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Does focal length of a lens depend on the colour of the light used? Does focal length of a mirror depend on the colour?

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#### Solution

Yes, the focal length of a lens depends on the colour of light.

According to lens-maker's formula,

\[\frac{1}{f} = (\mu - 1)(\frac{1}{R_1} - \frac{1}{R_2})\]

Here, *f* is the focal length, *μ* is the refractive index, *R* is the radius of curvature of lens.

The refractive index (*μ*) depends on the inverse of square of wavelength.

The focal length of a mirror is independent of the colour of light.

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