<p>Disclosed is a mirror which comprises a plurality of dielectric layers that are laminated in a first direction. The thickness of each one of the plurality of dielectric layers is 1/2 of the design wavelength. One of the plurality of dielectric layers has: a first portion that has a thickness of 1/8 of the design wavelength and a first partial refractive index; a second portion that is laminated on the first portion and has thickness of 1/8 of the design wavelength and a second partial refractive index that is lower than the first partial refractive index; and a third portion that is arranged between the first portion and the second portion and has a thickness of 1/4 of the design wavelength and a third partial refractive index that gradually decreases from the first portion side toward the second portion side. The dispersion of the third partial refractive index with respect to the thickness direction is larger than the dispersion of the first partial refractive index with respect to the thickness direction and the dispersion of the second partial refractive index with respect to the thickness direction. Consequently, the mirror has low loss and high reflection.</p>
