Question

$f_B$ and $f_R$ are focal lengths of a convex lens for blue and red light respectively and $F_B$ and $F_R$ are the focal lengths of the concave lens for blue and red light respectively. We must then have :

• A. $f_B>f_R$ and $F_B<F_R$
• B. $f_B<f_R$ and $F_B>F_R$
• C. $f_B>f_R$ and $F_B>F_R$
• D. $f_B<f_R$ and $F_B<F_R$

Answer 1

The correct option is D $f_B<f_R$ and $F_B<F_R$

Blue light has less wavelength than the red light, so, it will deviate more as compared to red so, it will converge at a shorter distance for parallel beams passing through a convex lens or appear to have short focal length in concave lens. So, blue focal length will be less for both convex and concave lenses.

So, $f_B<f_{R}and{F}_B<F_R.$