Question 36
Lenses and Optical InstrumentsA patient with a sight defect has a least distance of distinct vision of 150 cm. For him to be able to read a material placed at a distance of 25 cm, what is the focal length of the glasses he should wear?
- A.15.0 cm
- B.17.6 cm
- C.21.4 cm
- D.30.0 cm
Show the answer and explanation
Answer: D (30.0 cm)
Explanation
The converging lens must make a virtual image of the page (u = 25 cm) at his near point, 150 cm away on the same side. 1/f = 1/25 − 1/150 = 5/150, so f = 30.0 cm.