Read the following passage and answer the questions that follow &
: You had a convex lens of focal length 5m, and two lenses A and B of unknown focal lengths .You observed that if you placed lens A in contact with the convex lens, the combination acted like a plane glass slab.On the other hand, if lens B was Kept in contact with the convex lens the combination acted like a convex lens of focal length 10 cm.
Q1. What type of lens is lens A?
(a) convex lens (b) concave lens (c) either concave or convex lens (d) none of the above.
Q2. What is the power of lens A?
(a) 5D (b) 0.2D (c) -5D (d) -0.2D
Q3. What is the power and type of the lens B?
(a) -5D, concave lens (b) 0.1D, convex lens
(c) -0.1D, concave lens (d) 5D, convex lens
Q4. If lens A and lens B are kept in contact , what will be the focal length of the combination?
(a) -3.33m (b) 5m (c) -7.5m (d) 10m
Q5. If all three lenses are kept in contact, with A in the middle of the convex lens and the lens B, then what would be the focal length of the combination?
(a) 5m (b) -10m (c) 15m (d) -20m
Answers
Q1) B) concave lens
Q2) B) 0.2D
Q3) B) 0.1D, convex lens
Q4) D) 10m
Q5)C) 15m
Answer:
1.(a)concave lens, 2.(d)-0.2D, 3.(b)0.1D,convex lens, 4.(d)10m, 5.(15m)
Explanation:
1.since the combination of lens A and the convex lens acted like a plane glass slab , the type of lens A is concave lens .
so option (a) concave lens.
2. the power of lens is reciprocal of its focal length P=1/focal length.
=1/5=0.2
since power of a concave lens is negative , P=-0.2D, D is the SI unit of power.
so, option (d) -0.2D
3. the power of lens B=1/f=1/10(here , 10 is the focal length)
=0.1D and
type of lensB is convex.
so, option(b)
4. when two lenses are in contact, the focal length is given by,1/F=1/f₁₊1/f₂
1/F=f₁₊f₂/f₁f₂
F=f₁f₂/f₁₊f₂
F=f₁f₂/f₁₋f₂
=5*10/10₋5
=50/5
=10
so,option(d)10m
5.focal length of the combination of three lenses=15m
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