Question
The equivalent focal length of the two lenses in contact is 80 cm . If the focal length of one of the lenses is 20 cm. The power of the second lens is :
Answer: Option C
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The focal length of the combination is given by
`1/F = 1/f_1 + 1/f_2`
Here : F = 80 cm = 0.8 m., `f_1` = 20 cm = 0.2 m.
`:.` `1/f_2` =` 1/F - 1/f_1`
or `1/f_2 `= `1/0.8 - 1/0.2`
or `1/f_2` = `(1 - 4)/(0.8) = -3/0.8 = - 3.75`
But ` p = 1/f`
So, power `p_2 = 1/f_2`
Hence , `p_2` = - 3.75 D
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