$A$ man can see objects clearly between $15 \ cm$ and $30 \ cm$. He uses a lens to see far objects. Due to the lens used,the new near point will be at........$cm$.

$A$ 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 is suffering from a myopic defect. He is able to see clear objects placed at $15 \,cm$. What type and of what focal length of lens should he use to see clearly the objects placed $60 \,cm$ away?

$A$ man can see only between $75 \, cm$ and $100 \, cm$. The power of the lens required to correct the near point will be:

If the least distance of distinct vision for a boy is $35 \ cm$,then the lens to be used by the boy for correcting the defect of his eye is

Give the name of a natural optical instrument.