see the diagram.
Normally our eye is able to see any object clearly between (adults) 25 cm and
infinity. The distance 25 cm is called the near point. The far point is the
distance up to which we can see clearly(normally infinity). The eye muscles
focus (by contracting and expanding themselves) the incoming light rays so that
they they converge on the retina. Normal focal length of eye lens is 2.5 cm.
A person may not be able to see the objects nearby clearly. That is like objects between 25 cm and 50 cm
for example. The reason is that the eye muscles are able
to focus light rays only behind the retina. They are not able to adjust focal length of
the eyelens. The converging power has
become less and focal length is > 2.5 cm. This defect is called
HyperMetropia. We need to wear a
converging lens to focus rays on to retina. This is called farsightedness or
For the person who is not able to see objects nearer than 50 cm:
we use convex lens to create a virtual object of an object at 25 cm at 50 cm. The objects at distances between 25 cm and 50 cm will result in virtual images placed erect at distances more than 50 cm. So the person will see them clearly.
v = -50 cm u = -25 cm then focal length of convex lens is:
1/v - 1/u = -1/50 + 1/25 = 1/50 => f = 50 cm and power is +2 Dioptre.
eye donations are important in this world. Some persons due to illness or due to accidents may lose their vision. For them, any person who dies could donate their eyes. The doctors (opthalmologists) extract the cornea of the dead person and arrange it on the eyes of a person whose cornea is damaged or diseased. This is called corneal transplantation.
Eye donation is very important deed one could do , as one has no use for the cornea after death. These could be used for eye sight of another person to live more meaningfully. So this is much more than money donation.
Eye donation advertisements will spread awareness about this fact and people will know more about it and know also that there is no harm in doing it, rather it is prestigious to do that.