a person is unable to see objects can be placed within 50 cm from his name the defect of vision the person is suffer from and list it to possible causes draw a diagram to show the defect in the above case mention the type of lens used by him for correct of the defect and calculate its power assume that the near point from normal eye is 25 CM
Answers
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 long sightedness.
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.
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