In general, the refractive index of a certain medium tends to decrease with increasing wavelength.  But are there any materials in which the opposite phenomenon occurs? That is, in which the refractive index increases with increasing wavelength (which would lead to an "inverted" spectrum, if a light beam reached said material)?
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I was studying a  problem where optical light with a wavelength of 500&nbsp;nm travels through a gas with refractive index $n$.  If we look at the equations for wave motion and index of refraction  $$c=\lambda_0\nu\quad\text{(in vacuum)}$$  $$v = \lambda\nu\quad\text{(in medium)}$$  $$n = c/v$$  we see that, if the frequency is constant, the wavelength decreases in the medium compared to vacuum. Is this a consistent property at all frequencies and for all mediums with refractive index real and greater than 1?  Are there dielectrics which change the frequency (still for n > 1), and is there an example of that?
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If we have light of a particular phase that is incident on a beam splitter, I assume the transmitted beam undergoes no phase change. But I thought that the reflected beam would undergo a phase change of $\pi$. I have, however, read that it undergoes a phase change of $\pi/2$.  Which is it, and why?
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Optical density is a measure of the refracting power of a medium. In other words, the higher the optical density, the more the light will be refracted or slowed down as it moves through the medium. Optical density is related to refractive index in that materials with a high refractive index will also have a high optical density. Light will travel slower through a medium with a high refractive index and high optical density and faster through a medium which has a low refractive index and a low optical density. So to restate the question, why do optical media have different refractive indices?
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The book I'm reading about optics says at some point that "each color (wavelength) contained in the white light interferes only with itself". But why is this so?  Edit: I moved the rest of the question .
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This asked:   And also this:     The second question is contradictory; maximum frequency -> minimum wavelength.   I am asking the very opposite;    What is the minimum frequency and maximum wavelenght of electromagnetic radiation?   The lowest measured/defined seems to be 3 Hz;  Which means a wavelenght 1/3 of the speed of light; ~100 000 000 m.   But this can't be the physical limit for the wavelenght. Does such a physical limit for the wavelength exist? (Similar limit like the speed of light is for velocity).
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This is a summary from Physics World of the paper: L J Wang et al. 2000 Nature 406 277-- "Wang and colleagues begin by using a third continuous-wave laser to confirm that there are two peaks in the gain spectrum and that the refractive index does indeed change rapidly with wavelength in between. Next they send a 3.7-microsecond long laser pulse into the caesium cell, which is 6 centimetres long, and show that, at the correct wavelength, it emerges from the cell 62 nanoseconds sooner than would be expected if it had travelled at the speed of light. 62 nanoseconds might not sound like much, but since it should only take 0.2 nanoseconds for the pulse to pass through the cell, this means that the pulse has been travelling at 310 times the speed of light. Moreover, unlike previous superluminal experiments, the input and output pulse shapes are essentially the same."   I realise that the velocity of light in a medium is comprised of the phase velocity, the group velocity, and the front velocity. While the group velocity can exceed the value of c in a vacuum, the front velocity is not supposed to. The way this sounds, the front velocity is exceeding c in a vacuum by 310 times.    blue dot=phase velocity, green=group, red=front (wiki)  The wavepacket seems to exit the cell well before it enters, but the negative refractive index "forward shifts" the leading edge of the pulse. It is argued that although this is superluminal, information cannot be transmitted faster than c.  introduced a jump discontinuity into the waveform.Its max speed was c Here is a popular account from  . What if you used one photon? What if the photon itself were the information and not a carrier?
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When light travels through a material, it gets "slowed down" (at least its net speed decreases).  The atoms in the material "disturb" the light in some way which causes it to make stops on its path.  This is expressed in the material's permittivity and permeability:  Its ability to transmit electromagnetic waves.  But then what causes the limited permittivity and permeability of vacuum?  There are no atoms there to disturb the light, so what is keeping it from travelling at infinite speed?  Is there a special property of space itself which determines the speed of light?  Or is it the virtual particles in vacuum which interact with the light?
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I'm a high school teacher trying to teach my students (15year olds) about refraction. I've seen a lot of good analogies to explain why the light changes direction, like the marching band analogy, that the light "choose" the fastest way etcetc, and for most of my students these are satisfying ways to explain the phenomenon. Some students, however, are able to understand a more precise and physically correct answer, but I can't seem to find a good explanation of why the lightwaves actually changes direction.   So what I'm looking for is an actual explanation, without analogies, of how an increase/decrease in the speed of a lightwave cause it to change direction.   Thanks a lot
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I'm confused, does the absorption and emission determine the color of something? Or does that only happen when something is emitting energy?  When light hits an object, the photons get absorbed, then emitted with a different wavelength right?
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Why do different wavelength get impeded more or less when in different materials? Moving with the same speed, but a longer physical distance would imply that the fields oscillate less times in the material, but I don't know why a difference in the number of oscillations would impede the wave- I don't even know why things slow down in general. Why some electromagnetic wave would slow down just because it's entering other electromagnetic fields... It would seem to me that the only factor would be time taken to physically move some electron or something in the direction of the fields... But that seems to simple of an explanation to me.
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When I searched on the Internet for the reason of formation of , I got many explanations like  one &amp; . All the explanations consider only one spherical water droplet (like ).  Why don't the refracted waves coming from the adjacent droplets interfere with each other? For example, why doesn't a red ray coincide with a violet ray coming from another droplet at a suitable position?   How are we able to see rainbows with distinct colors? Since there are thousands of raindrops in the sky when it is raining, shouldn't it be a mess due to overlapping of different wavelengths?  Please explain.
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First of all, I know that light does indeed travel slower in a medium like air or water, but that's because the photons are bouncing off of the medium's particles and in different directions so the light beam as a whole is slowed down (or at least that's the classical explanation I've read somewhere).  But my question is about light traveling freely in the vacuum. What if an astronaut during a spacewalk switches on a flashlight towards the opposite direction he is moving. Will that light travel slower than normal?  If not, is there another way to make light move slower? For example, can you make a beam of light move so slow that you can visually perceive it being affected by gravity?
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My somewhat basic understanding of the concept comes from lectures I've attended about the Bohr-model, which explains the phenomenon as arising from the fact that certain configurations of an atom can only absorb certain wavelengths of light and other configurations can emit the same wavelength and change into the first configuration.  Now what I cannot understand is why these effects do not cancel out, and why in some instances absorption wins out and we observe absorption lines, and in other instances it is the other way around. Also, if I haven't misunderstood there are also some instances where we observe both absorption and emission at once.  So my question boils down to this: Why?
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Recently I was asked to explain the difference between reflection and total internal reflection from a purely conceptual standpoint (no math).    Let me explain what I already know.  Reflection and refraction at the quantum level are the same thing.  Light is a photon.  A photon is a discrete particle that has wave characteristics (a finite wave traveling like a bullet).  As the photon travels it collides with electrons in the matter of the medium it is traveling in.  Depending on the energy of the photon and the allowed energy bands of the medium the photon cause the electron to jump up a level.  If the photon is absorbed then the medium will increase its motion (at the macro scale increasing its temperature).  If the photon is not absorbed it will be re-emitted (really as a new photon). I have read and watched Feynman's QED lectures and book and have a pretty good understanding of his process for determining how all these paths come together to give the net path of the photon.  The general rule of thumb is that the photon wants to take the path which requires the least amount of time.  I understand how this principle goes to explain refraction and reflection.    What I don't seem to understand is why does one material seem to cause a higher percentage of refraction compared to another (metal vs. glass).  What about the electron configuration of a the medium changes the net effect of the absorption and re-emissions of the photons?  Is there a change in the probability of the photon being re-emitted in a reverse direction?  Is there a farther distance the photon can travel before being incident onto an electron?  This is the part where my understand breaks down.    When you have hit the critical angle in a medium that refracts and the light completely reflects, are the photons moving is the same manner as they would be in a material that always reflects? How does this connect to the question in the previous paragraph?  I know I have a bunch of mini questions embedded in answering this one larger question.  Any help on any of the parts would be greatly appreciated?
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Almost in every book on physics we can find a statement like "diffraction gets stronger when the size of the slit is comparable to the wavelength". Let's say we have a wall in a bathtub with a slit in it. When water waves reach the slit, the books usually come up with the Huygens' principle to explain that the points on the wavefront which are near the edges interfere in some fancy way so that the water waves spread out radially. However I do not see any connection between the size of a slit and the wavelength. I do understand that if we have a really tiny slit, there are very few points on the wavefront which produce radial waves, and if the slit gets bigger, there are more points that produce the same secondary waves which eventually interfere in an interesting way. But what it has to do with the wavelength? If we have a tiny slit and even more tiny wavelength, how does this change the game?  So for example, if we have a big wavelength, the diffraction would be:    But if the wavelength gets smaller while the slit remains the same, I'd expect the very same diffraction with the only difference that the resultant wave will have a shorter wavelength:
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I have a quick question about the refraction of light, and I'm sorry if it seems a bit simplistic or even stupid, but I'm still learning.  We know that when light passes through a denser medium, it slows down. However, is there a limit to that slowing down? If no, wouldn't it be possible, with some type of medium we make ourselves (like a crystal), to decrease the light's speed down to a point where we would be able to "see the photons moving forwards"?  I know it is way more complicated than that and that you can't see light unless some other light bounces off it, what I mean is slowing down light to an extreme and unusual speed). That sounds completely absurd but I can't seem to find what is wrong. Can anyone can explain it to me? :)
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The intensity of light (as calculated from time average of the poynting vector) is given by $I = (1/2) \epsilon v E_0^2$. Here the intensity is dependent on the velocity of light in the medium. The refractive index also depends on the velocity of light. So is it safe to say that the intensity of light depends on the refractive index of the medium?