Answer:
27.2 cm
Explanation:
Note: For the student to see an object clearly at 25 cm, from the eye the image must be formed 309 cm on the same side of the lens, at his near point.
From mirror Formula,
1/f = 1/v+1/u
Where f = focal length of the glasses, v = image distance, u = object distance.
f = vu/(v+u) ................. Equation 1
Given: u = 25 cm, v = -309 cm ( virtual image)
Substitute into equation 1
f = [25×(-309)]/[25+(-309)]
f = -7725/-284
f = 27.2 cm.
Hence the focal length of the glass = 27.2 cm.
