I've read that all electromagnetic waves travel in the same speed in vacuum. But what about air, water and other materials, is their speed different in those materials, if yes then why?
[Physics] Do electromagnetic waves travel in the same speed only in vacuum? Why so
electromagnetic-radiationrefractionspeed-of-light
Related Solutions
The particles associated with the electromagnetic waves, described by Maxwell's equations, are the photons. Photons are massless gauge bosons, the so called "force-particles" of QED (quantum electrodynamics).
While sound or the waves in water are just fluctuations (or differences) in the densities of the medium (air, solid material, water, ...), the photons are actual particles, i.e. excitations of a quantum field. So the "medium" where photons propagate is just space-time which is still there, even in most abandoned places in the universe.
The analogies you mentioned are still not that bad. Since we cannot visualize the propagation of electromagnetic waves, we have to come up with something we can, which is unsurprisingly another form of a wave, e.g. water or strings.
As PotonicBoom already mentioned, the photon field exists everywhere in space-time. However, only the excitation of the ground state (the vacuum state) is what we mean by the particle called photon.
Jon Custer hinted at something, which I think is best explained via an analogy.
Imagine you can walk along a pavement at 4mph. When the pavement is empty, it takes you an hour to travel four miles. But when the pavement is crowded, you're dodging around people and bumping into them. You're still walking at 4mph, but it takes you an hour and a half to travel the four miles. And if you're a little old lady with short little steps walking at 4mph, you're held up more than if you're a big guy with long strides walking at 4mph. Now let's look at your questions again:
But I wonder why their speeds differ in any other medium?
Because the light interacts with the material, and those interactions are wavelength dependent.
And why Red light travels faster while it has less energy than blue light?
Because the light interacts with the material, and those interactions are wavelength dependent!
Best Answer
Your question can be answered in a variety of ways that are quite different from each other depending on whether a classical or quantum type answer is most meaningful to you. However, I will take a very simple Classical argument to answer your question.
Wave propagation in Free space is the Speed of Light usually denoted by the letter $c$. Maxwell's equations tell us that this value of $c$ can be computed from the following relationship with the permeability of free space $\mu_0$ and the permittivity of free space $\epsilon_0$: $$ c = \frac{1}{\sqrt{\mu_0 \epsilon_0}} $$ However, in material mediums, both of these values of permeability and permittivity can change in such a manner that the resultant speed computed is less than the standard speed of light in a vacuum.
These values of permeability and permittivity are sort of like an impedance to the flow of the electromagnetic waves. As a result, the medium is sometimes denoted as having a velocity factor for electromagnetic waves which is a percentage of the speed of light for that medium.