Answer:

Fermat’s Theorem

The principle of least time: Light always takes the quickest path between any two points (which may not be the shortest path).

Rectilinear propagation of light and the law of reflection [∠i=∠r] can be validated by Fermat’s principle of least time.

Plane mirror

Any flat and polished surface that has almost no irregularities on its surface that reflect light is called as a plane mirror.

Characteristics of images

Images can be real or virtual, erect or inverted, magnified or diminished. A real imaDust of Snow’ is a beautiful short poem written by Robert Frost. This poem tells that even a simple moment has a large significance. The poet has mentioned crow and hemlock tree in this poem. Crow signifies his depressive and sorrowful mood and hemlock tree is a poisonous tree. Both these signify that the poet was not in a good mood and so he describes the dark, depressive and bitter side of nature to present his similar mood.