Q30 of 234 Page 279

What is presbyopia? Write two causes of this defect. Name the type of lens which can be used to correct presbyopia.

The defect of vision due to which an old person cannot see the nearby objects clearly due to loss of power of accommodation of the eye is known as presbyopia.

The two cases are :-


1) ciliary muscles gets weaken gradually with time.


2) Eye lens loses its flexibility.


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28

The near-point of a person suffering from hypermetropia is at 50 cm from his eye. What is the nature and power of the lens needed to correct this defect? (Assume that the near-point of the normal eye is 25 cm).

 29

A person needs a lens of power, - 5.5 dioptres for correcting his distant vision. For correcting his near vision, he needs a lens of power, +1.5 dioptres. What is the focal length of the lens required for correcting

(i) distant vision, and (ii) near vision?

 31

When is a person said to have developed cataract in his eye ? How is the vision of a person having cataract restored?

 32

Fill in the following blanks with suitable words:

A person is short-sighted if his eyeball is too ______ Spectacles with a ______ lens are needed. A person is long-sighted if his eyeball is too ______ Spectacles with a ______ lens are needed. These focus light rays exactly on to the ______