Mahmood is farsighted and cannot see objects clearly that are closer to his eye than 80.0 cm.What is the focal length of the contact lenses that will enable him to comfortably see objects at a distance of 25.0 cm from his eyes?
A)+36.4 cm
B)-36.4 cm
C)-21.2 cm
D)+21.2 cm
E)+32.5 cm

Question

Quizplus · Accepted Answer

To correct farsightedness and allow Mahmood to see objects at 25.0 cm, we use the lens formula: 1/f = 1/v - 1/u. Here, v (image distance) is the least distance of distinct vision (comfortable viewing distance), typically 25 cm for a normal eye, and u (object distance) is -25 cm (negative because the object is in front of the lens). We want to find f (focal length of the lens) that makes the image appear at the least distance of distinct vision, which is 80 cm away for Mahmood. So, we set v = -80 cm (negative because the image formed is virtual and on the same side as the object). Plugging the values into the lens formula gives us 1/f = 1/(-80) - 1/(-25), solving which gives f = +36.4 cm.

Mahmood Is Farsighted and Cannot See Objects Clearly That Are