A convex lens is needed to produce an image that is 0.55 times the size of the object and located 30 cm from the lens on the other side. What focal length should be specified?

Answer 1

Thin Lens Equation
1/d +1/f = 1/F,
where d is the object distance,
f is the image distance
F is the focal length
A is the size of the object
B is the size of the image.
B = 0.55•A,
From the similarity of triangles
d/A = f/B
d = (A/B) •f = f/0.55
F = d•f/(d+f) =f/1.55=0.3/1.55 = 0.194 m