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Two lenses of power $+ 3 and - 1$ dioptre are placed in contact. Which of the following is the focal length of the combination of lens?

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100 cm

25 cm

50 cm

33.3 cm

answer is C.

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Detailed Solution

Two lenses of power

+ 3 and - 1

dioptre are placed in contact. The focal length of the
combined lens is

50 cm

.
Given
Two lenses of power

+ 3 D and - 1 D

are placed in contact.
To find:
The focal length of combined lens
Calculation:
The general expression of the power of a combination of lenses is:

P_{net} = P_{1} + P_{2}

= + 3 - 1

= 2

\Rightarrow F_{net} = \frac{100}{P_{net}}

\Rightarrow F_{net} = \frac{100}{2}

\Rightarrow F_{net} = 50 cm

So, the focal length of the combination of lenses is

50 cm

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