Hi Dibyajyoti Mukherjee ,

since the human body is made up of water in large part, it is save to assume that the body's electromagnetic properties resemble strongly that of water.

The attached picture, from J, D. Jackson: "Classical Electrodynamics", 2nd ed., 1975, page 291, shows the dependency of the absorption coefficient of water on frequency. In the THz range, i. e. from 1012 to 1015 Hz there is a steep decrease by about 8 orders of magnitude in a narrow interval from near infrared to visible light.

So, we can expect that humans are quite opaque to THz radiation below about 300 THz but would be virtually transparent for visible light if they would consist exclusively of water. Of course, blood, pigments, bones etc. prevent complete transparency but it is possible with simple equipment (light source, light dependent resistor (LDR), and amplifier) to measure the variation of the transparency due to blood circulation in a fingertip, for example.

In the domain of medical imaging (of parts of the human body), a sophisticated method employing near infrared light is Optical Coherence Tomography (OCT). Please see as introduction this Wikipedia article:

https://en.wikipedia.org/wiki/Optical_coherence_tomography

Please study following links:

Article The interaction between electromagnetic fields at megahertz,...

https://bwn.ece.gatech.edu/papers/2014/c7.pdf

https://par.nsf.gov/servlets/purl/10080953

The absorption of the water molecules to the electromagnetic waves is due to the polarization mechanism where there is orientation polarization at relatively low frequencies after which comes the absorption due to ionic polarization at the long infrared and after that comes the electronic polarization at extremely high frequency. There is also the attenuation due to the conductivity of the water because of the salt contained in it. May be the salty water is very similar in response to the human body.

Best wishes