A student can see clearly only up to 65 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 student can see clearly only up to . What kind of lens does she need, and what are its focal length and 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 kind of lens. Far objects blur: short sight, so a concave lens.
Step 2. The focal length. .
Step 3. The power. .
Answer: a concave lens, , .
Check. The nearer the far point, the stronger (more negative) the lens.
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