When a light beam approaches you, its frequency is greater and its wavelength less. Does this contradict the postulate that the speed of light cannot change? Defend your answer.
Solution 10E Step 1 When light is approaching me, the light is moving towards the higher gravitational field, that is the gravitational potential is decreasing (since the gravitational field is attracting). According to the general theory of relativity, if light travels towards the higher gravitational potential, the energy of the light will decrease. Now, in this case, the light is actually travelling towards the lower potential energy. Hence the energy of light will increase. Since the speed of the light remains same, the frequency of the light will increase and hence the wavelength of the light will decrease. Hence it is clear that the phenomenon does not violate the postulate of the special theory of relativity, that the speed of the light is a universal constant.
