Final answer:
Monochromatic light retains its frequency when reflected or refracted because it acts as a forced oscillation of constant frequency despite changes in speed or wavelength due to different media. This constancy is rooted in the properties of electromagnetic waves and the conservation of energy for photons.
Step-by-step explanation:
When monochromatic light is incident on a surface separating two media, the reflected and refracted light both have the same frequency as the incident frequency. The frequency of the incident light does not change during reflection or refraction because the light wave is considered a forced oscillation, which must maintain the original source's frequency. When light passes from one medium to another, its speed and wavelength may change, but its frequency remains constant. The speed of light in a medium is v = c/n, where c is the speed of light in a vacuum and n is the index of refraction of the medium.
The phenomenon can be partially explained by considering that the wave's frequency is determined by its source and the nature of light as an electromagnetic wave, which exhibits wave-particle duality. Furthermore, the laws of conservation of energy imply that the energy of photons (quantized light packets) is determined by their frequency, and since the energy remains constant when light is reflected or refracted, the frequency must remain constant as well.