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JEE Advanced 2018 Paper 1
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Question : 13
Total: 54
Sunlight of intensity
1.3
k
W
m
−
2
is incident normally on a thin convex lens of focal length
20
c
m
. Ignore the energy loss of light due to the lens and assume that the lens aperture size is much smaller than its focal length. The average intensity of light, in
k
W
m
−
2
,at a distance
22
c
m
from the lens on the other side is
[JEE Adv 2018 P1]
Your Answer:
Validate
Solution:
r
R
=
2
20
=
1
10
  ∴ Ratio of area
=
1
100
Let energy incident on lens be E.  ∴ Given
E
A
=
1.3
So final,
E
a
=
?
?
E
=
A
×
1.30
Also
a
A
=
1
100
∴Average intensity of light at
22
c
m
=
E
a
=
A
×
1.3
a
=
100
×
1.3
=
130
k
W
/
m
2
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