Verified Solution • 2025–26 Session moderate

QJEEM25Ph9f03: JEE Main 2025 Physics - Class 12 JEEM25Ph9f03 Solution: A concave mirror of focal length in air is dipped in a liquid…

A concave mirror of focal length ff in air is dipped in a liquid of refractive index μ\mu . Its focal length in the liquid will be:

Multiple Choice Options

  • A μf\mu f
  • B ff
  • C fμ\frac{f}{\mu}
  • D f(μ−1)\frac{f}{(\mu-1)}

Step-by-Step Verified Solution

When a concave mirror is dipped in a liquid, its focal length is still determined by the geometry of the mirror. For a spherical concave mirror, the focal length is given by

f=R2f = \frac{R}{2}

where RR is the radius of curvature of the mirror. This derivation comes solely from geometrical considerations and the law of reflection, which states that the angle of incidence equals the angle of reflection. Since reflection is independent of the medium in which the mirror is immersed, the focal length remains unchanged.

Thus, even when the mirror is in a liquid of refractive index μ\mu , its focal length is still

f.f.

The correct answer is Option B: ff .

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