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Manipal Entrance Test 2020 Physics Paper
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© examsnet.com
Question : 49
Total: 50
A man who wears glasses of power 3 D must holds a newspaper at least
25
c
m
away to see the print clearly. How far away would the newspaper have to be if he took off the glasses and still wanted clear vision?
10 {cm}
25 {cm}
1 {m}
-1 {m}
Validate
Solution:
Given, power
P
=
3
D
f
=
1
P
=
1
3
m
=
100
3
c
m
u
=
−
25
c
m
.
Applying lens formula, we can write
1
v
−
1
u
=
1
f
⇒
1
v
+
1
25
=
3
100
⇒
1
v
=
3
100
−
1
25
=
3
−
4
100
=
−
1
100
v
=
−
100
c
m
Now, again using lens formula we can write
1
v
−
1
u
=
1
f
As, focal length of a far sighted person deceases
⇒
1
−
100
−
1
u
>
1
1.7
=
10
17
[
∵
f
<
1.7
c
m
]
⇒
1
u
<
−
1
100
−
10
17
=
−
17
−
1000
1700
=
−
1017
1700
⇒
u
>
1700
1000
≈
−
1.7
=
−
1
m
(nearest option)
© examsnet.com
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