How does one focus on an airplane in the sky immediately after taking the eyes off from the pages of a book?

1 Answer
Jul 10, 2017

in such cases lens adapts by Accommodation, which involves changing optical power of the lens. It is done by reflex and ciliary muscles contract or relax depending on the distance of object from eye.

Explanation:

When someone is reading, the lens is involved in projecting an image on retina of printed words, held near the eyes.

To see an aeroplane in the sky, the lens must focus an image on retina of a distant object.

When light comes from a nearby source, light rays are more divergent while light rays coming from distant object tends to become parallel. Converging the two different types of rays on retina will require the lens to change its shape.

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When the object is nearby, lens becomes more rounded due to contraction of ciliary muscles.

On the other hand lens remains flattened when distant object is seen, and ciliary muscles remain relaxed.

(The pupil in the above mentioned case may also undergo changes in diameter, to adjust in different light conditions. To know more read this answer.)