Which one of the following is the correct order of decreasing optical density?

Which of the following is responsible for production of shadow ?

An object is placed at a distance of 15 cm in front of a concave mirror with a radius of curvature of 20 cm. Which one among the following statements is correct for the image formed?

The focal distance of a plane mirror is

Twinkling of a star is due to :

As the diameter of objective lens of a telescope increases, the resolution of telescope