An optical system comprising: a light source; a photodetector; a first light-receiving system for causing the photodetector to receive first reflected light with a first angle of reflection from a surface; and a second light-receiving system for causing the photodetector to receive second reflected light with a second angle of reflection, different from the first angle of reflection, from the surface is provided. Here, an incident area on the surface, in which light generating the first reflected light is incident, is spaced apart from an exiting area on the surface, which light, to be incident on the photodetector from the surface via the second light-receiving system, exits.