A fisherman tying flies desires a magnifier that produces an angular magnification of $2.00$ when used with eyes in the relaxed (unaccommodated) state. What focal length lens should be used ? How close to the fly does the sportsman's eyes have to be.?

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Solution

Angular magnification $α = \frac{d}{u}$ where $d$ is distance of near point and $u$ is distance of object from eyepiece.
when eyes are relaxed $⟹$ image formed at infinity
which means object is placed at focal point. $u = f$ and $d = 25 c m$ is known.
$α = \frac{d}{f}$
$2.0 = \frac{25 c m}{f}$
$f = \frac{25 c m}{2}$
$f = 12.5 c m$
focal length of lens is 12.5 cm and object is also at focal length i.e 12.5 cm

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