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Question

A lady uses +1.5D glasses to have normal vision from 25cm onwards. She uses a 20D lens as a simple microscope to see an object. Find the maximum magnifying power if she uses the microscope together with her glass? Do the answers suggest that an object can be more clearly seen through a microscope without using the correcting glasses?

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Solution

The lady uses glasses of +1.5 D to have a normal vision from 25 cm onwards.
Least distance of clear vision,D=25cm
Focal length of the glasses, f=1 power=11.5 m
She should have the greater least distance of distinct vision without the glasses.
Take:
u=25 cm=0.25 m
The lens formula is given by
1v+1u=1f

Putting the values, we get:
v=0.4m=40cm

Near point without glasses =40cm
The focal length of the magnifying glass,f=0.05 m=5 cm

(a) The maximum magnifying power if she uses the microscope together with her glass is given by
m=1+Df
Here,
D=25cm
f=5cm
On substituting the values, we get:
m=6

(b) Without the glasses, D=40cm.
m=1+Df=1+405=9


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