What would happen to the focal length of a spherical mirror and lens if they are immersed in water?

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Question

If a spherical mirror is immersed in water, does its focal length change?

Asked by: Amanda Answer

Mirrors, spherical or otherwise, operate on the principle that the angle of reflection of a ray of light equals the angle at which it strikes the mirror's surface. That behavior is not affected by the medium in which the light travels, so the mirror's focal length would be no different in air or water. If you had asked about the focal length of a LENS, however, the answer would be different. Lenses work by REFRACTION, not reflection, and the angle of refraction is based on the difference between the speed of light in the lens compared to the surrounding medium. When immersed in water, the difference between light's speed in the lens and the surrounding medium is less than the difference when it's in air, and the focal length of a lens would be greater under water.
Answered by: Paul Walorski, Part Time Physics Instructor

What happens to the focal length of a convex lens when it is immersed in water?

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CONCEPT:

A spherical mirror is a mirror whose reflecting surface is a part of a hollow sphere of glass.
One side of the mirror is well polished and reflecting, and another side of the mirror is opaque (often painted red).
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.

EXPLANATION:

In case of spherical mirrors, there will be no change in the focal length of the mirrors when immersed in water.
It is so because the focal length of mirrors does not depend on the external medium in which it is held.

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Hint The lenses use the phenomenon of refraction to converge or diverge a beam of light. A spherical mirror uses the phenomenon of reflection to converge or diverge a beam of light. The refractive index of water is different from the refractive index of air.

Complete Step by step answer

A concave mirror is a spherical mirror that is polished on the surface which is curved inward. The polished curved surface reflects any light falling on it according to the law of reflection, which states that the angle of incidence should be equal to the angle of reflection. The focus of a concave mirror is situated at a distance which is half of the radius of curvature of the mirror.When kept inside water, the laws of reflections are still valid, thus the focal length of the concave mirror does not change.A convex lens is an optical lens, which is curved outwards from one side or both sides. It is known as a converging lens because its shape deviates a beam of light in such a way that the rays get converged after refracting from the convex lens. The refraction of light when it travels from one medium to the other depends on the refractive index of both mediums. We know that the refractive index of air is different than that of water, the converging capacity of a convex lens will change when it is immersed in water.Since water has a greater value of refractive index than air, the converging power of the lens will be less as compared to air. Thus the focal length of the convex lens will increase.

Note The direction of light changes only when it travels through two mediums of different refractive indexes. Otherwise, the light propagates in a straight line when the medium has a constant value of refractive index. Thus the light will travel in a straight line, be it air or water.