Question

Two lenses of focal lengths 0.20 m and 0.30 m are kept in contact. Find the focal length of the combination. Calculate powers of two lenses and combination.

Answer 1

Focal length of each lens is given as $f_1=0.20$ m and $f_2=0.30$ m.

Focal length of the combination $\frac{1}{F}=\frac{1}{f_1}+\frac{1}{f_2}$

$\therefore$ $\frac{1}{F}=\frac{1}{0.20}+\frac{1}{0.30}$

$⟹$ $F=0.12$ m

Power of lens $P=\frac{1}{f}$

Power of first lens $P_1=\frac{1}{0.20}=5D$

Power of second lens $P_2=\frac{1}{0.30}=3.33D$

Power of combination of lens $P_c=\frac{1}{F}$

$⟹$ $P_c=\frac{1}{0.12}=8.33D$