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Question

The focal length of lens I is + 5 cm, lens II is + 10 cm and lens III is + 20 cm. The total power of the combination of the lens is.

A
+ 0.1 D
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B
+ 0.35 D
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C
+ 0.75 D
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D
0.35 D
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Solution

The correct option is B + 0.35 D

Given,
Focal length of lens I = + 5 cm
Focal length of lens II = + 10 cm
Focal length of lens III = + 20 cm
We know, Power of lens P=1focal length.
Total Power = P| + P|| + P||| ; P = 1f
So, P| = 15 D ; P|| = 110 D ; P||| = 120 D
Total Power of combination Pt=P| + P|| + P||| ;
Pt=15 + 110 + 120 = 720
Pt = + 0.35 D.
Therefore, the total power of combination is + 0.35 D.


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