The least distance of distinct vision for a young adult with normal vision is about ...... $cm$.

What is meant by the power of accommodation of the eye?

Explain why the planets do not twinkle.

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

Why do stars twinkle?

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 $25 \, cm$.