Question

Effective focal length. ( write a short note)
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Answer 1

Answer:

Effective focal length is the distance between the rear principal point and the rear focal point of the lens.

Any light ray traveling parallel to the lens axis, but not along the axis itself, will be refracted (bent) toward the axis in passing through the lens.It will intersect the axis at a point behind the lens called the rear focal point.

When the distance of separation between the two lenses is equal to the sum of the individual focal lengths, the combined focal length is infinite. Simply it means:—

$d=f1+f2= \infty$