Question

Achromatic combination of lenses comprises of the two lenses of same material placed $4$cm apart. If the focal length of one lens is $5$cm, the focal length of the other lens is:

• A. $2$cm
• B. $4$cm
• C. $6$cm
• D. $3$cm

Answer 1

The correct option is D $3$cm

Given distance between the lenses =$4$ cm and focal length of first lens = $5$ cm and let the focal length of second lens = $y$ cm

Thus we know,

Distance between the lenses = $\frac{f_1+f_1}{2}$

$4\times 2=5+y$

$y=3$ cm