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Class 12 NEET Physics Optics Questions Part 2
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Question : 34
Total: 50
Two similar thin equi-convex lenses, of focal length f each, are kept coaxially in contact with each other such that the focal length of the combination is
F
1
. When the space between the two lenses is filled with glycerine (which has the same refractive index (µ = 1.5) as that of glass) then the equivalent focal length is
F
2
. The ratio
F
1
:
F
2
will be :
[NEET 2019]
1 : 2
2 : 3
3 : 4
2 : 1
Validate
Solution:
👈: Video Solution
Equivalent focal length in air
1
F
1
=
1
f
+
1
f
=
2
f
When glycerin is filled inside, glycerin lens behaves like a diverging lens of focal length
(
–
f
)
1
F
2
=
1
f
+
1
f
−
1
f
=
1
f
F
1
F
2
=
1
2
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