Are concave mirror is a part of sphere of radius 40 cm What is the focal length of the mirror?

Option 2 : 60 cm from the mirror on the side of the object

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Concept:

  • Spherical mirrors are of two types - Convex mirror, and Concave mirror.
    • The image formed by the spherical mirrors depends on the position of the object.

  • The relationship between the focal length (f) and the radius of curvature (R) is given by -

\(f = \frac{R}{2}\)

  • Concave mirror: If the inner surface of the spherical mirror is the reflecting surface, then it is called a concave mirror. It is also called a focusing mirror/converging mirror.
    • The size of the image produced by these mirrors can be larger or smaller than the object, depending upon the distance of the object from the mirror.
    • The concave mirror can form both real as well as virtual images of any object.
  • Mirror formula: The expression which shows the relation between object distance (u)image distance (v), and focal length (f) is called mirror formula.

\(\frac{1}{f} = \frac{1}{u} + \frac{1}{v}\)

Calculation:

Given,

Radius of curvature of concave mirror (R) = -40 cm

So focus = R/2 = -40/2 = -20 cm

Object is placed in front of mirror (u) = -30 cm

Image position (v) =?

We know that,

\(\frac{1}{f} = \frac{1}{u} + \frac{1}{v} \Rightarrow \frac{1}{v} = \frac{1}{f} - \frac{1}{u}\)

\( \Rightarrow \frac{1}{v} = \frac{1}{{ - 20}} - \frac{1}{{ - 30}} = \frac{{ - 3 + 2}}{{60}} = - \frac{1}{{60}}\)

\(\frac{1}{v} = - \frac{1}{{60}} \Rightarrow v = - 60\;cm\)

The image is formed at 60 cm in front of mirror or on the side of object.

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A concave mirror is a part of a sphere of radius 40cm. What is the focal length of the mirror?

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