If the focal length of a double convex lens for red light is fR its focal length for the violet light is:

Question

If the focal length of a double convex lens for red light is fR its focal length for the violet light is: (A)  fR  (B) greater than fR  (C) less than fR  (D) 2 fR

Tardigrade Admin · Accepted Answer

In a double convex lens the speed of violet light is minimum while that of red light is maximum. So, the refractive index (n) of glass is maximum for violet light and minimum for red light (nV-nR). Hence according to the formula (1/f)=(n-1)( (1/R1)-(1/R2) ) the focal length fv of the lens for the violet light is least and the focal length fR for the red light is largest i.e., fR>fV.

Q. If the focal length of a double convex lens for red light is fR its focal length for the violet light is:

$f_{R}$

greater than $f_{R}$

less than $f_{R}$

2 $f_{R}$

Q. If the focal length of a double convex lens for red light is fR its focal length for the violet light is:

fR​

greater than fR​

less than fR​

2 fR​

$f_{R}$

greater than $f_{R}$

less than $f_{R}$

2 $f_{R}$