Ncert class 12 9.38

Dear Swapan,

Focal length of the convex lens, f₁ = 30 cm

The liquid acts as a mirror. Focal length of the liquid = f₂

Focal length of the system (convex lens + liquid), f = 45 cm

For a pair of optical systems placed in contact, the equivalent focal length is given as:

Let the refractive index of the lens be and the radius of curvature of one surface be R. Hence, the radius of curvature of the other surface is −R.

R can be obtained using the relation:

Let be the refractive index of the liquid.

Radius of curvature of the liquid on the side of the plane mirror =

Radius of curvature of the liquid on the side of the lens, R = −30 cm

The value of  can be calculated using the relation:

Hence, the refractive index of the liquid is 1.33.

Best Of luck

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