A man cannot see clearly anything closer than 110 cm…
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
A man cannot see clearly anything closer than . What lens lets him read a book at ? Find its focal length and its power.
Need a reminder? The key ideaShow
Short sight (myopia): far objects blur; a concave lens with . Long sight (hypermetropia): near objects blur; a convex lens that makes a page at appear at , from . Power with in metres. Old age (presbyopia) may need bifocals.
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. The distances. and .
Step 2. The focal length. , so .
Step 3. The power. .
Answer: a convex lens of focal length and power .
Check. Long sight needs a positive power.
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