Focal length of a convex lens will be maximum | vedclass

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(D) According to the lens maker's formula,the focal length $f$ of a lens is given by $\frac{1}{f} = (\mu - 1) \left( \frac{1}{R_1} - \frac{1}{R_2} \ri

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Red light

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(D) According to the lens maker's formula,the focal length $f$ of a lens is given by $\frac{1}{f} = (\mu - 1) \left( \frac{1}{R_1} - \frac{1}{R_2} \right)$.This implies $f \propto \frac{1}{\mu - 1}$.According to Cauchy's dispersion formula,the refractive index $\mu$ is related to the wavelength $\lambda$ as $\mu = A + \frac{B}{\lambda^2}$. Thus,$\mu$ is inversely proportional to $\lambda$ $(\mu \propto \frac{1}{\lambda})$.Combining these,we get $f \propto \lambda$.Since the wavelength of red light $({\lambda_r})$ is greater than the wavelength of blue,green,or yellow light,the focal length $f$ will be maximum for red light.

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