night blindness-vitamin A deficiency
color blindness - trouble distinguishing colors
hyperopia-eyeball too short
myopia-eyeball too long
astigmatism-light refracted aspherically.
Night blindness is defined as the disease which involves the mal production of retinol in the rod cells of retina. This leads to the decreased sensitivity of eye to dim light.
Similarly, colour blindness is a genetic disease which is characterised by absence of a particular pigment in cone cells. This restricts the vision of a particular colour.
Hyperopia or hypermetropia is the shortening of eyeball which leads to the focusing of light rays behind the retina. So convex lenses are required to focus them on retina.
Myopia is the opposite of hypermetropia where the eyeball is elongated and the light rays doesn't reach the retina. So concave lens are required to focus the rays on retina.
Astigmatism is the case where the curvature of lens isn't correct, and the lens required for the correction is cylindrical lens
