Talk:Light
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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
See plea for help in article....
- 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] (http://www.bartleby.com/173/7.html)
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
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.
"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.
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
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.
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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)
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)
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.
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.
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)
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|]] 22:13, Dec 14, 2004 (UTC)
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)