We consider the radiation emitted by the human body. Which of the following statements is true :
A
The radiation emitted is in the infrared region
B
The radiation is emitted only during the day
C
The radiation is emitted during the summers and absorbed during the winters
D
The radiation emitted lies in the ultraviolet region and hence is not visible
The answer is A.
Solution
Every body above 0 K emits thermal radiation all the time (day and night, summer and winter), and also absorbs radiation from its surroundings. The wavelength of maximum emission is given by Wien's law, $\lambda_m T = b = 2.9 \times 10^{-3}$ m K. For the human body, $T \approx 310$ K: $\lambda_m = \dfrac{2.9 \times 10^{-3}}{310} \approx 9.4 \times 10^{-6}$ m $\approx 9.4\ \mu m$ This wavelength lies in the infrared region (longer than visible light, not shorter like ultraviolet). In general, a body at ordinary temperature emits radiation of longer wavelength than the visible and UV radiation it absorbs from sunlight. So the radiation emitted by the human body is in the infrared region.
ConceptHuman body emits infrared
Thermal Properties of Matter in past papers
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