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To find the focal length of a convex mirror using a convex lens.

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To find the focal length of a convex mirror using a convex lens.

Convex Mirror – Focal Length:

The Theory: Convex mirror

A convex mirror is a curved mirror in which the reflecting surface bulges towards the light source. Convex mirrors reflect light outwards; therefore they are not used to focus light. A convex mirror is also known as fish eye mirror or diverging mirror.
The image formed by a convex lens is virtual and erect, since the focal point (F) and the centre of curvature (2F) are both imaginary points “inside” the mirror that cannot be reached. As a result, images formed by these mirrors cannot be projected on a screen, since the image is inside the mirror. Therefore, its focal length cannot be determined directly. The image is smaller than the object, but gets larger as the object approaches the mirror. The ray diagram of a convex mirror is shown below.

The focal length of a convex mirror can be determined by introducing a convex lens between the object and the convex mirror. An image can be obtained with the help of a convex lens side by side with the object when the convex mirror reflects the rays along the same path, i.e., when the rays fall normally on the mirror. Then, the radius of curvature, R, of the mirror is the distance between the screen and the mirror.

The focal length f of the convex mirror is calculated using the formula

Learning Outcomes:

Students understand the following terms:

  • Convex mirror
  • Focal point
  • Radius of curvature

Students are better able to do the experiment in the real laboratory once they understand the procedure.


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Materials Required:

  • Illuminated wire gauze
  • Stand
  • Screen
  • Meter scale
  • Convex lens
  • Given convex mirror

Real Lab Procedure:

  • Place a convex lens on a stand facing the illuminated wire gauze at a fixed distance, say 30 cm, away from the wire gauze.
  • Place a screen, at the other side of the lens so that the wire gauze, lens and screen are in a straight line.
  • Adjust the position of screen to get a clear image of the wire gauze.
  • Now fix the given convex mirror to another stand and place it in between the convex lens and screen with its reflecting face facing the wire gauze.
  • Now place another side by side with the wire gauze.
  • The position of the convex mirror is adjusted so that a clear image of the wire gauze is formed on the screen placed side by side with the wire gauze.
  • Measure the distance between the mirror and first screen and take as the radius of curvature of the mirror R.
  • The focal length of the mirror is calculated as ,
  • Repeat the experiment by moving the convex lens towards the object by 2 cm each time and record the values in a tabular column.