Why does the sky appear dark instead of blue to an astronaut?

Why do planets not twinkle?

Why are normal eyes not able to see clearly objects placed closer than 25 cm?

Make a diagram to show how hypermetropia is corrected. The near point of a hypermetropic eye is 1 m. What is the power of the lens required to correct this defect? Assume that the near point of the normal eye is 25 cm.

The far point of a myopic person is 80 cm in front of the eye. What is the nature and power of the lens required to correct the problem?

A person needs a lens of power –5.5 dioptres for correcting his distant vision. For correcting his near vision, he needs a lens of power +1.5 dioptre. What is the focal length of the lens required for correcting : (i) distant vision, and (ii) near vision?