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If a concave mirror of focal length 10 cm is immersed in water its focal length will ______________
1. be reduced
2. be increased
3. remain unchanged
4. changes sign

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Correct Answer - Option 3 : remain unchanged

Concept:

  • Spherical Mirrors: The mirrors having spherical reflecting surfaces are known as spherical mirrors. There are two types of spherical mirrors, concave mirrors, and convex mirrors.
    • Concave mirror: The mirror in which the reflecting surface is curved inward is called a concave mirror. The concave mirror tends to focus parallel rays of light coming to it at a single point,
    • Convex mirror: The mirror in which the reflecting surface is curved outward is called a convex mirror. The convex mirror tends to diverge parallel rays of light coming to it.
  • Focus of the Concave mirror: The point where parallel rays of light coming to the mirror converge at a point is called the focus of the mirror. 
  • Focal length (f): The distance between the focus and center of the mirror (pole) is called the focal length of the mirror.
  • The radius of curvature(R): The radius of the sphere from which the mirror has been carved out is called the radius of curvature.

R = f / 2

The focal length of the mirror depends only upon the curvature of the mirror and not on the medium in which it is kept.

Explanation:

  • So the focal length of the mirror does not depend upon the medium in which it is kept. This is because the light reflects back in the same medium and does not changes its medium. Also, the focal length of the mirror depends upon its radius of curvature only. 
  • So, When the concave mirror is kept in water, its focal length will not change.

  • The case will be similar to any spherical mirror (both concave and convex).
  • The focal length will change in the case of lenses.  This is because the light changes its medium of propagation in that case. The focal length will depend upon the refractive index of the medium in which lenses are kept.

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