Ordinary mirrors, made by silvering the back face of a sheet of glass, suffer from the disadvantage that extra images are formed by reflection from the front surface of the glass and also by multiple internal reflection inside the glass. . Ordinarily these images are very much weaker than the image formed by the silvered surface and go unnoticed. But if silvered glass mirrors are used in certainoptical instruments, the extra images are a nuisance. For this reason plane mirrors in reflex cameras, and the curved mirrors of telescopes are aluminized on the front surface. Care should be taken when handling these, as the metallized surface is easily scratched.