Karnataka Board PUCPUC Science 2nd PUC Class 12

A Person Has Near Point at 100 Cm. What Power of Lens is Needed to Read at 20 Cm If He/She Uses - Physics

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Answer in Brief

A person has near point at 100 cm. What power of lens is needed to read at 20 cm if he/she uses (a) contact lens, (b) spectacles having glasses 2.0 cm separated from the eyes?

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Solution

Given: A person has near point of 100 cm.
According to the question, it is needed to read at a distance of 20 cm.
(a) When the contact lens is used:
 u = – 20 cm = – 0.2 m,
 v = – 100 cm = – 1 m
The lens formula is given by

`1/v -1/u =1/f`

On substituting the values in the above formula, we get:

`1/f = 1/((-1)) -1/((-0.2)) = 4`

∴ Power of the lens, P = `1/f` = 4 D
(b) When the person wears spectacles at a distance of 2 cm from the eyes:
u = – (20 – 2) = – 18 cm = – 0.18 m,
v = – 100 cm = –1 m
The lens formula is given by

`1/v-1/u=1/f`

On substituting the values, we get:

`1/f = 1/((-1)) -1/((-0.18)) = 4.53`

∴ Power of the lens, P = `1/f` = 4.53 D

Concept: Lenses
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Chapter 19: Optical Instruments - Exercise [Page 432]

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HC Verma Class 11, Class 12 Concepts of Physics Vol. 1
Chapter 19 Optical Instruments
Exercise | Q 22 | Page 432

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