Light is made up of a spectrum or broad range of wavelengths. The human eye perceives different wavelengths as different colors. Different wavelengths or colors can transmit or pass through various substances differently.

Consider the ocean. It appears blue because blue lights transmit through water better than other colors. Colors like red or green can transmit a short distance, but get absorbed much faster than blues. The human body can transmit a whole spectrum of wavelengths. The human eye can’t perceive most of those wavelengths but modern technology can. 

The way we can test blood glucose is by shining light (painless–you would feel as much as if a flashlight was shining on you) through your skin. We measure the amount of light going into your skin, and the amount of each wavelength of light that is reflected back. And by those ratios we can determine the quantity of glucose in your blood. The measurement and analysis of wavelengths of a spectrum is called spectroscopy.