Question

I am trying to make sense of nearsightedness and farsightedness. I conducted an experiment in which I determined the focal length in two corrective lenses. I found that the focal length of the lenses to correct farsightedness were bigger. I know that convex lenses are used to correct farsightedness and concave lenses are used to correct nearsightedness. Can anyone explain to me why my result makes sense or would be expected? I do not know how to relate my result to the big picture or real life.

Answer 1

Nearsightedness is the effect in which image is formed in front of the retina due to high converging power of human eye. So to form the image at the retina we use diverging lens (concave lens). The focal length of the lens depends on the converging power of human eye. Basically, in this far point is shifted near to eye, so the focal length of the lens to be used is given by

Which gives where D is new far point.

In case of farsightedness, the image is formed behind the retina due to low converging power of human eye lens. So we use convex (converging) lens for the correction. Again the focal length depends on the extend of defect. In other words, Near point shifted away from the eye, so

Where d_new is near point of defected eye and d is near point of normal eye, d=25cm.