Question

The closest distance a book can be read from a pair of reading eyeglasses (Power = 2.06 dp) is 27.5 cm. What is the near distance? (Assume a distance between the eyeglasses and the eyes to be 3.00 cm)

Answer 1

We can not see objects clearly once they are closer than a particular distance from the eye.

This distance is known as near point of the eye.

For some people, due to some problems in the eye, the near point shifts too much, that they can not read even for a fair distance.

For such people, an extra lens is used to see clearly.

See the image. Here, the distance d is what we have to find.

For the lens, power, P = 2.06 D

For the lens, the object is placed at a distance of 27.5-3 = 24.5 cm = 0.245 m

The image distance can be found using the thin lens formula.

Where P = 1/f

The - sign means that the image is on the left side of the lens.

So, this distance is 49.47 cm

This is the distance from the lens.

The distance from the eye is

d = 49.47+3 = 52.47 cm