An optical system includes a light sending section that sends light to an object having a scattering medium and a lower medium positioned below the scattering medium, where the scattering medium scatters light and the lower medium feeds back polarized light in response to light incident thereon, a light receiving section that receives (i) light that is sent from the light sending section and then scattered by the scattering medium and (ii) light from the lower medium, and a thickness calculating section that calculates a thickness of the scattering medium, by referring to at least one of a non-polarization component and a polarization component of the light received by the light receiving section.
