Give the name of a natural optical instrument.

$A$ myopia patient uses:

$A$ person cannot see objects clearly beyond a distance of $40 \, cm$. What power of lens should be prescribed for them?

$A$ short-sighted person can see distinctly only those objects which lie between $10\, cm$ and $100\, cm$ from him. The power of the spectacle lens required to see a distant object is

$A$ myopic adult has a far point at $0.1\, m$. His power of accommodation is $4\, D$.
$(i)$ What power lenses are required to see distant objects?
$(ii)$ What is his near point without glasses?
$(iii)$ What is his near point with glasses? (Take the image distance from the lens of the eye to the retina to be $2\, cm$.)

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