Maran, at the end of Siman 302, states, "It is prohibited to gaze into a metal mirror, which is sharp as a blade, as we are concerned he might use it to cut dangling hairs. This applies even to a mirror affixed to a wall. However, if it is not sharp, it is permitted to gaze into it."
While this type of mirror is not found today, and therefore it is permitted to look into our mirrors, Hacham Bension learns from this Halacha a practical application. He rules that one may not use a metal knife as a mirror on Shabbat, out of concern that he may cut his hair with it, based on what he sees.
SUMMARY
It is permitted to look into a mirror on Shabbat.
One should not use a knife as a mirror on Shabbat.
