The focal length of a lens is $20 \ cm$ and its dispersive power is $0.08$. What is its longitudinal chromatic aberration in $cm$?

$A$ parallel beam of white light falls on a convex lens. Images of blue,yellow,and red light are formed on the other side of the lens at distances of $0.20 \ m$,$0.205 \ m$,and $0.214 \ m$ respectively. The dispersive power of the material of the lens will be:

The Cauchy's dispersion formula is:

$A$ medium is said to be dispersive,if

Explain dispersive and non-dispersive medium.

Consider the four different cases of dispersion of a light ray which has all the wavelengths from ${\lambda _1}$ to ${\lambda _2}$ (where ${\lambda _1} > {\lambda _2}$). The dotted line represents the light ray of wavelength ${\lambda _{avg}}$. Which ray diagram shows the maximum dispersive power?