Question

Why is near infrared spectroscopy used?

Answer 1

Because many organic compounds absorb in the infrared.

Explanation:

The length and tightness of a bond determines the frequency at which a certain bond will resonate/vibrate. (Compare this to strings in sound).

Each frequency corresponds to a wavelength (colour) in the electromagnetic spectrum. Many bonds in organic compounds resonate with wavelengths just beyond those of the visible spectrum, i.e. infrared.