Question

A bug which is 4.8 mm in diameter flies in front of a converging (convex) lens with a focal length of 6.0 cm. Take the lens to be located at x = 0 cm.

a. Does the lens have a positive focal length or a negative focal length?

b. At the moment the bug is located at x = 7.0 cm, where is its image?

c. At the moment the bug is located at x = 7.0 cm, how big is its image (diameter)?

d. If the bug flies from x = 7.0 cm to x = 9.0 cm, how far does its image move?

Answer 1

a) the lens has a positive focal length as it is a converging lens.

b) using thin lens equation

1/f = 1/di + 1/do

1/6 = 1/di + 1/7

di = 42 cm

therefore, image of bug is located 42 cm behind the lens.

c) using magnifcation = di/do = Hi/Ho

42/7 = Hi/4.8 mm

28.8 mm = Hi

therefore diameter of bug = 28.8 mm

d) using thin lens equation

1/f = 1/di + 1/do

1/6 = 1/di + 1/9

di = 18 cm

therefore image becomes closer to lens by a distance of = 42-18 = 24 cm

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