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JEE Main Physics Class 12 Ray Optics and Optical Instruments Part 1 Questions
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© examsnet.com
Question : 52
Total: 99
To find the focal length of a convex mirror, a student records the following data :
Object Pin
Convex Lens
Convex Mirror
Image Pin
22.2cm
32.2cm
45.8cm
71.2cm
The focal length of the convex lens is
f
1
and that of mirror is
f
2
. Then taking index correction to be negligibly small,
f
1
and
f
2
are close to :
[Online April 9, 2016]
f
1
=
7.8
c
m
,
f
2
=
12.7
c
m
f
1
=
12.7
c
m
,
f
2
=
7.8
c
m
f
1
=
15.6
c
m
,
f
2
=
25.4
c
m
f
1
=
7.8
c
m
,
f
2
=
25.4
c
m
Validate
Solution:
Taking
f
2
=
12.07
Using Mirror's formula
1
f
=
1
v
+
1
u
⇒
1
12.7
=
1
25.4
+
1
u
⇒
1
12.7
−
1
25.4
=
1
u
u
=
25.4
=
v
′
Now using Len's formula
1
f
=
1
v
−
1
u
⇒
1
f
1
=
1
25.4
+
13.6
+
1
10
⇒
1
f
1
=
1
39
+
1
10
⇒
f
1
=
390
49
=
7.96
The closest answers is (a) as option (c) and (d) are not possible.
© examsnet.com
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