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Reflection in a Plane Mirror

A highly polished plane surface is called a plane mirror. When you stand before a plane mirror, your image is seen in the mirror. The image appears to be as far behind the mirror as you are in front of the mirror. This image cannot be caught on the screen. Such images are called virtual images (Figure 9.4). Now raise your right hand. It appears as though you have raised your left hand in the image. This reversing of the image, that is, right side appearing as left side and vice versa is called ‘lateral inversion’. We also notice that size of the object and size of the image are equal.
Description: Description: 87238.png
Figure 9.4 Formation of Image of an Extended Object in a Plane Mirror

Characteristics of the Image Formed by a Plane Mirror

The image formed by a plane mirror is
  1. upright,
  2. virtual,
  3. of the same size as the object,
  4. laterally inverted and
  5. as far behind the mirror as the object is in front of it.
Effect of Rotation of the Mirror
If a plane mirror is rotated at an angle θ, with the incident ray fixed, then the reflected ray rotates through an angle 2θ.

Multiple Reflection

For an object kept between two mirrors, we get many images of the object. This is because of the multiple reflections between the two mirrors. The image formed by one mirror acts as an object for the other mirror. The number of images formed is generally odd and depends on the angle between the mirrors. If two plane mirrors make an angle θ with each other, the number of images formed ‘n’ is given by
Description: 82111.png
If Description: 87158.png is odd.
Description: 82126.png, if the object is situated symmetrically between the mirrors. (Here, n is even.)
Description: 82133.png, if the object is situated unsymmetrically between the mirrors.


Uses of Plane Mirrors
Plane mirrors are used
  1. as dressing mirrors,
  2. by opticians to increase the effective length of letter chart while eye testing,
  3. in saloons for rear view and
  4. in instruments such as periscope, kaleidoscope, etc.

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