D.

Note
As we know that the radiations emitted by the human body are not visible. The visibility of the radiations has been discussed in the option C. But along with this it is also saying that the radiations emitted are in the ultraviolet region, which is not true. So do not mark the answer solely on the basis of the visibility.