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

$A$ medium is said to be dispersive,if

Calculate the dispersive power for crown glass from the given data if $\mu_v = 1.5230$ and $\mu_r = 1.5145$.

Dispersive power depends on the following:

Let us list some of the factors,which could possibly influence the speed of wave propagation:
$(i)$ nature of the source.
$(ii)$ direction of propagation.
$(iii)$ motion of the source and/or observer.
$(iv)$ wavelength.
$(v)$ intensity of the wave.
On which of these factors,if any,does
$(a)$ the speed of light in vacuum,
$(b)$ the speed of light in a medium (say,glass or water),depend?

$A$ convex lens,a glass slab,a glass prism,and a solid sphere are all made of the same glass. The dispersive power will be: