Question

The speed of photon
A. May be less than speed of light
B. May be greater than speed of light
C. Must be equal to speed of light
D. Must be less than speed of light

Answer 1

Hint: Photons are massless particles that are thought to be constituents of light. Light gets emitted or absorbed in the form of photons. Due to the dual nature of light, there is also a characteristic wavelength and frequency associated with light.

Complete step by step answer:
The discovery of photons has been credited to Albert Einstein majorly, with the original idea coming from Max Planck's work. It was basically regarding the explanation of black body radiation that a theory of quantization of light came into existence. Later, Photoelectric effect proved the particle nature of the light. So, today light has been accepted to have a dual nature i.e., wave and particle. The particles of light are called photons. And light emitted from any body is emitted in terms of integral amounts of these photons (as was proved by photoelectric effect experiment). As photons are particles of light and we know that light moves at a definite speed c (which is fixed for all frames), photons must also move with the speed equal to the speed of light. Therefore the correct answer is option (C).

Additional information:
Different lights like violet, red, etc. have characteristic wavelength and frequency. As these lights move in vacuum, all (various frequencies) move with the same speed c. What characterizes their energy are their respective frequencies. More the frequency of a radiation more will be its energy (as given by Einstein's relation). In classical mechanics though, we have always known that kinetic energy of a body or particle is described by its velocity. Light is not just a particle but is also a wave.

Note:
In different media the speed of light changes from c to a smaller value (never higher) depending on the refractive index of the media. The photons will travel with the same speed as that of light in the media.