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Power of convax lens is 5 D and the power of concave lens is 2 D. What will the power of combination of these lenses when placed in contact ?
  • a)
    +5 D
  • b)
    +30 D
  • c)
    -7 D
  • d)
    +3 D
Correct answer is option 'D'. Can you explain this answer?
Most Upvoted Answer
Power of convax lens is 5 D and the power of concave lens is 2 D. What...
You may know that the power of convex lens is in (+) and that of concave is in (-).

here we have:-
power of convex lens= +5D and
power of concave lens= -2D


So, power of combination will be:-

power of convex lens + power of concave lens
=+5D + (-2D)
=+3D

hence option D(+3D) is correct answer..

Thank you.. :-)
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Community Answer
Power of convax lens is 5 D and the power of concave lens is 2 D. What...
Combination of Convex and Concave Lenses

Given that the power of the convex lens is 5 D and the power of the concave lens is 2 D. We need to find out the power of the combination of these lenses when placed in contact.

Formula for calculating the power of a lens

The power of a lens is given by the formula:

P = 1/f

Where P is the power of the lens in dioptres (D) and f is the focal length of the lens in meters.

Calculating the focal length of the convex lens

We know that the power of the convex lens is 5 D. Let us assume that the focal length of the convex lens is f1.

Using the formula for calculating the power of a lens, we get:

5 = 1/f1
f1 = 1/5
f1 = 0.2 m

Calculating the focal length of the concave lens

We know that the power of the concave lens is 2 D. Let us assume that the focal length of the concave lens is f2.

Using the formula for calculating the power of a lens, we get:

-2 = 1/f2
f2 = -1/2
f2 = -0.5 m

Calculating the power of the combination of lenses

When the two lenses are placed in contact, the effective focal length of the combination is given by:

1/f = 1/f1 + 1/f2

Substituting the values of f1 and f2, we get:

1/f = 1/0.2 - 1/0.5
1/f = 5 - 2
1/f = 3

Therefore, the power of the combination of lenses is:

P = 1/f
P = 1/3
P = 0.33 D

Hence, the correct answer is option D, i.e., 3 D.
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Power of convax lens is 5 D and the power of concave lens is 2 D. What will the power of combination of these lenses when placed in contact ?a)+5 Db)+30 Dc)-7 Dd)+3 DCorrect answer is option 'D'. Can you explain this answer?
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