Question

The focal length of a thin convex lens for red and violet colour are `44.6cm` and `42.5cm`. Calculate focal length for the mean colour and dispersive power of the lens.
A. Focal length `=43.53cm` dispersive power `=0.048`
B. Focal length `=28.53cm` dispersive power `=0.048`
C. Focal length `=63.53cm`, dispersvie power `=8.48`
D. Foca length `=30.43cm,` dispersive power `=4.8`

Answer 1

(a) Mean total length `=f=sqrt(f_(r)f_(v))`
`f=sqrt(44.6xx45.5)=43.53cm`
`:.` Dispersive power `=(f_(r)-f(v))/f=(44.6-42.5)/43.33=0.048`