Ordinary glasses are worn in front of the eye and usually 2.00 cm in front of th

Question

Ordinary glasses are worn in front of the eye and usually 2.00 cm in front of the eyeball. A certain person can see distant objects well, but his near point is 54.0 cm from his eyes instead of the usual 25.0 cm . Suppose that this person needs ordinary glasses.

Part A

What focal length lenses are needed to correct his vision ?

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Part B

What is their power in diopters?

Ordinary glasses are worn in front of the eye and usually 2.00 cm in front of the eyeball. A certain person can see distant objects well, but his near point is 54.0 cm from his eyes instead of the usual 25.0 cm . Suppose that this person needs ordinary glasses.

Part A

What focal length lenses are needed to correct his vision ?

f = cm

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Part B

What is their power in diopters?

D = diopters

Explanation / Answer

A converging or positive lens is required, with
a power such that when an object is placed 25 cm in front of the
lens, the lens forms a virtual image.
The image distance di= 54 cm, so that this image serves as an
object for the eye at the eye’s near point.
Taking into account the distance 2 cm
di =0.56 m, do =0.27m
Applying the thin lens equation, we nd
1/F =1/do +1/di = 1/0.27 +1/0.56 =
F = 0.182m= 18.2cm

What is their power in diopters?

D =1/F = 1/18.2 = 0.0549 diaptors

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