It was mentioned in the Light and Color unit that each color is characteristic of a distinct wave frequency; and different frequencies of light waves will bend varying amounts upon passage through a prism.
In this unit, we will investigate the dispersion of light in more detail, pondering the reasons why different frequencies of light bend or refract different amounts when passing through the prism.
Earlier in this unit, the concept of optical density was introduced. Different materials are distinguished from each other by their different optical densities. The optical density is simply a measure of the tendency of a material to slow down light as it travels through it. As mentioned earlier, a light wave traveling through a transparent material interacts with the atoms of that material. When a light wave impinges upon an atom of the material, it is absorbed by that atom.
The absorbed energy causes the electrons in the atom to vibrate. If the frequency of the light wave does not match the resonance frequency of the vibrating electrons, then the light will be reemitted by the atom at the same frequency at which it impinged upon it. The light wave then travels through the interatomic vacuum towards the next atom of the material. Once it impinges upon the next atom, the process of absorption and re-emission is repeated. The optical density of a material is the result of the tendency of the atoms of a material to maintain the absorbed energy of the light wave in the form of vibrating electrons before reemitting it as a new electromagnetic disturbance.
Thus, while a light wave travels through a vacuum at a speed of c 3. The index of refraction value n provides a quantitative expression of the optical density of a given medium. Materials with higher index of refraction values have a tendency to hold onto the absorbed light energy for greater lengths of time before reemitting it to the interatomic void. The more closely that the frequency of the light wave matches the resonant frequency of the electrons of the atoms of a material, the greater the optical density and the greater the index of refraction.
A light wave would be slowed down to a greater extent when passing through such a material. What was not mentioned earlier in this unit is that the index of refraction values are dependent upon the frequency of light. For visible light, the n value does not show a large variation with frequency, but nonetheless it shows a variation. This creates a dipole field that opposes the incident field of the electromagnetic wave, and reduces the perpendicular component of the electromagnetic field, and this changes the direction of the wave.
Fermilab have a good video explaining this, which is not too detailed. They don't talk about the atoms, though -- they just say the charges are randomly placed in the material. But these are the details they have skipped over.
Most undergraduate electromagnetic courses will also cover this in detail. David Tong's lecture notes are quite good. You want chapter 7, Electromagnetism in Matter. You can also search online for any other set of notes that you like better: if they are posted, they will usually be open for anyone to read.
Now, this explains why light refracts, but why does blue light refract more than red? The light ray is composed of oscillating electric and magnetic fields. So the charges in the atoms are not just moved in one direction and that's it, they are being oscillated because the field itself is oscillating. As such, the induced dipole field acts like a damped harmonic oscillator that is being driven by the external field the light ray. And like all damped harmonic oscillators, the response of the oscillator depends on what the driving frequency is with respect to the natural frequency of the oscillator.
Going back to Semoi's answer, for glass, for example, the natural frequency of the atomic dipole is much higher than the frequency of visible light, so the closer the driving frequency the frequency of the light ray gets to it, the more in phase the incident field the light ray will be with the induced dipole field. And the more in phase these two fields are, the stronger they will cancel. And the stronger this cancelling is, the more the ray will be refracted, as per the explanation above.
A cursory search shows what looks like a few university videos: again, pick whichever seems easiest to you to understand. Where Semoi talks about transitions, this is the modern semiclassical understanding of the process, whereby rather than shifting the position of the electron cloud, the electrons are instead excited into higher orbits. The higher-energy orbits are located further away from the nucleus, so effectively this is like moving the electron cloud.
The only difference here is a more accurate description of the atomic dynamics -- the end result is basically the same. It has a good description of the basic physics, but, again, it may be too advanced.
It will cover all the concepts though. If you can get your hands on a copy, then I recommend giving it a read. You're specifically asking about the case of normal dispersion, where a decrease in index of refraction leads to an increasing wavelength, which is the case for most transparent media.
On a microscopic level an incoming electromagnetic wave displaces particles of different charge and thus are creating dipole moments polarization. The strength of this effect is characterized by the electric susceptibility of the material, which again depends on the wavelength. Maybe think of it in this way. The particles are inert, they don't want to be oscillated. So they take a bit of time to move, after the EM-wave passed.
If you're now applying light of different wavelengths and thus different frequencies, they will move faster or slower, depending on how fast the EM-wave oscillates.
Edit2: It is important to notice that the velocity of a light-source can't affect the speed of light in vacuum, because of special relativity. The slowing of the speed in the material is due to the Ewald-Oseen extinction. Picture walking towards a plowed field, which you meet at an angle to the edge of the field.
The furrows run parallel to the edge of the field at which they meet. The furrows make it harder to walk over the furrows at an angle, and you compensate by turning into the field. However, longer legs mitigate this effect so that you don't need to turn as much.
In this analogy, you are a photon, the plowed field is a substance with a higher index of refraction, and the length of your legs is the wavelength of the photon. Sign up to join this community. The best answers are voted up and rise to the top. The secondary rainbow that can sometimes be seen is caused by each ray of light reflecting twice on the inside of each droplet before it leaves.
This second reflection causes the colours on the secondary rainbow to be reversed. Red is at the top for the primary rainbow, but in the secondary rainbow, red is at the bottom.
Learn more about the many different kinds of rainbows and how they are formed from the Atoptics website — Rainbows reflect and Rainbow orders. Learn more about human lenses, optics, photoreceptors and neural pathways that enable vision through this tutorial from Biology Online.
Add to collection. Activity ideas Use these activities with your students to explore refration further: Investigating refraction and spearfishing — students aim spears at a model of a fish in a container of water. When they move their spears towards the fish, they miss! Angle of refraction calculator challenge — students choose two types of transparent substance. They then enter the angle of the incident ray in the spreadsheet calculator, and the angle of the refracted ray is calculated for them.
Light and sight: true or false? This activity can be done individually, in pairs or as a whole class. Useful links Learn more about the many different kinds of rainbows and how they are formed from the Atoptics website — Rainbows reflect and Rainbow orders. Go to full glossary Add 0 items to collection.
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