The Basics of Light Reflection
Any mirror's job is to reflect light. Magnifying mirrors, otherwise known as concave spherical mirrors, not only reflect light, but also concentrate it in a way that makes the image larger. As with any mirror, magnifying mirrors work on the principle that the angle of incidence of a beam of light is equal to the angle of reflection of that same beam of light. This means that a beam of light that shines at a 45-degree angle to the surface of the mirror will reflect at a 45-degree angle in the other direction.
The Reflecting Properties of a Magnifying Mirror
A magnifying mirror is shaped similarly to a satellite dish in the sense that it is a concave mirror. This means that it, similarly to a satellite dish, focuses all the beams of light that hit it directly on to a central focal point. This means that the light--say, what reflects off your face as you look in the mirror--hits the mirror straight on, facing the face of the mirror and not at an angle to the mirror. All the beams of light hit the mirror and reflect off toward the focal point. This is a property of the parabolic shaping of the mirror. All parabolic-shaped dishes have this same property. This focal point of the mirror is different depending on the size of the mirror, and, by definition, it is where all beams of light reflected off the mirror converge.
The Magnifying Properties of a Magnifying Mirror
Because of the focusing of the light caused by the parabolic shape of magnifying mirrors, the light is concentrated more and more as it approaches the focal point. This concentration of the beams of light results in a magnification of the image reflected by the mirror. As the beams of light get closer and closer to the focal point, they become more and more magnified, which accounts for the increase in magnification as you step away from the mirror. However, once you pass the focal point, the magnification will start to reverse itself, and the image reflected will be upside-down. This is due to the fact that as the beams of light pass through the focal point of the mirror, the ones that were on the bottom start to be the beams on the top, meaning that every photon (packet of light) in the image is flipped from its original position.