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Question
A convex lens has refraction of 1.5 and 20 cm length of radius of both plane. What is the power of lens?

Answer
Assuming that n=1.5 is the index of refraction and that this is a double convex lens with both radii of curvature of R=20cm, the focal length of this lens will be given by the "Lensmaker's Formula":
1/f=(n-1)*(1/R1+1/R2)=(1.5-1)*(1/20+1/20)=0.5*(2/20)=0.5*(1/10)=0.05
Therefore the focal length ofthe lens f will be:
F=1/0.05=20cm
The corresponding "power" Mx of the lens is equal to 100cm divided by the focal length of the lens in centimeters:
For this lens the power will be:
Mx=100cm/20cm=5 diopters
It is worth noting that this is only an approximation since the actual magnifying power of a lens also incorporates the human eye which varies from individual to individual.

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James J. Kovalcin

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I am teaching or have taught AP physics B and C [calculus based mechanics & electricity and magnetism] as well as Lab Physics for college bound students. I have a BS in Physics from the University of Pittsburgh and a Master of Arts in Teaching from same. I have been teaching physics for 27 years.

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