The substance present in the choroid is:

The least distance of distinct vision of a long-sighted person is $60 \ cm$. By using a spectacle lens,this distance is reduced to $12 \ cm$. The power of the lens is

$A$ person is suffering from the defect astigmatism. Its main reason is

$A$ person has a near point of $50 \, cm$ and a far point of $3 \, m$. What power of lens is required to $(i)$ read and $(ii)$ see distant stars?

Myopia is a defect of the eye due to which a person experiences difficulty in seeing .......

Astigmatism (for a human eye) can be removed by using