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What happens to the focal length of a concave mirror when it is immersed in water?
It increases
It decreases
It remains the same
It becomes zero
It remains the same
The focal length of a concave mirror remains unchanged when it is immersed in water. Unlike lenses, the reflection of light by a spherical mirror depends entirely on the laws of reflection, which are independent of the refractive index of the surrounding medium.
The focal length of a concave mirror remains unchanged when it is immersed in water. Unlike lenses, the reflection of light by a spherical mirror depends entirely on the laws of reflection, which are independent of the refractive index of the surrounding medium.
Think of a shiny metallic spoon acting as a mirror; if you submerge it in a bowl of water, its physical shape and curvature do not change, meaning its ability to focus light remains exactly the same.
f=2RтАЛ тАФ Relationship between focal length and radius of curvature for a spherical mirror
The focal length of a spherical mirror is determined solely by its radius of curvature (R), as given by the formula f=2RтАЛ. Because reflection occurs at the surface boundary and relies on the angle of incidence equaling the angle of reflection, changing the surrounding medium from air to water does not affect the focal length.
A mirror's focal length depends only on its radius of curvature.
The refractive index of the surrounding medium affects refraction (lenses), not reflection (mirrors).
Immersing a mirror in water, oil, or any other transparent medium has zero effect on its focal length.
Design of optical instruments used in various fluid or underwater environments where focusing stability is required.
For lenses, the focal length changes when immersed in water because refraction depends on the relative refractive index of the lens and the medium.
Options A, B, and D are incorrect because the surrounding medium has no effect on the reflection geometry of a mirror.
C is correct тАФ The focal length of a concave mirror remains the same when immersed in water because reflection is independent of the surrounding medium's refractive index.
Always distinguish between mirrors and lenses in competitive exams! Lens focal length changes with the surrounding medium (Lens Maker's Formula), whereas mirror focal length (f=R/2) remains constant regardless of the medium.
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