a lence when placed on a plane mirrorthen objects needle and its image coincide at 15 cm the focal length of lence is
Answers
Answer:
Explanation:
Figure shows a convex lens placed in contact with a plane mirror. An axial point object is placed at a distance O at a distance, OC = 20cm. As image I of the object coincides with O, the rays refracted first from the lens and then refracted by the plane mirror must be retracing their path. This would happen when rays refracted by the convex lens fall normally on the mirror I.e, the refracted rays form a beam parallel to principal axis of the lens. Hence the object O must be at the focus of the convex lens. f = CO = 20 cm.
OR
From case (i) we get f
lens
=15cm since μ
lens
=1.5∴f
lens
=R
lens
=15cm
In case (ii) focal length of the combination =40 cm [combination of lens +combination of (planoconcave) liquid lens.]
40
1
=
15
1
+
f
liquidlens
1
or
f
liq.lens
1
=−
15
1
+
40
1
f
liq.lens
=−24cm
f
liq.lens
1
=(μ−1)[
R
1
−
∞
1
]or
−24
1
=(μ−1)[
15
1
]or
μ=
8
13