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Question

# Focal length of a lens depends on the (a) Medium constituting lens only. (b) Medium constituting the surroundings only. (c) Medium constituting the lens as well as surroundings. (d) Wavelength of incident light.

A
only (a)
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B
(a) and (d) both.
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C
(c) and (d) both
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D
only (b)
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Solution

## The correct option is C (c) and (d) bothFrom lens maker's formula, 1f=(μ2μ1−1)(1R1−1R2) Where μ1 is refractive index of surrounding and μ2 is refractive index of lens having radius of curvature R1 and R2 respectively. ⇒R1 and R2 being the radii of curvature are fixed for a given lens. Also, μ=cv=cfλ i.e. μ∝1λ Refractive index of a material is inversely proportional to the wavelength of incident light. ∴(c) is correct. Why this question?Tip: For a comprehensive examination offocal length of a lens, analyse the parametersinvolved in lens maker's formula.

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