Give the name of a natural optical instrument.

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

$A$ man can see clearly up to $3 \, m$. Prescribe a lens for his spectacles so that he can see clearly up to $12 \, m$.

The image formed on the retina is:

Myopia is due to

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