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Question

A convex lens of power 5 D and a concave lens of power 7.5 D are placed in contact with each other. Which of the following is/are correct regarding the combination of these lens.

A
Power of combination of lens is 12.5 D
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B
Power of combination of lens is -2.5 D
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C
Focal length of the combination of lens is 8 cm
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D
Focal length of the combination of lens is -40 cm
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Solution

The correct options are
B Power of combination of lens is -2.5 D
D Focal length of the combination of lens is -40 cm
Let P represents power of lens and f represents focal length of lens.
Given: Power of convex lens, P1=5 D
Power of concave lens, P2=7.5 D (power of concave lens is taken as negative)
So power of combination of lens = Ptotal
Ptotal = P1 + P2 = +5 + (-7.5)
Ptotal = -2.5 D
Since we know that P = 1f (in metre)
f=1P=12.5=0.4 m=40 cm


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