What is the least distance of distinct vision for a normal adult with normal vision?

What is the twinkling of stars due to?

$A$ glass slab is placed over a page on which the word $VIBGYOR$ is printed with each letter in its corresponding colour. $(i)$ Will the image of all the letters be in the same place? $(ii)$ If not, state which letter will be raised to the maximum. Give a reason for your answer.

$A$ person has to use a concave lens in his spectacles. Which defect of vision is he suffering from?

Why is the blind spot so called?

What is a blind spot?