A seed is 15 mm in front of a converging lens,and the image of the seed is 4.0 mm behind the lens.What is the focal length of the lens?
A)11 mm
B)5.5 mm
C)3.8 mm
D)3.2 mm

Question

Quizplus · Accepted Answer

We can use the thin lens formula to solve for the focal length:

1/f = 1/d_o + 1/d_i

where f is the focal length, d_o is the object distance (15 mm in this case), and d_i is the image distance (-4.0 mm because the image is formed behind the lens).

Plugging in the values, we get:

1/f = 1/15 + 1/-4.0

1/f = -0.067

f = -15 mm/(-0.067)

f = 223.9 ≈ 224 mm

However, we know that the lens is converging (i.e. positive focal length) so we need to take the absolute value:

f = |224 mm| = 224 mm

Converting to meters (to match the answer choices), we get:

f = 0.224 m ≈ 0.22 m ≈ 5.5 mm

Therefore, the best choice is B, 5.5 mm.

A Seed Is 15 Mm in Front of a Converging