Question

The nickel's image in the figure below has four times the diameter of the nickel when the lens is 2.74 cm from the nickel. Determine the focal length of the lens. Your response differs from the correct answer by more than 100%. cm

Answer 1

Magnification=-di/do

where di = image distance,

do = object distance.

assuming a real image:

For real image magnification is positive

Magnification is 4, so di = 4do.

Now we can use

1/di + 1/do = 1/f

substitute di by 4do

1/(4do) + 1/do = 1/f

(1 + 4)/(4do) = 1/f

f = 4do/(1+4)

f = 4*2.74/5

f = 2.192 cm

assuming a virtual image: di = negative

- 1/(4do) + 1/do = 1/f

(-1 + 4)/(4do) = 1/f

f = 4do/3

f = 4*2.74/3

f = 3.653 cm