The dispersive powers of glasses of lenses used in an achromatic pair are in the ratio 5 : 3. If the focal length of the concave lens is 15 cm , then the nature and focal length of the other lens would be

Options

A. convex, 9 cm
B. concave, 9 cm
C. convex, 25 cm
D. concave, 25 cm

Answer

A

Explanation

(a) From condition of achromatism,
`omega_(1)/(f_(1))+omega_(2)/(f_(2))=0omega_(1)omega_(2)=-f_(1)/(f_(2))rArr5/3=(-(-15))/(f_(2))`
Focal length, `f_(2)`= 9 cm (convex)