A given individual is unable to see objects clearly when they are beyond 100 cm.What focal length lens should be used to correct this problem?
A)−100 cm
B)−33.3 cm
C)−20 cm
D)75 cm
E)100 cm

Question

A given individual is unable to see objects clearly when they are beyond 100 cm.What focal length lens should be used to correct this problem?
A)−100 cm
B)−33.3 cm
C)−20 cm
D)75 cm
E)​100 cm

Quizplus · Accepted Answer

The individual has myopia (nearsightedness), meaning they can see objects clearly only when they are close. To correct this, a diverging lens is needed, which has a negative focal length. The far point of the individual is 100 cm, so the lens should have a focal length that allows them to see at infinity. Using the lens formula 1/f = 1/v - 1/u, where v is the image distance (infinity in this case, making 1/v = 0) and u is the object distance (-100 cm, negative because the object is on the same side as the light entering the lens), we find that the focal length f should be -100 cm to correct the vision to see at infinity.

A Given Individual Is Unable to See Objects Clearly When