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

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?

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

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 ______

What is short-sightedness ? State the two causes of short-sightedness (or myopia). With the help of ray diagrams, show:

(i) the eye-defect short-sightedness.

(ii) correction of short-sightedness by using a lens.