Question

A 50 mm lens is focused on an object 5.0 m from the lens. The image position is fixed (as in a camera). How far does the lens need to be moved to focus on an object 3.0 m from the lens? 0.34 mm

Answer 1

By lens formula,

1/f = 1/u + 1/v

here, f = 50 mm = 0.05 m

initially,

u = 5.0 m

then, 1/v = 1/0.05 - 1/5.0

v = 5*0.05/(5-0.05)

v = 50.5 mm

now, u = 3.0 m

then, 1/v = 1/0.05 - 1/3.0

v = 3*0.05/(3.0-0.05)

v = 50.84 mm

So, lens is moved by distance(d) = 50.84 - 50.5

d = 0.34 mm

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