Question

Which of the following rays is emitted by a human body?
A. X-rays
B. Visible rays
C. UV rays
D. IR rays
E. none of these

Answer 1

Hint: Electromagnetic radiations (EMRs) include- gamma rays, x rays, UV rays, visible rays, IR rays, microwaves, and radio waves. The human body also emits radiation in the form of thermal radiation because anybody or object that has non-zero temperature will always emit EMR.


Complete answer: Option (A) is incorrect. X-ray is ionizing radiation with a smaller wavelength. Thus, it possesses higher energy and can pass through objects including the human body. It has been used to examine or visualize most areas of the human body such as bones or joints, etc.
Option (B) is incorrect. Visible rays or light rays are those rays in EMR that are visible to the naked eye. They possess a wavelength in the range of 400-700 nm. Visible light is very important to humans because, without these rays, a human cannot see any images of the object.
Option (C) is incorrect. UV rays mostly come from the sun. These rays can cause sunburn, skin cancer, etc. as they are very harmful. They have wavelengths shorter than that of visible light and longer than that of X rays.
Option (D) is correct. IR rays are the rays that are emitted by a human body. Since no human body can have a non-zero temperature; therefore, radiations are emitted in the form of thermal radiations. These radiations include all the radiations given off by an object because of its temperature. Hence, these include IR rays, radio waves, and even visible light. Thus, the wavelength of radiation emitted depends on the temperature of the body. In the case of a human body, the radiation emitted is mainly at the wavelength of about ${12}$ microns and the wavelength of IR rays falls within the range of $0.75 - 1000$micron.
Option (E) is incorrect. As an option (D) is correct, therefore ‘none of the above’ option cannot be correct.

Hence, the correct answer is option (D).

Note: Thermal radiations, as the name indicates, only transfer heat that indicates the temperature of the source. IR rays are mostly emitted by the human body because the thermal radiation that is emitted by the human body mainly falls within the range of $0.75 - 1000$micron (wavelength of IR rays) i.e., ${12}$ microns.