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  4. A parallel beam of white light fall on a combination of a concave and a convex lens, both of the same material. Their focal lengths are 15 cm and 30 cm respectively for the mean wavelength in white light. On the same side of the lens system, one sees coloured patterns with violet colour nearer to the lens.

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Q. A parallel beam of white light fall on a combination of a concave and a convex lens, both of the same material. Their focal lengths are 15 cm and 30 cm respectively for the mean wavelength in white light. On the same side of the lens system, one sees coloured patterns with violet colour nearer to the lens.

IIT JEEIIT JEE 1988

Solution:

Focal length of concave is less, i.e. power of concave lens
will be more. Hence, the combination will behave like a
concave lens. Further, $\mu_v$ is greater than all other colours.
Hence, $f_v$ will be least.

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