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For a normal eye, the cornea of eye provides as converging power of 40 D and the lens least converging power of the eye lens behind the cornea is 20 D. Using this information , the distance between the retina and the cornea – eye lens can be estimated to be:

structure of human eye human eye and colourful world

  1. 1.5 cm

  2. 5 cm

  3. 2.5 cm

  4. 1.67 cm

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Correct Option: D
Explanation:

Given that,

Power ${{P} _{1}}=40\,D$

Power ${{P} _{2}}=20\,D$

Total power of the combination

  $ P={{P} _{1}}+{{P} _{2}} $

 $ P=40+20 $

 $ P=60\,D $

Now, focal length of the combination

  $ f=\dfrac{1}{60}\times 100 $

 $ f=\dfrac{5}{3}\,cm $

Now, for minimum converging state of the eye lens object is at infinity

  $ u=-\infty  $

 $ f=\dfrac{5}{3} $

Now, from lens equation

  $ \dfrac{1}{f}=\dfrac{1}{v}-\dfrac{1}{u} $

 $ \dfrac{3}{5}=\dfrac{1}{v}-\dfrac{1}{\infty } $

 $ \dfrac{1}{v}=\dfrac{3}{5} $

 $ v=\dfrac{5}{3}\,cm $

So, the distance between retina and cornea – lens system $1.67\ cm$