Hypermetropia is a:

Myopia is due to

The least distance of distinct vision for a long-sighted person is $x \ m$. He wants to read a newspaper placed at a distance $(x/2) \ m$. The power of the spectacle lens used by him is $x \ D$. What is the value of $x$?

$A$ lens of power $0.66 \ D$ is used to correct myopia. What is the far point of the eye in $cm$?

$A$ person cannot see objects beyond $3 \, m$. What should be the focal length of the lens required to see objects at $12 \, m$?

Give the name of a natural optical instrument.