A girl wears spectacles of power -2.25 D.
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
A girl wears spectacles of power .
- What is her defect?
- What is the focal length of her lenses?
- What is her far point, without the spectacles?
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. (a) The power is negative, so the lens is concave: myopia (short sight).
Step 2. (b) .
Step 3. (c) For short sight, , so the far point is .
Answer: (a) myopia. (b) . (c) .
Check. Without spectacles she sees clearly only up to about half a metre: quite short-sighted.
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