Question
what do you mean by combined focal length?
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Answer
The reciprocal of combined focal length is equal to the sum of the reciprocals of focal lengths of the lenses placed together.
$\therefore \frac{1}{ f }=\frac{1}{ f _1}+\frac{1}{ f _2} \text { where } f \text { - combined focal length, }$
$f _1 \text { - focal length of } 1^{\text {st }} \text { lens, }$
$f _2 \text { - focal length of } 2^{\text {nd }} \text { lens. }$
$\therefore \frac{1}{ f }=\frac{1}{ f _1}+\frac{1}{ f _2} \text { where } f \text { - combined focal length, }$
$f _1 \text { - focal length of } 1^{\text {st }} \text { lens, }$
$f _2 \text { - focal length of } 2^{\text {nd }} \text { lens. }$
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