Optics Question 822
Question: 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
Options:
A) +1.5
B) -1.5
C) +6.67
D) -6.67
Show Answer
Answer:
Correct Answer: B
Solution:
[b] $ f _{1}=+40cm=0.4m $ (for convex lens) $ f _{2}=-25cm=-0.25m $ (for concave lens)
$ \therefore $ Focal length (f) of the combination $ \frac{1}{f}=\frac{1}{f _{1}}+\frac{1}{f _{2}}=\frac{1}{0.40}-\frac{1}{0.25}=\frac{0.25-0.4}{0.40\times 0.25} $
$ =-\frac{0.15}{0.1}=-1.5\text{ dioptre}\text{.}\Rightarrow \text{P=}\frac{1}{f}=-1.5\text{ dioptre} $
