What is the power of a plane glass sheet?

What can be measured using a dioptre meter?

What is the focal length of a convex lens having power $+5.0 \,D$?

$A$ girl was playing with a thin beam of light from her laser torch by directing it from different directions on a convex lens held vertically. She was surprised to see that in a particular direction the beam of light continues to move along the same direction after passing through the lens. State the reason for this observation.

In which of the following,the image of an object placed at infinity will be highly diminished and point-sized?

Under what condition in an arrangement of two plane mirrors,will the incident ray and the final reflected ray always be parallel to each other,regardless of the angle of incidence? Show the same with the help of a diagram.