Plane mirror

Definition of the plane mirror

A plane mirror is a mirror that has a flat surface, where one of its surfaces is coated with reflective metal so that the surface of the plane mirror can reflect more than 95 percent of the light that strikes it. If you shine a flashlight toward a plane mirror surface, the plane mirror reflects the light so that your eyes glare if plane mirror reflects the flashlight to your eyes. Conversely, if you stand in front of a plane mirror surface during the day, your face reflects sunlight toward the surface of the plane mirror, and at the same time, a plane mirror reflects light back toward you. If you stand in front of a plane mirror surface at night, then your face reflects the light of the electric light towards the plane mirror surface, and at the same time, the plane mirror reflects the light of the electric light back towards you.

Why does your face appear behind a plane mirror and what is the process of the image formation?

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The image formed by the plane mirror

Plane mirror 1Observe an example of image formation by a plane mirror as shown in the figure. Objects (green) are in front of a surface of the plane mirror. The object reflects the beam of light towards the surface of the plane mirror but is drawn as only two narrow or two beams of light. The plane mirror reflects the two rays affecting the surface of the plane mirror.

The incident light ray 1 is perpendicular or forms an angle of 90o with the normal line (normal line = an imaginary line perpendicular to the surface of the plane mirror that the light passes through) hence according to the law of reflection of light, reflected light ray 1 is also perpendicular or forms an angle of 90o with the plane mirror. The incident light ray 2 forms an angle of 30o with a normal line therefore according to the law of reflection of light; the reflected light ray two also forms an angle of 30o with a normal line.

The light coming from P to the plane mirror is reflected. When the light is reflected by a mirror, the direction of the reflected light seems to come from P’. If the object is replaced with a flashlight, the light emitted by the flashlight is also reflected by the plane mirror. The eye of the observer who only saw the reflection of the flashlight and did not know that the light reflected came from P’. The dashed line in the image represents the “path” behind the mirror and P’ is the image of the object. Because the light does not pass trough the plane mirror, the P’s image is a virtual image. If you place the screen at point P, there is no image of the object on the screen.

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