For a normal eye,the least distance of distinct vision is......$m$.

$A$ person wears glasses of power $-2.5 \, D$. The defect of the eye and the far point of the person without the glasses are respectively

If the far point of a short-sighted person is $400 \ cm$, then the power of the lens required to enable him to see very distant objects clearly is

$A$ far-sighted person has a near point of $60\, cm$. What power lens should be used for eyeglasses such that the person can read a book at a distance of $25\, cm$?

If the distance of the far point for a myopia patient is doubled,the focal length of the lens required to cure it will become

$A$ man suffering from short-sightedness (myopia) is unable to see objects distinctly at a distance greater than $2\,m$. The power of the lens required to correct this defect should be.......$D$