When an object 1.15 cm tall is placed 12 cm from a lens,the lens produces an upright image of the object that is 5.75 cm tall.What is the focal length of the lens?
A) 24 cm
B) 18 cm
C) 60 cm
D) 15 cm
E) 9.0 cm

Question

Quizplus · Accepted Answer

Using the formula:
m = -v/u = h'/h
where,
m = magnification
v = image distance
u = object distance
h' = height of image
h = height of object

Rearranging,
v = -m*u
and,
f = (u*v)/(u+v)

Substituting the given values,
m = 5.75/1.15 = 5
u = -12 cm
h' = 5.75 cm
h = 1.15 cm

Therefore,
v = -5*(-12) = 60 cm

and,
f = (-12*60)/(-12+60) = 15 cm

Hence, the correct choice is D) 15 cm.

When an Object 1