A normal eye can see objects clearly at a far off distance down to about 25cm from the eye. But for many people it is not possible. There are two main defects of human eye.
- Short sightedness 2. Long Sightedness
- Short Sightedness
A man suffering from short sightedness cannot see distant objects clearly but can see near objects clearly. This defect may be due to that either the lens of the eye ball is thick (i.e., short focal length) or the eye ball is longer than the suitable size. For this reason the image of distant object is formed in front of the retina instead of at retina (fig. 14. 19-a). The actual far point of such an eye is near the eye instead of infinity (fig. 14. 19-b). Short sightedness can be corrected by wearing spectacles having suitable concave lens. Light rays coming from the distant object are now diverged by the concave lens so that they appear to be coming from the eye’s own far point F. therefore rays are focused on the retina, thus forming a sharp image (fig. 14. 19-c)
- Long sightedness
A person suffering from long sightedness cannot see the near objects clearly but can see the distant objects clearly. This defect may be caused by an eye ball which is shorter than the normal size or eye lens is thinner i.e., less converging. This means that the light rays from near objects are not focused on the retina but behind the retina (fig. 14.20-a). a man suffering from long sightedness is unable to read the printed page of a book held at the normal near point of 25 cm from such an eye because the near point of such an eye is greater than 25 cm (fig. 14.20-b)
longsightedness can be corrected by wearing spectacle having convex lens of such a focla length which forms a virtual image of the object placed at normal 25 cm at the near point O of the eye (fig. 14.20-c) hence a clear image of the object is formed on the retina.
Example 14.6: the near point of a person is 50 cm and his far point is 200 cm. calculate the power of the lenses which his spectacles should have (i) for reading (ii) to see clearly the distant objects.
Solution:
The person is suffering both from long sightedness and short sightedness.
- Correction of long sightedness
Here
Near point of the person = 50 cm
Normal rear point of the eye = p = 25 cm
Substituting q = 50 cm, p = 25 cm in lens equation.
Thus for the correction of long sightedness, the person should use a convex lens of +2D power.
- Correction of short sightedness
Here q = -200 cm , p = x
Using lens equation
Thus for the correction of short sightedness, the person should use a concave lens of 0.5D power.
Observe And Tell
If you or your friend uses spectacles, observe its lens and tell whether it is convex lens or concave lens and also whether it is being used for long sightedness or short sightedness.
Please care
In these days, some people use contact lens instead of spectacles. A specific cleaning is essential for their usage to avoid any possible infection.