The refractive index of light in glass varies with its wavelength according to equation
`mu (lambda) = a+ (b)/(lambda^(2))` where a and b are positive constants.

A nearly monochromatic parallel beam of light is incident on a thin convex lens as shown. The wavelength of incident light is `lambda_(0) +-Delta lambda` where `Delta lambda lt lt lambda_(0)`. The light gets focused on the principal axis of the lens over a region AB. If the focal length of the lens for a light of wavelength `lambda_(0)` is `f_(0)`, find the spread AB.

Answer

`f_(0) [(4b Delta lambda)/(lambda_(0) (a lambda_(0)^(2) +b - lambda_(0)^(2)))]`