This behavior is often explained by supposing light to be a wave phenomenon. Waves change speed depending on the density of the medium they are moving through, and light seems to behave in this same way.
However, there is an interesting alternative explanation that does not rely on waves. In this explanation, photons take longer to travel through the medium because they bump into atoms on their way through it. The photons don't slow down, they merely take a longer path as they zigzag their way past atoms they encounter on their way. All the bouncing about averages out to a straight line, but the journey is longer, and so it appears as if the photons have slowed down.
But if this is how light moves why does red light move faster than blue light through the same medium? What is it about blue light that makes it take a longer path than red light? After all, if all photons move equally fast, and the time photons take to move through a medium vary only to the extent that they take different paths through it, then there must be a physical difference between red and blue photons.
The only explanation I can think of is that red photons must be smaller than blue ones.
Being smaller than the blue photons, the red photons bump into fewer atoms on their way through the medium, so the red photons end up traveling in a straighter line. Blue photons bump into more atoms due to their larger size. They end up taking a longer path.
Red photon taking a straighter path than a blue photon
Red light refracts at a smaller angle than blue light when traveling through a prism. The usual explanation for this is again that light is a wave phenomenon. However, if red photons are smaller than blue photons, there is a perfectly good alternative explanation.
Note that the refraction happens at the interface between the prism and the air around it. This means that refraction only happens to a photon while it is in this transition zone. If photons are different in size, while traveling at the same speed, the smaller photons will be affected by this zone for a shorter duration than the larger ones.
Red Photon refracting less than the larger blue photon
Interesting. Certainly there is a 'problem' with the slow-down-speed-up concept. Once slowed it should stay slowed.
ReplyDeleteIf your idea about photons hitting more things in a denser medium is correct, then one would expect there to be exceptional degree of scattering on exit. For example a narrow beam of light passing through a lens is seen to be quite coherent on exit - not exhibiting any marked level of scattering (apart from what may arise from imperfections in the entry/exit surfaces etc). So I respectfully don't think your explanation is correct, tho I do not know what is the right answer here.
That's a very good point you are making. However, there is a way around your conclusion about the scattering.
DeleteIf we imagine the photons doing down hill slalom through the lattice of the medium, then everything would depend on the entry and exit conditions. Transparent media may be so constituted that photons always enter or leave the medium according to Snell's law.
The slalom photons enter and exit the medium at the angle determined by the angle of the interface. The fact that the blue ones take a longer route through the medium makes no difference.
This would be similar to real life slalom in which the skiers enter and leave the course the same way, even if they take different paths down the hill.
Staying with the slalom analogy, this is how transparent mediums may work:
DeleteA photon enters the medium rolling past the first electron it encounters. This is a half roll towards one side.
Subsequently the photon takes full rolls to alternating sides as it encounters electrons to the left and the right.
As the photon leaves the medium, it takes a final half roll, leaving at the exact same angle as it entered, provided the entry surface is parallel to the exit surface.
Since large photons will roll around electrons with its geometrical center at a larger distance from the electron than a smaller photon, larger photons end up taking a longer path even when they take the exact same rolls through the lattice.
The more rolling the photons do around electrons, the bigger the time delay between the large and the small photons. This difference would be what we refer to as the refractive index.