Define 1 Dioptre of power of a lens.

You are given kerosene, turpentine and water. In which of these the light travels fastest?

The refractive index of diamond is 2.42. What is the meaning of this statement?

A convex lens forms a real and inverted image of a needle at a distance of 50 cm from it. Where is the needle placed in front of the convex lens if the image is equal in size to the object? Also find the power of the lens.

Find the power of a concave lens of focal length 2 m.