Talk:Light

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Help with this template Comments: edit – history – watch – refresh Please note the following: "PISS OFF - U R A FUCKER The speed of light in a vacuum is exactly 299,792,458 meters per second (fixed by definition). Although some people speak of the "velocity of light", the word velocity is usually reserved for vector quantities, which have a direction." As you can see somebody is trying to be a smart ass by inserting profanity in the definition. (see original article, as of Feb 13th 2007) This should be corrected ASAP

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Restructure according to the guidelines proposed in Wikipedia:WikiProject Science, ie. with the following main sections: Introduction : importance of light for life and humans; brief history of the understanding of light; sources of light (man-made or natural) Light in practice: (avoid theory here) Types of light: normal(incandscent, fluorescent) HID (football stadium lights) New eco-watt fluorescent lights and how they change they way colors look., laser; visible vs infrared NO SUCH THING AS INRARED "LIGHT", in optics fiber, LED is the future, man. Properties of light: Spectral characteristics, brightness, color, frequency, coherence, modulation, speed of light Light in the Universe: appearance of light after big bang, sources of light, ... Light in everyday life: Hw different light effect humans (depression), everyday sources, human eye Light in industry: sources, conversion from/to electricity; some applications of light (laser, ...); application of optics Light in science: spectroscopy, astronomy, interferometry (If tht were important, than it should be in those sections) Light in theory: introduce quantum electrodynamics, explain how the current theory explains light and its applications History Older theories of light History of measurement of the speed of light
Priority 1 (top)

Contents 1 Music faster than the speed of light 2 Help re velocity faster than c 3 Light years 4 Refraction 5 V for velocity? 6 Infrared 7 Explanation on using Jupiter's moons to calculate speed of light confuses me 8 Restructure 9 lightyear as an unit of measure 10 Quantum Electrodynamics 11 Does fluorescence go here? 12 Scientific notation or Engineering notation? 13 A question about light 14 roomfilling density of lightbeams from everywhere ? 15 Gravity 16 WP:FAC 17 Visible Light No such thing 18 Visible Light is "oily oil" 19 Weight of light 20 GA nom has failed 21 history 22 1st Line 23 Under: Optical theory 24 light and mass 25 what are the ways light travels 26 Frequency or wavelength 27 Light as EM of any wavelength 28 merge tag 29 Wave theory

[edit] Music faster than the speed of light

I saw a show on PBS where a scientist transmitted music faster than c. He recorded it and played it back on a tape recorder. It was staticy, but recognizable.

--Alan D

?

[edit] Help re velocity faster than c

See plea for help in article....

gogi "Light has a mass. The foton has a mass, i know this because the light slow down when moving trought sea, and large object like eart bend the light when passing near bay. Bending of the light does not have nothing with bending of the space."

As far as I understand it, the evanescent wave coupling stuff is the same as the other group velocity > c stuff in the case of extreme absorption.

There seems to be a lot of interest and half-explainations of this topic scattered around the wikipedia, I may try to write a superluminal communication article, In My Copious Free Time.

What's really interesting is the effect that occurs between the plates of a Casimir Force experiment, that's the only legitimate v>c stuff I know about. -- DrBob

How is "velocity" incorrect? It was good enough for Einstein:

In short, let us assume that the simple law of the constancy of the velocity of light c (in vacuum) is justifiably believed by the child at school. [1]

I suppose we should make it clear that we are referring to the velocity of the propagation of light. --TheCunctator

Well, if the velocity of light was a constant, then it would always move in the same direction. (Velocity being a vector quantity.) This is not true for all observers (the direction of light propogation being different in different reference frames, e.g. the light-clock thought experiments), and not even true for a single observer (light being able to go in any direction). However, the speed of light, being the magnitute of the velocity, is the same for all observers.

Maybe I'm being nit-picky but it seems that Einstein was being loose with the terminology in the article you reference. I still think 'speed' is the most technically correct. -- DrBob

I agree, velocity is generally taken to be a vector quantity and speed is a scalar. The speed of light in vacuum is constanct and equal to c; the velocity vector of light in vacuum is not constant, because light can travel in different directions; the magnitude of the velocity vector is c. --AxelBoldt

Fine with me. I think it would be fair to note that Einstein used "velocity", and define speed. --TheCunctator

[edit] Light years

Are light-years really the prefered unit in Astronomy? I have heard that they are mostly used in popular science articles, but real astronomers use Parsecs instead. I'm not a real astronomer, so i can't vouch for this. -- Geronimo Jones.

The star and galaxy catalogs tend to use parsecs rather than light-years, so I'd guess it's a safe bet.

Yep, astronomer use parsecs (just over three light years) for light year sized measurements, with MegaParsecs, KiloParsecs etc also being used. This is due to parsecs coming from the parallactic measurements techinique of determining distances from angles of parallax - ParSec = one Prallatic Second of arc, where a second here is one sixtieth of a minute, which is one sixtieth of a degree. For larger scales, doppler shifts (z) are used.--MilleauRekiir 22:53, 12 May 2006 (UTC)

[edit] Refraction

"Light passes through liquids such as water or solids such as glass at reduced volocity."

Good point, but I have three reservations:

1 - It's "velocity", not "volocity"

2 - I think it should be "speed", not velocity. Velocity is a vector, right? (Somebody help me out here :-) )

3 - Maybe this could go in the paragraph here about the speed of light, or in the entry on speed of light linked there

- Thanks :-)

All good points, I had a lot of trouble that day getting on to Wikipedia. If I can keep getting in I'll try to do something better. My question is why you took all this time on talk and didn't just do a better job including the material.

[edit] V for velocity?

In the section on speed of light, the symbol v - for velocity? :) - renders in my browser as a greek letter nu, which could cause confusion as that is commonly used to represent frequency. This happens throughout except in the last equation, v=c/n , where it is rendered correctly as v, but the whole equation is in larger type. I couldn't find anything to explain this in the markup, except that the last equation had spaces around the "=" whereas the first two did not.

Irrelevant as this seemed, I put spaces in the first two and, in preview, the problem was fixed - v rendered as v, and all equations similar (large) font. However, when I saved the changes, it reverted to the earlier rendering, apart from the added spaces.

Anyone have an idea what's going on and how to fix it? Do others see the same problem or does it display normally for you? (I'm using Opera 7.23, Windows). I've also had a look in Internet Explorer and seen similar, except a slight difference in font appearance makes me wonder whether I'm seeing, not a Greek "nu" but a cursive "v". Either way, it is confusing. --Richard Jones 16:16, 13 Mar 2004 (UTC)

I think it is not a nu but a v in italics; that is what TeX rendered as PNG gives, as opposed to TeX rendered as HTML. For more uniformity specify HTML if possible in the preferences. --Patrick 20:56, 13 Mar 2004 (UTC)

Thanx - that fixes it. - Richard

[edit] Infrared

The discussion of infra-red (IR) and infra-red cameras is incomplete. Near infra-red is just outside the range of humans vision (730nm - 1100nm), and most infra-red (night-vision) cameras detect these wavelengths. Actually all CCD - CMOS cameras will detect these wavelengths, though normally a filter is put on top of the image sensor to stop wavelengths greater than 700nm from reaching the cameras sensor. Infra-red (forget the exact wavelength, believe between 2000nm - 5000nm?) is detected as heat. It requires a completely different kind of camera / sensor to detect these wavelengths, and these cameras are quite expensive.

[edit] Explanation on using Jupiter's moons to calculate speed of light confuses me

I was confused by this explanation:

when Earth and Jupiter were not as close, the moon's revolution seemed to be more. It was clear that light took longer to reach Earth when it was farther away from Jupiter. The speed of light was calculated by analyzing the distance between the two planets at various times.

Why would the moon's revolution seem to be more? Let's take a revolution when Jupitar is near Earth starting at n1 and ending at n2. Then, consider a revolution and when Jupiter is far from Earth starting at f1 and ending at f2. Ok, so it takes at extra delay d for the light of the start of the revolution to reach Earth when f1 so, in fact, the revolution started at time f1-d. But so what? The revolution ends at nearly time f2-d also. Thus:

(f2-d) - (f1-d) = f2 - f1 = period of revolution = n2 - n1

Distance from Earth has no bearing.

After thinking for a moment, I have a guess at what the explation was trying to say.

When Jupiter and Earth are moving apart, the revolutions of the moon would seem to take more time. Similarly, the revolutions would seem to speed up as Jupiter and Earth become closer. That is, a distant Jupiter moving closer would seem to have faster moons whereas a nearby Jupiter moving away would seem to have slower revolutions.

That is, relative motion and not distance is what changes the apparent period of the moon's revolution. If I am correct, then a better explanation would be:

when Earth and Jupiter were moving apart from each other, the moon's revolution seemed to take longer. It was clear that light took longer to reach Earth because Earth and Jupiter had moved apart during the start and the end of the moon's revolution. Similarly, as Earth and Jupiter came closer together, the moon's revolution seemed to take less time. The speed of light was calculated by analyzing the distance between the two planets at various times.

However, maybe I'm being a bit too wordy. How about:

when Earth and Jupiter were moving apart from each other, the moon's revolution seemed to take longer. It was clear that light took longer to reach Earth as Earth and Jupiter moved apart. Similarly, as Earth and Jupiter came closer together, the moon's revolution seemed to take less time. The speed of light was calculated by analyzing the distance between the two planets at various times.

So, would some astronomer confirm/reject my suggestion?

WpZurp 02:00, 8 Sep 2004 (UTC)

Anyway, I've put in what I believe to be a correction. Makes sense to me. Hope it's right and hope you all like it.

WpZurp 20:13, 19 Sep 2004 (UTC)

[edit] Restructure

I've restructured the article somewhat, to move the Theories of light down to the bottom, as the todo and WP:FAC comments suggest, plus other general tidying, also deleting one of the prism images. Some parts are clearly in need of more work. -- ALoan (Talk) 14:43, 11 Nov 2004 (UTC)

Should we place the headlines in the article from the Todo-list above? That way it would be easier to edit the article into new subsections. Thechamelon 12:52, Apr 19, 2005 (UTC)

[edit] lightyear as an unit of measure

light-year is a deprecated but accepted unit. The only unit that should be used in SI should be the meter. The unit parsec is strongly preferred to light-year. Since light-year unit is, to some degree, common used, it worth to explain what it is.

[edit] Quantum Electrodynamics

I think a brief explanation of this theory--or at least a link to the related Wikipedia entry--would be appropriate, given that it has resolved the wave-particle duality of light, which this article incorrectly describes as the modern theory of light.

[edit] Does fluorescence go here?

Hey, in the section about ways to quantify light or measure light: plese don't forget the Spectral power distribution. It is a graph/plot generated by a spectroradiometer reading the Watt/Flux...something at 31 regions along the spectrum from 400 to 700nm.--done

I can help. I need the definition bad.--[[User:Dkroll2|Dkroll2]] 22:13, Dec 14, 2004 (UTC)

[edit] Scientific notation or Engineering notation?

The article currently contains the following statement:

Light is the visible portion of the electromagnetic spectrum, between the frequencies of 3.8×10¹⁴ hertz (abbreviated 'Hz') and 7.5×10¹⁴ Hz. Since the speed (v), frequency (f or ?), and wavelength (?) of a wave obey the relation...

I don't know about you, but I always find I have a much faster "intuitive" grasp of a quantity if it's stated in "Engineering Notation" (such as "380 THz" or maybe "380×10¹² Hz") rather than pure Scientific Notation ("3.8×10¹⁴ Hz"). I think it's because I routinely work with kilo, mega, giga, tera, and with nanometers and the rest of the SI units and prefixes but not with pure scientific notation. Does anyone else agree with this? And do you agree with it to the extent that we change the article to use this form?

Atlant 12:57, 23 Apr 2005 (UTC)

• I don't think there can be any objections to using SI-standard abbreviations. Not so sure about Engineering notation in the sense of using the raw unit but constraining the exponent of ten to be a multiple of a 3. It's offered e.g. on HP calculators, but is it really used in engineering publications? Dpbsmith (talk) 15:11, 23 Apr 2005 (UTC)

I've only seen use of prefixes. - Omegatron 17:37, Apr 23, 2005 (UTC)

I guess I'm arguing in favor of using the SI prefixes rather than ANY scientific or engineering notation.

Atlant 00:36, 24 Apr 2005 (UTC)

How about: "Light is the visible portion of the electromagnetic spectrum, between the frequencies of 380 THz (3.8×10¹⁴ hertz) and 750 THz (7.5×10¹⁴ hertz)." The THz link goes to the SI multiples of Hertz, so one could even omit the info in parenthesis. Splarka 01:09, 24 Apr 2005 (UTC)

Ah. Cutting the Gordian knot, eh? Using both works for me. Dpbsmith (talk) 11:30, 24 Apr 2005 (UTC)

That's great -- let's do that.

Atlant 12:52, 24 Apr 2005 (UTC)

Snip Snip. (Since we all seem to be in agreement) Splarka 19:28, 24 Apr 2005 (UTC)

[edit] A question about light

Someone had already posted "Visible Light: No Such Thing" above, and it turned out to be a bit different than what I want to ask:

Is light actually visible? I'm looking at my desk lamp now. The light bulb is illuminated and casts light onto the desk and the various items thereon. But I don't actually see the light emitting from the bulb: I see the bulb itself, and the items that are lit. This causes me to guess that we don't actually see light, we see objects that either emit or receive light. (The only exception I can think of would be a laser). Maybe someone here can tell me if I'm way off. -- Gyrofrog (talk) 07:49, 24 August 2005 (UTC)

You "see" an object when your brain interprets the signals coming from your eyes. The signals come from the rods and cones in your eyes when they detect photons. The photons come from objects that emit light, either direct from the light source or after the photons are bounced off or pass through an intermediate object or medium. -- ALoan (Talk) 10:59, 24 August 2005 (UTC)

I'm not sure this is a very meaningful question. However, it may be worth noting that light, being a vector field, always has directionality associated with it. For example, you do not see a "beam" of light directly when viewed from the side. If a laser beam is shining in a vacuum, you cannot see it at all from the side, no matter how bright it is. When a light source emits a beam of light, you can see it directly if it is shining straight at you, but it does not look alike a beam, it looks like a very bright starlike point. From the side, you only see a "beam" if there is something in the beam that can scatter or reflect it out of the beam. Perhaps this is what you are thinking of.

Decades ago, when it was usual for people to smoke in movie theatres, you could look up and see a very solid-looking cone of light, with moving cones of light, dark, and color within it. Now the projector beam is almost invisible. Dpbsmith (talk) 15:28, 24 August 2005 (UTC)

Thanks, I probably could have worded my question in a better way. But you both confirmed what I was thinking. -- Gyrofrog (talk) 19:19, 24 August 2005 (UTC)

This was on Qi, Stephen Fry said that light is NOT visible only its effects are visible. The easiest way to think about it is if you could see light, that's all you'd see. Think about it......

[edit] roomfilling density of lightbeams from everywhere ?

What keeps on striking me is that lightbeams (in a lighted room or in the landscape) come reflected from everywhere to everywhere. At any point lightbeams must be crossing one another from every direction (except of course inside objects). It's like connecting every point on a sheet with every other with a pencil .. just a big mess .. In just one point all reflected beams meet, and alike in every other point .. How is it possible, that all those beams don't hinder / interact / mess up / interfere (?) one another when thinking of them as electromagnetic wave 'packages' (packs, packets, light quantum )? Is it because they are 'constituted' - as far as actual theory is concerned - .. constituted only of the abstract notion of their 'fields' (electromagnetic field) propagating through space? If so, then .. What 'is' a lightbeam? Or, what is it thought of by theory? Is it because they all have a slightly different direction of propagation? (The corresponding article: electromagnetic interaction is very meager when read on this special behalf of free travelling electromagnetic waves (i'd rather cling to everyday's "lightbeams") crossing one another, and other quantum electrodynamics -articles mostly deal with charged particles, rather than em-waves)

Or do I just not understand , what a field is or how it can constitute a real lightbeam?

thk for reply

roeighty 217.248.62.13 217.248.62.13 00:38, 30 September 2005 (UTC) 00:38, 30 September 2005 (UTC)

You're exactly right, and that is a good question, in fact a superb question. I'm going to answer your question with another question. What does a single ray of light look like? It does not look like a sci-fi movie of a laser beam. That's not the beam itself. When you "see" a beam of light from the side, and it looks like a bright line, what you are really seeing is millions of other beams of light resulting from the beam itself being scattered by air molecules or particles of dust or smoke. Dpbsmith (talk) 10:41, 28 September 2006 (UTC)

I think the big reason you don't see all sorts of interesting interference effects is that most light that fills most rooms is completely non-coherent (uncorrelated in phase, frequency, or polarization), so any interference effects that occur happen at a microscopic scale where you never notice them.

I'd imagine that it would be pretty interesting to experiment with a room full of well-correlated coherent light. A small-scale example of this occurs when you defocus the beam from a laser pointer so that it creates a large-ish spot on the wall. You will see all sorts of interesting "grain" effects in that spot; these are a result of constructive and destructive interference. (I don't know if it's true, but I've been told that near-sighted (mypoic) people see the grain pattern move one way when they move their heads and far-sighted (hyperopic) people see the grain structure move the other way, but I can never remember which type of vision gets which type of motion.)

Atlant 13:00, 3 October 2005 (UTC)

• Unless you're at extremely high intensities (when effects from quantum electrodynamics become important), light beams in a vacuum don't interact except by a linear superposition of their electric fields. So at the point they meet you can get interference effects (if the beams are sufficiently coherent), but there's no lasting effect on the beam and one will pass straight through the other as if it wasn't there. As Atlant says, for ordinary light sources the effects of interference aren't very noticable. If you illuminate a large area with a laser, you tend to see speckle patterns, which is simply due to inteference between different parts of the beam.

Of course if you're working in a medium, light beams can and will interact with one another, to produce nonlinear optics and things such as the photorefractive effect. -- Bob Mellish 15:54, 3 October 2005 (UTC)

Ok, incoherence, no linear superposition, interference only in the meeting point, but passing it unchanged .. that explains it.
Light is a wonderous phenomenon, trying to understand the nature of the universe.

[seeing unclear under water stuff deleted by roeighty] roeighty

13:50, 5 October 2005 (UTC)

I'd say that the concept of "light beams" should be used to help students learn about angles of refraction and reflection, and should then be discarded. If you shine some light from a torch/flashlight in a dusty room, you see a "beam" reflected in the dust particles. This beam is a cone-shaped volume of space that is filled with a series of spherical wavefronts radiating from the source. The wavefronts are the aggregates of the amplitudes of the wave functions of countless photons. You can make this cone as narrow as you like, but it never turns into a pure linear "beam"; it's always a cone. Even laser beams are just very narrow cones. It's only classical optics experiments that fool us into thinking that light consists of "beams". Thinking in terms of "beams" makes it harder to understand what's going on in the world of photons. --Heron 20:53, 22 July 2006 (UTC)

I disagree. Beam is the layman way of says k-vector, which is very fundamental in the world of photons. If your beam is really a cone, it has several k-vectors and therefore is not a 'beam'. What you are saying is similar to saying we shouldn't use the word frequency, because most light is actually has some finite spectral bandwidth and therefore using that word makes understanding 'the world of photons' difficult. Waxigloo 18:13, 27 September 2006 (UTC)

A light wave?

That's an interesting explanation. I didn't know about k-vectors, so I didn't know that beams had physical reality. Are you saying, then, that a photon occupies only one spatial dimension? And is a picture of a 'light wave', like the one opposite, actually a picture of a photon, or part of a photon as it whizzes through the picture from left to right, or of the sum of lots of photons, or something else? --Heron 20:12, 27 September 2006 (UTC)

I think bringing the word photon into the picture just makes the discussion more complex that it needs to be. There are very few cases where quantization of the field need be applied, and a discussion of k-vectors is not one of them.

The k-vector is basically the momentum of the light and usually points in the direction of propagation. So, you run into an uncertainty principle (x and p are conjugate related by Fourier transform) kind of issue here: if the k-vector is very well defined, then the position x is not. For example: a plane wave has a single k-vector, but occupies all space (a very poorly defined x -- hence the name: its wavefronts are infinite parallel planes).

My previous explanation was a bit simplistic, and to some extent you are right: there is no light occupying a single x and a single p and thus acting like a perfect beam. But geometric optics (based on beams) is very handy, because all the ray matrices developed there can be applied to Gaussian beams, a much more realistic description of beam propagation (under the paraxial approximation).Waxigloo 22:06, 27 September 2006 (UTC)

Thanks again. I'm happy with the classical explanation, now I know that it does not claim the existence of perfectly one-dimensional beams. --Heron 19:53, 28 September 2006 (UTC)

[edit] Gravity

how is light afected by gravity?

Beams of light follow "straight lines" ("light-like" [geodesic]]s) in space-time. Massive objects effectively cause space-time to be bent, so the "straight lines" are themselves curves. For example, where a massive object, such as a cluster of galaxies falls along the sightline from Earth to a distance astronomical object, such as a quasar, the massive object can bend the light like a lens, a phenomenon known as gravitational lensing. HTH. -- ALoan (Talk) 12:30, 22 November 2005 (UTC)

There must be equations for the bending of light that don't use curved geometry. Can the bending of light by gravity be directly related to its energy. For example, if I was going to make an educated guess of how light bends, I would start with the equation accelleration = m G/r^2 and transform it into A = E G/(rc)^2. Then using that I could predict how light would be bent. Using relativity, the speed wouldn't change, but the direction would. It would also predict that the bending of light depends on its frequency (which is proportional to its energy). I have no idea if this is even near right, but the equations for how light is affected by gravity would be very useful to have somewhere around here. Fresheneesz 19:17, 12 April 2006 (UTC)

[edit] WP:FAC

I think this article is pretty good, and I put it on Wikipedia:Featured article candidates over a year ago (the disussion from November 2004 is in the WP:FAC edit history). I would like to get this article to featured status, like speed of light. Any comments for what needs doing? -- ALoan (Talk) 12:30, 22 November 2005 (UTC)

[edit] Visible Light No such thing

I was taught in imaging science to never use these 2 words together ; Visible Light It is redundant. If it is Light then it is by definition "Visible" If it is visible then there is some light going on in the equation.

Visible portion or region of the electromagnetic spectrum is ok. Or just Light. Light Source, etc. I know it seems short and lonely, but it is correct.

I think it is too common to use "light" when referring to any type of EM radiation used for the purpose of illumination, not just directly visibly. For example, we say "infrared light" often, "infrared radiation" is kind of cumbersome when referring to the frequencies near to visible. If an invisible source of IR radiation is used in conjunction with night vision goggles, it becomes a light source for the camera. Some insects can see "UV light". "UV Radiation" is used when describing ionizing UV that causes cancer, sun tans, and sterilizing of bacteria. "UV Light" is used when referring to near-visible illumination (as in fluorescence, or when describing the visible spectrum of some insects). If you want to argue semantics into the ground then "Visible Light" is incorrect. But if you want to get by outside a physics class, just accept that the phrase "Visible Light" == "Light visible to native human eyes", and that other forms of light exist. 64.162.10.163 21:52, 28 Mar 2005 (UTC)

The word "light" is not limited only to wavelengths visible to human beings. Technically, light is electromagnetic radiation of optical frequencies, which include not only visible wavelengths, but also ultraviolet and infrared. It is perfectly natural to speak of "ultraviolet light" or of "infrared light." Moreover, does the term "sunlight" really refer only to the wavelengths that humans can see? As we all know, that is simply not true. Sunlight includes electromagnetic radiation from the entire spectrum, with the vast majority of the energy density coming from infrared, visible, and ultraviolet light. From a scientific point of view, it is rather parochial of us humans to say that light includes only wavelengths that we can see, as if the others don't really exist. As it turns out, not all animals are limited to the same region of the spectrum that we are. Honeybees, for example, can see ultraviolet light, and many flowers exhibit different "color" patterns in UV from what we see in the visible region. First Harmonic 12:07, 7 September 2006 (UTC)

Technically speaking, the entire electromagnetic spectrum consists of three major regions: (1) radio, (2) optical, and (3) high energy. Radio includes long waves, short waves, and microwaves. Optical includes infrared light, visible light, and ultraviolet light. And high energy includes X-rays and gamma-rays. Although establishing the exact boundaries between the regions is somewhat arbitrary (the regions overlap to some degree), the distincition between the regions is not arbitrary. There are real and fundamental differences in the physical properties between these three regions. By comparison, the only real difference between red light of wavelength 650 nm and infrared light of wavelength 850 nm is the wavelength itself, and the fact that human beings can detect one but not the other. First Harmonic 12:07, 7 September 2006 (UTC)

[edit] Visible Light is "oily oil"

I absolutely agree with the first definition: "LIGHT is electromagnetic radiation with a wavelength that is VISIBLE to the EYE". But NOBODY from scientists use the word "Light" as "electromagnetic radiation of any wavelength". UV radiation is NOT light! IR radiation is NOT light! Radiowaves is NOT light! X-ray is NOT light! But all of them are electromagnetic radiation with different wavelengths.

But NOBODY from scientists use the word "Light" as "electromagnetic radiation of any wavelength".

UV radiation is NOT light! ... X-ray is NOT light!

You're going to have an awfully hard time explaining the naming of the National Synchrotron Light Source then, as it emits infrared, vacuum UV light, and soft X-rays, none of which are called "light" by real scientists according to you. I guess the guys at Brookhaven National Laboratory aren't real scientists, ehh? ;-)

Atlant 17:25, 1 March 2006 (UTC)

My laser group, in fact, frequently calls laser light of any wavelength light. Be careful with the word nobody. — Laura Scudder ☎ 16:41, 7 September 2006 (UTC)

In my experience scientists and engineers frequently do talk about "ultraviolet light" or "infrared light"; but on the other hand I don't know anyone who uses the term light to encompass all electromagnetic radiation. I would say in a technical context, light is the subject of the study of optics. It's em radiation at wavelengths that historically were studied by optical methods.

--The Photon 03:23, 1 May 2006 (UTC)

Well, there's always the dictionary:

"1. Physics a. Electromagnetic radiation that has a wavelength in the range from about 4,000 (violet) to about 7,700 (red) angstroms and may be perceived by the normal unaided human eye. b. Electromagnetic radiation of any wavelength." [2]

Just to shed some invisible light on the subject, the near infrared and near UV certainly are visible. The sensitivity of the eye tails off gradually and slowly without a sharp cutoff. Probably one reason for limiting it to the conventional 400-700 nm is that the sensitivity outside this range is very variable from person to person. Of course, astronauts perceive cosmic rays as flashes of light when they hit the retina,[3] so in a sense cosmic rays are visible, too. Dpbsmith (talk) 15:50, 7 September 2006 (UTC)

You reminded me of a story from a professor of mine. While he was in graduate school they had a beam off an accelerator. They used to check that their beam was on by sticking an eyeball down in its path and looking for flashes of light from bremsstrahlung. — Laura Scudder ☎ 16:41, 7 September 2006 (UTC)

[edit] Weight of light

because light has infinite mass does it have a weight of zero?

Weight is a force. "Weight" is usually used to mean the force which an object feels as a result of gravity. I've never heard anyone refer to the weight of light, but light does NOT have infinite mass - just the opposite. Light has zero rest mass - a slightly misleading term for light, since light is never at rest. I'm a little fuzzy on the "mass of light" issue myself, but "weight" is never ever discussed in the context of light or particle physics, because it simply isn't used as a technical term. Fresheneesz 19:27, 12 April 2006 (UTC)

I believe that the increase in mass with speed is a multiplicative process, and not an additive one; thus, if a particle has a rest mass of zero, it will have no mass per se, no matter what its speed is. However, I am only an amateur physicist (I'm a high school freshman), so I would not take this as definitive. Geekman314^{(contact me)} 02:20, 18 February 2007 (UTC)

The rest mass is zero, and the multiplier at speed c is infinity, so the mass is indeterminate from that approach. You need to use E=mc² instead, to get the mass from the energy of the photon, or from the total light energy. That mass is attracted by gravity like ordinary mass, so you can compute the weight of light. Dicklyon 05:14, 18 February 2007 (UTC)

Since the gravitational field of stars deflects light, my guess is that light does have weight, but I'm not speaking from much knowledge. Dpbsmith (talk) 15:53, 7 September 2006 (UTC)

Physical Review Focus has an article called The Weight of Light which references articles in a journal about the history of physics entitled Weighing Photons, so apparently the definition of "weight" used by physicists is consistent with the idea that light has weight. Dpbsmith (talk) 15:57, 7 September 2006 (UTC)

[edit] GA nom has failed

The Good article nomination for Light has failed for the following reason:

Although good articles aren't as polished as featured articles, they should at least have a few lines per topic. However, this article lacks essential information in the following sections: Optics, Hellentistic theories, The 'plenum', Particle theory revisiteds. Though not required for GA (this is just for a future FA nom), it would be good to add in-line references, external links, and a few more pictures. -- King of Hearts ^talk 02:36, 8 April 2006 (UTC)

[edit] history

I was wondering what everyone thought of putting the "classical theories" section into a couple chunks of history - say "Before 1000 CE", "from 1000 CE to 16XX", "from 16XX to 18XX", and "from 18XX to 19XX" - or something to that effect. Listing by culture or by theory seems drawn out and not as useful as a date-sequential progression. Fresheneesz 02:32, 13 April 2006 (UTC)

[edit] 1st Line

Should: I think people need light, there isnt enough light in the world. so sad. so sad. why are you so bitter? really be here? raptor 10:05, 19 September 2006 (UTC)

I removed it, if anyone objects, please say so here. raptor 10:07, 19 September 2006 (UTC)

[edit] Under: Optical theory

"He understood mathematically why a spherical mirror produces aberration." How do you know?

[edit] light and mass

light have a mass it is very small but this is true. Can you tell me an explenation way light when passing trought sea slow down. I will tell you because of the mass of the light, without mass the light has no reasone to slow down or to reflect from object. Light whitout mass would simply pass trought any object and our ayes wouldn't see the light. Because of the mass the gravity bend the light. Without mass the light would be without any particle in it that particle is a foton.

Every person who read this comment i ask to check this with the formulas because this is not my area of expertise. And please don't delete this comment so that the other's may enjoy in little things like discovering a truth about everything.

If you can send me a mathematics proof to my email "nesmetami@net.hr". All that i tell here is true and i don't mind if some of you take a nobell prize for this discovery, i will be pleased that truth has win once again. I thanks all the people who will work on this matter. Thank you.

Light has zero rest mass, but has nonzero relativistic mass which can be defined as p/c, where p is momentum and equals E/c (E is energy, equal to hf per photon, where h is Planck's constant and f is the frequency of the photon; c is the speed of light.

Anyway, light in water does not slow down because photons slow down, but because they are continuously absorbed and re-emitted. --Army1987 22:45, 24 November 2006 (UTC)

[edit] what are the ways light travels

light travels in 4 ways but i cant find what they are?? —The preceding unsigned comment was added by 74.109.193.19 (talk) 21:58, 8 January 2007 (UTC).

[edit] Frequency or wavelength

in one of the first paragraphs it says Frequency (or wavelength)...who posted this article? frequency and wavelength are absolutely NOT the same thing. —The preceding unsigned comment was added by 128.8.79.53 (talk • contribs).

Right, they are not the same. But you can use them interchangeably to specify the light characteristics; probably this would be more clear if not done with parentheses? Dicklyon 00:06, 23 February 2007 (UTC)

Though they are interchangable, those 2 factors are critical for mathematical analysis. You can determine one from the other, but fundamentally they are different and the introduction be edited. —The preceding unsigned comment was added by 67.68.40.31 (talk) 05:08, 5 March 2007 (UTC).

[edit] Light as EM of any wavelength

Just came across this article, and the first line troubles me:

" in a technical or scientific context, electromagnetic radiation of any wavelength[1]. "

This is surely not accurate. Light is specifically EM radiation in the visible spectrum .The reference cited does not seem to back up this statement and I am curious to know who thinks it is accurate

—The preceding unsigned comment was added by 138.40.24.189 (talk • contribs).

Er...from the reference...

Speaking broadly, light refers to the electromagnetic (light) spectrum

Sure seems that the reference backs up the notion in question. For the record, I've always been taught in my educated layman's physics education that, as the reference says, light can refer to the entire spectrum, and I came to this article to find out about electromagnetic radiation, not merely the visible portion (though I would not object to making this article just about the latter, if that was made clear at the top of the article).

The killer argument, though, is...well...has anyone ever heard of "the speed of electromagnetic radiation"? No. It's always the speed of light. Surely this isn't meant to only refer to what's normally thought of as the visible portion of the spectrum? Let's put this silly argument to bed. -- Calion | Talk 02:32, 6 April 2007 (UTC)

[edit] merge tag

My concerns looking at the electromagnetic radiation and light are that it is not clear that light is talking about visual light, while electromagnetic radiation is talking about electromagnetic radiation. Feel free to take the tag if the definitions are defined distinctly.100110100 07:29, 3 April 2007 (UTC)

[edit] Wave theory

As waves are not affected by gravity, it was assumed that they slowed down upon entering a denser medium

Um...huh? This doesn't make any sense, does it? -- Calion | Talk 02:25, 6 April 2007 (UTC)