Without glasses,a person can comfortably read a magazine when it is held at 60 cm from his eyes.What should be the focal length of his contact lenses to allow him to read the magazine comfortably at a distance of 30 cm?
A)-30 cm
B)+30 cm
C)-60 cm
D)+60 cm
E)+90 cm

Question

Quizplus · Accepted Answer

From the given problem, we can use the lens formula:

1/f = 1/v - 1/u

where f is the focal length of the lens, u is the object distance (60 cm when reading comfortably without glasses), and v is the image distance (30 cm when reading comfortably with the contact lenses).

Plugging in the values, we get:

1/f = 1/30 - 1/60

Simplifying this equation, we get:

1/f = 1/60

Therefore, f = +60 cm, which means the person needs contact lenses with a focal length of +60 cm to read comfortably at a distance of 30 cm. The positive sign indicates that the contact lenses should be converging, which is the most common type of contact lens used for correcting nearsightedness.

Without Glasses,a Person Can Comfortably Read a Magazine When It