The Human Eye And The Colourful World

Planets do not twinkle because they are much closer to Earth and appear as extended sources of light, not point sources like stars. The variations in light from different parts of the planet cancel out, so the planet appears steady.

Presbyopia is caused by the gradual weakening of the ciliary muscles and the reduced elasticity of the lens, making it difficult to focus on nearby objects. It is corrected using bifocal or progressive lenses.

The near point is the closest distance at which an object can be seen clearly, typically about 25 cm for a normal adult. The far point is the farthest distance at which objects can be seen clearly, which is at infinity for a normal eye.