Optics Ques 71
- An eye specialist prescribes spectacles having combination of convex lens of focal length $40$ $ cm$ in contact with a concave lens of focal length $25$ $ cm$. The power of this lens combination in diopters is
(1997, 2M)
(a) $+1.5$
(b) $-1.5$
(c) $+6.67$
(d) $-6.67$
Show Answer
Answer:
Correct Answer: 71.(b)
Solution:
Formula:
- The focal length of combination is given by
$\frac{1}{F} =\frac{1}{40}-\frac{1}{25} \quad (\frac{1}{F}=\frac{1}{F _1}+\frac{1}{F _2})$
$\text { or } \quad F =-\frac{200}{3} cm$
$ =-\frac{2}{3} m$
$\therefore$ Power of the combination in dioptres,
$ \begin{aligned} P & =-\frac{3}{2} & [P=\frac{1}{F(m)}] \\ & =-1.5 & \end{aligned} $
BETA
