$A$ man is suffering from colour blindness for green colour. To remove this defect,he should use goggles of

$A$ person who can see things most clearly at a distance of $10 \, cm$ requires spectacles to enable him to see clearly things at a distance of $30 \, cm$. What should be the focal length of the spectacles?

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

$A$ person wears normal spectacles in which the distance between the glasses and the eyes is approximately $2 \, cm$. The power required is $-5 \, D$. If he wears contact lenses,then the required power is ......... $D$.

$A$ person can see objects clearly when they lie between $40 \ cm$ and $400 \ cm$ from his eye. In order to increase the maximum distance of distant vision to infinity, the type of lens and power of the correction lens required respectively are

$A$ man who cannot see clearly beyond $5 \, m$ wants to see stars clearly. He should use a lens of focal length ....... $m$.