# Calculate the focal length of the spectacle lens

gungo

## Homework Statement

A hyperopic eye has a near point of 1.10 m. Calculate the focal length of the spectacle lens required to correct the hyperopia (assume that the near point of the normal eye is 25.0 cm.)

1/f=1/p+1/q

## The Attempt at a Solution

I just used .25 m as p and 1.1 m as q and inverted it to solve for f, which gave me 0.20 m, but the answer given is 0.324 m.

Homework Helper
Gold Member
In order for the person to be able to clearly see an object placed 25 cm away, where should the image of the object formed by the lens be located? Should the image be on the same side of the lens as the object?

gungo
In order for the person to be able to clearly see an object placed 25 cm away, where should the image of the object formed by the lens be located? Should the image be on the same side of the lens as the object?
It should be on the opposite side, so q is negative,right? That would give the right answer.

Homework Helper
Gold Member
It should be on the opposite side, so q is negative,right? That would give the right answer.
q is negative when the image is on the same side as the object.

Try to draw a diagram showing the location of the person’s head, the lens, the object, and the image. You don’t have to post the diagram, but drawing it might help.