A scanning optical system using a short-wavelength light of 500 nm or less
uses a reflecting mirror having a higher absolute reflectivity and having
reduced wavelength and angle dependences. Divergent ray of light emitted
from a semiconductor laser is converted into an approximately parallel
light beam by a collimator lens and the diameter of the light flux is
reduced by an aperture before travel to a polygon mirror. The light beam
from the polygon mirror passes through scanning lenses to form a small
spot at any point in the entire scanning area. The semiconductor laser is
a gallium nitride semiconductor laser having an oscillation wavelength of
408 nm. The polygon mirror has such a characteristic that, if the complex
refractive index N of a metallic film contributing to a reflection
characteristic of the reflecting mirror is defined as
N(.lambda.)=n(.lambda.)-ik(.lambda.), then
k(.lambda.)>(-n(.lambda.).sup.2 +18n(.lambda.)-1) is satisfied.
