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IMAGING &
APPLICATIONS

Mirror - a reflective surface with a silver-polished back

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Concave mirrors - sections of a glass, hollowed sphere which are polished from the outside, and whose inner side reflects

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Concave mirrors convert all parallel beams of light to a single point known as the principal focus or converging point, in which rays of light will reflect and converge on a point 'F'. ​

  • Center of curvature (C) - the point where, if the curved mirror was spherical, the center would be

  • Radius of curvature - the distance between the center of curvature and the mirror

  • Focal length (f) - the distance between the mirror and the focal point

    • focal length = 1/2 radius of curvature

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IMAGES FORMED BY CONCAVE MIRRORS

If an object is distant (left of/beyond C on Fig. 2), the concave mirror forms an image that is

  • smaller

  • inverted

  • appears to float when looked at​

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If the object is even more distant, the reflected rays are nearly parallel and the image is formed near the principal focus. As the object is brought closer to the mirror, its reflected image moves further away from the mirror and gets larger.

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Reflected rays actually meet to form this image, which can be picked up by a screen. This is known as a real image, distinct from a virtual image, which can not be picked up by a screen. In a virtual image, the rays don't come to focus after the reflection, but instead diverge, appearing to come from a point behind the mirror

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The rules of reflection for a concave mirror are as follows:

  1. If the ray of light goes through C, it is reflected back through C
     

  2. If the ray of light is parallel to the principal axis, it is reflected back through F
     

  3. If the ray of light goes through F, it is reflected back parallel to the principal axis (inverse of rule two)
     

  4. If an object is between C and F, a concave mirror forms an image that is

    • placed at the center of curvature

    • real

    • inverted

    • the same distance from the mirror as the object
       

  5. If an object is between C and P, a concave mirror forms an image that is

    • magnified

    • upright

    • virtual​

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