Instructables

Do Ultraviolet rays travel faster than Light?

Because Ultraviolet Rays have higher wavelengths than the visual light spectrum. Does that mean that Einstein's wrong, or did I miss something and am I stupid?

orksecurity3 years ago
Ultraviolet rays are electromagnetic radiation. They travel at the speed of light. Wavelength has absolutely nothing to do with speed of propagation.

NOTHING moves faster than light. Period. (At least, nothing you'll ever encounter; there are some theoretical things like tacheons but but nobody has ever demonstrated that they exist or that they could interact with anything else if they did.)


Very, very close to true, but not 100%. In a transparent medium, the index of refraction is generally wavelength dependent. That's how rainbows are made.
Index of refraction, and speed of propagation, are different things though they are related. Refraction occurs because the speed is different in one medium than the other, causing the beam to be deflected as it crosses the boundary. The amount of _deflection_ depends on wavelength for reasons that aren't hard to model if you think about what's happening to the crossed B-field and E-field at the boundary. But in each medium, the speed of light is the speed of light is the speed of light.

Having said that, there are materials whose index of refraction can be altered by heat and the like, which may mean that the local speed of light may change if given time to do so. But that's beyond the scope of the original question, and "In any given medium, all light moves at the same speed" really is close enough.for a beginner.

(There's also the localized exception to "nothing can move faster than light" at the boundary between materials -- hence bremsstrahlung radiation, which you can websearch if you want to know more -- but that, again, is much more detail than Wasagi needed in order to answer the question.)

Part of giving a good answer is knowing how not to overload the questioner with unnecessary detail. I'm still working on that. Obviously.

Sorry, the speed of propagation is exactly c/n, where n is the index of refraction. The angle of deflection (the refraction from which the index gets its name) arises because of (a) the difference in propagation speeds, and (b) the angle of incidence on the boundary of the medium.

And "all light moves at the same speed in a medium" is not close enough for a beginner, since that statement directly predicts the nonexistence of rainbows. Without n(lambda), you would not get dispersion.

I don't need to do a Web search for bremsstrahlung. I have been a practicing particle physicist for 20 years, and brem is one of the real annoyances when trying to reconstruct electron trajectories. The brem photons tend to be fairly low energy (MeV or less), so they aren't well reconstructed in calorimeters, but they can change the electron momenta non-trivially.

Sorry; that parenthetical was a general observation for folks who didn't know but were interested in going deeper into the topic; I didn't intend to poke you. Obviously, you've been doing this much more recently than I have, so I'll assume you're right and concede the point. That does leave me wondering why I'm remembering it differently. Admittedly this was a few decades ago, but it was a certain small engineering school in Boston, so if there's an error it would be mine rather than theirs. Guess I'll have to hit the book for a refresher. Thanks for flagging this.
Hm. "The refractive index of a material may depend on the light's frequency, intensity, polarization, or direction of propagation; in many cases, though, it can be treated as a material-dependent constant." In a dispersive medium, refractive index depends on phase velocity, which does depend on frequency.... Ok, that's where I was misremembering the simplified case. And, yeah, I was also misremembering Cherenkov; thanks for that correction too. The one opportunity I've had to see Cherenkov radiation first hand was when Brookhaven Labs did an open house, back when I was a kid. If I remember correctly, they had an open-pool reactor going -- with not much between us and the pool but a don't-walk-past-here barrier -- and I was suitably impressed to see a blue glow at the bottom of the tank; I'd thought that was just a comic-book convention.

"They had an open-pool reactor going -- with not much between us and the pool but a don't-walk-past-here barrier"

Man, those were the days. That is a totally awesome idea (can you tell I'm a native Californian?), and the safety folks would have their knickers in such a twist now :-(

If you're really interested, you can look up "imaging Cherenkov detector" (the acronyms are RICH or DIRC); we had one on BaBar. Take a highly polished bar of something like glass or fused quartz: The Cherenkov cone when a particle hits the interface will totally internally reflect off the polished surfaces, and can be picked up as a ring by photodetectors at the end of the bar. Measure the diameter of the ring and you know the opening angle, from which you can get the particle's velocity (not momentum!). Get the momentum and trajectory (so you know where it hit), and you can figure out what kind of particle it was.

:-P No worries. I've done the same thing to experts in some of my comments.

I'm not sure you're remembering it differently. In almost every physics class, refraction is shown with a nice ideal optical ray, and a parameter n that looks like a constant. Then they pull out the plastic prism to make a rainbow. But almost never does the teacher point out that you can't make a rainbow if n is the same for all wavelengths! It's just impossible. So students are left with a fairly basic misunderstanding.

By the way, I think you meant Cherenkov radiation (see my self-reply, below) in your comment, not brem. Bremsstrahlung ("braking radiation") happens when an electron is deflected or passing near an atom in a material; the acceleration of the charged particle causes it to radiate, just like classical Maxwell's equations predict. The German name comes from the fact that electrons hitting material slow down ("brake") due to energy loss, including brem.

kelseymh3 years ago

You did miss something. In vacuum, all electromagnetic radiation (radio, microwaves, infrared, visible light, UV, X-rays, gamma rays) travels at exactly the same speed, c.

In a transparent medium, the speed of propagation is reduced by the index of refraction n, which is why light rays are bent passing into or out of such a medium (water, glass, diamond, fused quartz, whatever). In water, for example, light is travelling at just 0.75c (since n = 1.33).

Typically (despite what you learned in your high school physics class), the index of refraction varies with wavelength. so different wavelengths will travel at different velocities through that medium. It also means that different wavelengths are bent at different angles. I leave it as an exercise to the reader to name a commonly observed consequence of a wavelength-dependent refractive index.

... The last paragraph doesn't match what I learned in my college physics class. But then, I was EE, not materials science.
You can work it out trivially. If n is independent of wavelength, then for a given angle of incidence i, the beam will be bent by the same angle nsini for all wavelengths. Therefore, no dispersion, and therefore no spreading out of colors from a white-light beam, and therefore no discovery by Newton of the colors of light.

The fact that the index of refraction reduces the speed of light in a material has a very interesting consequence. Charged particles (like the electrons from radioactive beta decay) can travel at any speed less than c, depending on their mass and energy.

If a particle goes into a material where c/n is lower than the particle's own speed, it will produce an electromagnetic "shock wave," called Cherenkov radiation: This is a cone of light coming off the particle's trajectory, with the angle of the cone dependent on how much faster than c/n the particle is moving.

The radiation spectrum of Cherenkov light tends to be blue, violet, and UV. The glow you see (below) from radioactive materials stored underwater, such as the fuel rods at a nuclear power plant, is Cherenkov radiation from the beta decays.

Cherenkov glow

My answer is YES to the question, Do Ultraviolet rays travel faster than Light?

Please refer:
X-rays and gamma rays travel faster than light.
Fresh interpretation of Albert Einstein's mass-energy equivalence.
http://wiki.answers.com/Q/Does_x_ray_travel_faster_than_visible_light

I have explained mathematically why "EUV photon goes faster than an infrared photon".

M.A.Padmanabha Rao, PhD.
Former Professor of Medical Physics
While I answered yes to the question "Do Ultraviolet rays travel faster than Light" on March 9,2011, I furnish here a detailed explanation:

The famous Einstein's mass--energy equivalence is widely used for sub-atomic particles having a mass such as electron. The term mass (m) restricts its use to gamma, X-ray or light photon as they are said to have no mass. However, an attempt has been made here in explaining the puzzling experimental observations from cosmic sources on modifying the formula as follows. Energy (E ) of a gamma, X-ray or light photon is equal to square of its velocity (V). If this is really true, 40 keV X-ray photon goes 100 times faster than 4 eV light photon. Likewise, EUV photon goes faster than an infrared photon.

First of all, this article provides two reasons for experimental evidences on EUV or UV reaching much later than gamma or X-rays from cosmic sources. The following peer reviewed paper has disclosed that gamma ray causes EUV or UV from within one and the same excite atom of a radioisotope by a previously unknown atomic phenomenon. Likewise, characteristic X-ray causes EUV or UV from within one and the same excite atom of a XRF source, or a radioisotope predominant in XRF. Therefore, EUV or UV emission is expected to reach much later than a gamma or X-ray, though they all originate from a radioisotope or X-ray source.

(M.A. Padmanabha Rao, UV dominant optical emission newly detected from radioisotopes and XRF sources, Brazilian Journal of Physics, Vol.40, no.1, March 2010. http://www.sbfisica.org.br/bjp/files/v40_38.pdf


A fresh interpretation of Einstein's mass--energy equivalence holds the key for EUV or UV reaching much later than gamma or X-rays from cosmic sources. Although mass (m) restricts use of this formula to gamma, X-ray or light photon that are said to have no mass, it can be made useful on assuming that all these photons have negligible but equal mass. In other words, m can be given a value 1 to all these photons, for ease in calculation. For a light photon having mass (m) = 1, and energy at eV level, E equals to the square of light velocity C (Eq.1).

In the case of a X-ray photon having m equals to 1 like a light photon but with much higher energy Ex at keV or MeV level, its velocity is expected to be much greater than than the velocity of light C. Therefore, in the case of a photon, Einstein's mass--energy equivalence is modified by replacing velocity of light C with general term velocity (V) as follows.

Energy of a gamma, X-ray or light photon E is equal to square of Velocity (V) (Eq. 2).

If Eq.2 is really true, 40 keV X-ray photon goes 100 times faster than 4 eV light photon. I have considered light photon as a simple case for ease in the above explanation. However, on applying the actual energies for EUV and infrared radiations for E in the above formula, we realize that EUV photon goes faster than an infrared photon. Gamma ray or X-ray photon going faster than light photon could be the reason for deeper penetration into heavy metals like lead. Eq.2 explains the following two observations.

Solar flare on April 21, 2002 has shown strong, localized bursts of high energy X-rays coming from the base of the flaring region well before the initial brightening in the EUV. http://www.gsfc.nasa.gov/topstory/20020605rhessi.html

"Startlingly, Woods says, "the observatory has already found that the X-ray emissions of flares are followed, an hour or two later, by a pulse of extreme ultraviolet containing three times more energy than the initial X-ray burst". http://www.nature.com/news/2010/100724/full/news.2010.374.html

Further reference (in the nature.com) : "The observed delay in EUV emission to the X-rays has been attributed to X-rays traveling faster than EUV, against the traditional wisdom the X-rays and EUV travel at the same speed C". http://www.angelfire.com/sc3/1010/Solarfission.html

M.A. Padmanabha Rao, PhD
raomap@yahoo.com
lemonie3 years ago
No. UV is higher frequency and shorter wavelength than visible.
http://en.wikipedia.org/wiki/Planck_constant

L
rickharris3 years ago
Mmm No - although this answer is relative. Within any fixed framework the speed of light is fixed as the max you can attain. HOWEVER quantum physics does allow for speeds faster than the speed of light if you change the reference framework.

The answer isn't relative. One of the basic points in special (and general) relativity is that the laws of physics are absolute -- the same in every reference frame, regardless of the motion of the observer.

One of those laws is the constancy of the speed of light in vacuum. It does not matter how fast you are going, you will always measure light to travel at the same speed, c = 299 792 458 m/s, no more and no less. You wlll see the wavelength to be different than some other observer (the Doppler effect), but not the speed.

Regarding your comment about quantum theory, can you provide a reference? Are you thinking of the superluminal phase-velocity experiments from a few years ago?

http://en.wikipedia.org/wiki/Faster-than-light Yes -

Thanks for the link, Rick. I notice that most of the "scientific" claims are either unsubstantiated hypotheses, or unreplicated one-off experiments.

There have been some good experimental results with tunnelling through dielectrics which show "apparent FTL" propagation -- the phase velocities of particular frequency components can be computed to have travelled superluminally, but the group velocity (which is the speed at which energy and/or information are carried) remains subluminal, usually significantly so.