The ciliary muscles of an old man can raise the power of his eye lens to 42.8 D, and no more.
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
The ciliary muscles of an old man can raise the power of his eye lens to , and no more. The lens is from the retina. What is the nearest distance at which he can see clearly?
Need a reminder? The key ideaShow
The cornea and the eye lens form a real, inverted, small image on the retina, about behind the lens. The iris sets the size of the pupil. The ciliary muscles change the lens (accommodation). A normal eye sees clearly from its near point, about , to its far point at infinity.
The worked solution
Have you tried it?
Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Step 1. Far objects. With the retina away, a far object needs .
Step 2. The extra power. .
Step 3. The near point. .
Answer: his near point is .
Check. It is farther than : his lens cannot thicken enough. This is presbyopia.
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