A combination of two thin lenses in contact is to be made which has the same focal length for blue and red light. . (Such a combination is known as an ' achromatic doublet '). Show that the ratio of their focal lengths (for lengths (for yellow light ) must be equal in magnitude and opposite in sign to the ratio of the dispersive powers of the material of the two lenses. b. Use the results in (a) to suggest a way of removing chromatic aberration of the convex lens of focal length 15 cm , made of flint glass. You are given convexand concave lenses (made of crown glass ) of various focal lengths. The ratio of the dispersive powers of flint glass to crown glass is about 1.5

A combination of two thin lenses in contact is to be made which has th

1.	A combination of two thin lenses in contact is to be made which has the same focal length for blue and red light. . (Such a combination is known as an ' achromatic doublet '). Show that the ratio of their focal lengths (for lengths (for yellow light ) must be equal in magnitude and opposite in sign to the ratio of the dispersive powers of the material of the two lenses. b. Use the results in (a) to suggest a way of removing chromatic aberration of the convex lens of focal length 15 cm , made of flint glass. You are given convexand concave lenses (made of crown glass ) of various focal lengths. The ratio of the dispersive powers of flint glass to crown glass is about 1.5
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