In order to clearly read a book 18 cm away, a farsighted girl needs an image...
In order to clearly read a book 18 cm away, a farsighted girl needs an image to be 31 cm from her eyes. What focal length is needed for the lens in her eyeglasses? (answer is in cm)
Homework Answers
Using lens equation:
1/f = 1/u + 1/v
u = object distance = 18 cm
v = image distance = -31 cm (Since image is on the same side of object)
f = focal length of lens = ?
So,
f = u*v/(u + v)
f = 18*(-31)/(18 + (-31))
f = focal length = 42.9 cm
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