Liberal arts pleb here
I'm taking some low level physics course right now and I'm having a little trouble understanding dispersion/refraction.
I get the basic principle, that light propagates at different speeds through different mediums.
Refraction is the change in the light's vector/direction when it passes through a certain medium, correct?
And dispersion is the act or process of light being separated into its constituent components, right?
Why does light change direction when entering a different medium?
What exactly is refractive index? What does it apply to? What determines it?
Would it be correct to say that the physical properties of the prism determine (or influence) the refractive index? Or is that wrong?
>>8134362
Also, what is dispersion ration/ index of dispersion, and how does it differ from refractive index?
>Why does light change direction when entering a different medium?
The best way to explain this is with the wavefront theory of light. Imagine you have a point light source, and imagine the light radiating from the point is doing so as fronts of concentric spheres. Each sphere is one wavelength apart and these are known as wavefronts.
Now imagine a point source very far away, and to keep it simple lets only do 2 dimensions. The wavefronts are now circles, and far enough away from the point source the curvature is so large that small sections of the front appear to be flat. In physics, light coming from a point source infinitely far away is called collimated and the wavefronts are parallel lines.
So, now I can attempt to answer your question. Look the pic, The black line is the surface between the mediums, the red lines are the wavefronts, and the blue lines represent the ray or path of the photon (which is another common way to look at light).
See how the blue line to the right isn't as far as the one to the left because it's velocity went down in the second medium. You also may notice that the wavelength in the second medium is shorter. This is because speed of light= wavelength*frequency, so we also learn that the frequency of light remains the same no matter what medium (and is determined by the source).
Each wavefront represents a particular moment in time (this concept is known as coherence. Light can be coherent in time, space, both or neither). Because the speed is lower in n=1.5, the part of the wavefront that enters the medium doesn't travel as far, and this effect continuously happens and stacks down the wavefront as it keeps moving into the surface, sort of like a wave crashing and the crash moving down the line.
>>8134396
Alright I think I can follow that. How about the rest of my questions?
>>8134415
>Refraction is the change in the light's vector/direction when it passes through a certain medium, correct?
yes
And dispersion is the act or process of light being separated into its constituent components, right?
yes
What exactly is refractive index? What does it apply to? What determines it?
n=c/v
it's the ratio between the speed of light in vacuum and the speed of light in medium. You see it come up in snell's law and basically all optics.
>Would it be correct to say that the physical properties of the prism determine (or influence) the refractive index?
It doesn't have to be a prism, but yes, the refractive index depends on the molecule/atom itself, the phase/temperature/density, crystal structure if it's a solid, intermolecular forces play a role too. Refractive index is dependent on the wavelength of light as well, this is why dispersion is a thing. Large scale things like the shape of the prism have no influence on it.
>>8134362
This theory represents light as corpuscles. So it's incorrect.
>>8134429
If the shape doesn't matter then why are triangular prisms always used in these experiments?
And what is index of dispersion?
>>8134436
the angle and geometry of the prism determines the angle of dispersion. This depends on the index of refraction too, but the shape doesn't determine the index of refraction in any way.
What if take an empty prism?
>>8134362
In free space, light is just light. It has no mass and hence moves at the speed of light.
When the wave of light enters a medium, you're no longer dealing with just a wave of light, it becomes a mixture of a wave of light and the vibrations and excitations of the material, all moving together. The details of this involve quantum field theory (i.e particle physics and condensed matter physics) so it's fairly complicated, but the point is that when the light is inside the material, this composite vibration+light wave acts as if it has mass, which means it moves slower than the speed of light. Blue light has more energy and for a massive particle more energy means moving faster, which is why it is diffracted more.
Light particles are called photons, quantum vibration "particles" are called phonons (they aren't fundamental particles, they're vibrations in a material that behave like a quantum particle), and the mixture of the two is called a polariton, and these polaritons have mass. Pic related is a classical picture of a phonon.
All of this is a bit of a simplification, because excitons are more important here than phonons, but a phonon is easy to picture as a little (quantum) vibration, but an exciton is trickier - it's a bound state of an electron and an "electron hole".
Anyway, don't worry about that, the point is light -> light+vibration, and the light+vibration mixture has mass.
>>8134789
and direct the light beam to it
>>8134931
Jesus Christ, when this fucks finally admits the existence of the aether instead of pile up one bullshit after another?
