Give the name of a natural optical instrument.

$A$ man suffering from myopia can read a book placed at $10 \ cm$ distance. For reading the book at a distance of $60 \ cm$ with relaxed vision,the focal length of the lens required will be ...... $cm$.

$A$ man suffering from short-sightedness (myopia) is unable to see objects distinctly at a distance greater than $2\,m$. The power of the lens required to correct this defect should be.......$D$

Myopia is a defect of the eye due to which a person experiences difficulty in seeing .......

The retina of the eye acts like the ........ of a camera.

$A$ man cannot see clearly the objects beyond a distance of $20 \, cm$ from his eyes. To see distant objects clearly,he must use which kind of lenses and of what focal length?