In accordance to Einstein’s first postulate of special theory of relativity, the fundamental laws of physics have the same form for all inertial reference frames. But according to second postulate, the speed of light in free space has the same value c in all inertial reference frames that signifies that the velocity of light in vacuum is independent of the relative motion of the source and the observer. A beam from a laser falls at right angles on a plane mirror and reflects from it.
(a) Laser is an electromagnetic wave. In free space, electromagnetic wave travels with speed of light c. According to Einstein’s second postulate, the speed of light is a constant quantity irrespective of the frame of reference. Therefore if the mirror is fixed in the laboratory, the speed of reflected beam will be c.
(b) According to Einstein’s second postulate the velocity of light in vacuum is independent of the relative motion of the source and the observer. Since laser is an electromagnetic wave and the electromagnetic wave travels with speed of light c, therefore if the mirror is moving directly toward the laser with speed v, the speed of reflected beam will be again c.