Question
Physics Question on Ray optics and optical instruments
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
A
1.5
B
-1.5
C
6.67
D
-6.67
Answer
-1.5
Explanation
Solution
The focal length of combination is given by \hspace15mm \frac{1}{F}=\frac{1}{40}-\frac{1}{25}\hspace10mm \Bigg(\frac{1}{F}=\frac{1}{F_1}+\frac{1}{F_2}\Bigg) or \hspace15mm F=-\frac{200}{3}cm=-\frac{2}{3}m ∴ Power of the combination in dioptres, \hspace15mm P=-\frac{3}{2}\hspace15mm \Bigg[P=\frac{1}{F(m)}\Bigg] =-1.5