When a girl moves towards a plane mirror at a speed of 4.0m/s, the distance between the girl & her image reduces at a speed of

One newton × One meter equals?

A microscope is focused on a mark on a table, when the mark is covered by a plate of glass 2m thick, the microscope has to be raised 0.67cm for the mark to be once more in focus. Calculate the refractive index.

Radio waves belongs to the class of waves whose velocity is about

A single force which produces the same effect as a set of forces acting together at a point is known as the