A plane mirror is a mirror with a flat surface, i.e. a plane. Bathroom mirrors and dressing mirrors are both examples of plane mirrors. All plane mirrors share certain physical characteristics which they do not share with curved mirrors such as concave mirrors.
The Angle of Reflection is Equal to the Angle of Incidence
When a beam of light strikes a plane mirror, it bounces off again at an angle equal to the angle it struck at. For example, if a light ray struck the mirror at 45° from perpendicular, it would bounce off of the mirror at a 45° angle. If light strikes a mirror straight on (with an angle of incidence of 0°) it is reflected straight back. Scientists state this rule as "the angle of reflection is equal to the angle of incidence."
Mirrors Create Virtual Images
When light from an object hits a mirror straight on, it creates a virtual image. This image is called "virtual" because it appears in a place different from where the light comes from. The image appears to be behind the mirror, but the light actually comes from in front of the mirror.
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Mirrors Creat Proportional Images
Although the image behind the mirror is a virtual image, it is governed by rules. The distance of the virtual object behind the mirror is equal to the distance of the real object from the mirror. For example, if you stood 2 feet in front of a mirror and looked at your image, you would see a reflection of yourself 2 feet behind the mirror, or four feet in front of you. The proportions of that object are identical to the real object at that distance. So if a six-foot-tall man with shoulders two feet wide regards himself in a plane mirror two feet before him, he will see a six-foot-tall man with two-foot-wide shoulders four feet away.