Ray Optics and Optical Instruments - Result Question 67
71. For a normal eye, the cornea of eye provides a converging power of $40 D$ and the least converging power of the eye lens behind the cornea is $20 D$. Using this information, the distance between the retina and the eye lens of the eye can be estimated to be
(a) $2.5 cm$
(b) $1.67 cm$
(c) $1.5 cm$
(d) $5 cm$
[2013]
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Answer:
Correct Answer: 71. (b)
Solution:
- (b) $P _{\text{cornea }}=+40 D$
$P_e=+20 D$
Total power of combination $=40+20=60 D$
Focal length of combination $=\frac{1}{60} \times 100 cm$
$=\frac{5}{3} cm$
For minimum converging state of eye lens,
$u=-\infty \quad v=? \quad f=\frac{5}{3}$
From lens formula,
$\frac{1}{f}=\frac{1}{v}-\frac{1}{u} \Rightarrow v=\frac{5}{3} cm$
Distance between retina and cornea-eye lens
$=\frac{5}{3}=1.67 m$
