Why red light is slower than blue light in glass??

Question

Why red light is slower than blue light in glass??

Answer 1

A simple way to think of the first two answers may be:

Consider that light has both wave-like and particle-like attributes.  Consider the particle attributes ... as the "photon".

Red photons are "less energetic" than blue ones (that is what distinguishes them from each other).  Glass apparently presents a resistance to the progress of photons as they try to pass through it, like a person trying to "slog" through waist-deep water.

Just as less strong/energetic persons will "slog" through such water more slowly, so may the analogy work for photons of different energies "slogging" through glass.  The less energetic ones will make slower passage.

As with persons, the slowing isn't "permanent" (as long as, with persons, the distance to "slog" isn't very far!): as soon as the person ... or the photon ... gets to "the other side" of the resistant medium, he/she or it can resume his/her or its "normal pace".

Of course if a red and blue photon enter the glass at the same time, when they emerge the blue one will do so ahead of the red one.  So even if they emerge into a vacuum, where their speed of transit will be the same for red and blue again, the blue photon will remain ahead of the red one by the delay difference for the two caused by the glass. Thus the "spreading" of the colors in a "packet" of multicolored light "in time" or "by energy" that leads to the prism effect if the glass is not uniformly thick.

This is why in fiber optics, when sending digital/binary signals, it is necessary to use a light signal of a very narrow bandwidth (like from a laser) ... or a detector that has a very narrow response to different light frequencies.  Otherwise, over distance the signals become "garbled" by the different transit times of different wavelengths (photons of different energies) through the fiber optic material.  White light would only work for short distances (if the detector is sensitive to all colors, too), or alternatively, low "frequencies" of digital signal  ... because you have to wait for the last red photons to "get there" before you can detect a zero ... no light.

In the photon "horse race" for visible light, always bet on "Old Blue" (or purple if it's in the race) ... you'll win every time!

Answer 2

Color is caused by the decrease in light energy, if a color has lost less energy it will move at a different speed than light which has lost more energy and changed color

Answer 3

refractive index of red light is more than that of blue.

Answer 4

The index of refraction 'n' is defined as: n = c/v, where 'v' is the speed of the light in the medium in which it's traveling.

So....v = c/n

If 'v' were the same speed as 'c' (n = 1) the light would not slow down, but the index for glass say is about 1.5, which makes it a little slower due to interactions with the atomic structure of the material.

The index (n) is a function of wavelength, so it varies quite a bit from one wavelength to another. Most of the time in school its taught that glass has an index of 1.5 and that's it, but in reality the index has a slightly different index for all wavelengths.

But in essence it's this dependence on 'n' that gives the speed 'v' a lesser value than the speed of light 'c'.

Hope this helps.