MediumPhysicsHigh School (9–10)Optics › Lenses and Optical InstrumentsNumericalBoard ExamAEEE (Amrita)AMUEEEAP EAPCETBCECECOMEDK UGET
A doctor prescribes spectacles of power -3.5 D for a student who cannot see distant things clearly…
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
A doctor prescribes spectacles of power for a student who cannot see distant things clearly. What kind of lens is it, and what is its focal length?
Need a reminder? The key ideaShow
Power with in metres, in dioptres (D). Convex lenses have a positive power, concave lenses a negative power. For thin lenses in contact, the powers add: .
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. The power is negative, so the lens is concave.
Step 2. The focal length. .
Answer: a concave lens of focal length (written ).
Check. A concave lens spreads light out. The next chapter shows why that helps such an eye.
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