1308: Christmas Lights

Explain xkcd: It's 'cause you're dumb.
Revision as of 11:09, 25 December 2013 by (talk) (Explanation: Star spectra don't look like that; also the red and green ones are too monochromatic to be incandescent bulbs so they're probably LEDs too)
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Christmas Lights
Merry Christmas from xkcd!
Title text: Merry Christmas from xkcd!


Ambox notice.png This explanation may be incomplete or incorrect: Check grammar and add details about the star spectrum
If you can address this issue, please edit the page! Thanks.

Understanding this comic requires knowing about electromagnetic spectrums. In a nutshell, an electromagnetic spectrum is the scientific way of representing the amounts and type of radiation than a specific source emits. In this comic apears a subtype of these, a light spectrum, that shows the visible radiation, some infrarred and some ultraviolet. More detailed info on wikipedia(Electromagnetic spectrum).

There are 4 graphs in this comic:

In the center of the image, between the couple Cueball and Megan and Beret Guy appears a light spectrum of a fire, notably because it emits a lot of energy in the infrared band (The left zone of the spectrum), and in the red and orange zone (typical colors of a fire)

In the right of the comic appears some spectrums shaped in the form of a Christmas tree. There are 3 different spectrums in this "Christmas tree":

In the top appears a complex spectrum, probably that of a white LED (see here), representing the tradition of putting a star in the top of the Christmas tree, representing the Star that guided the Three Wise Men in Christianity.

In the "leaves" there are are two simpler spectrums, one with a peak in the green zone, representing a green LED, and other with a peak in the red zone, representing a red LED.


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Is it possible the spectrums are how beret guys sees colors? It would fit in with him seeing atoms 04:24, 23 May 2018 (UTC)

That is not the right spectrum for the sun. 20:44, 27 December 2013 (UTC)

I think the spectrum at the top of the tree is a specific star. The Sun. 16:21, 26 December 2013 (UTC)

If I am reading the graphs right, except for the very top there is no blue lights. From the guy with his eye on the sky. (talk) 06:41, 25 December 2013 (UTC)

Presumably the top is gold? 07:47, 25 December 2013 (UTC)

Also the spike in the near IR of the large graph is likely to be a mercury line. I think fires would have a smoother curve of a black body. 06:47, 25 December 2013 (UTC)

Could someone make a coloured version?Guru-45 (talk) 07:22, 25 December 2013 (UTC)

Also the light at the top of the tree seems to be emitting in the UV range. Perhaps it’s supposed to be a fluorescent lamp? 07:30, 25 December 2013 (UTC)

I'm thinking it's a "white" LED. 07:50, 25 December 2013 (UTC)

I think the one at the top matches the profile of a star... 08:18, 25 December 2013 (UTC)

The spike is not in the "near" infrared, this is water and/or carbondioxide line. (talk) (please sign your comments with ~~~~)

From a rough pixel-measurement and assuming a linear scale, the peak on the large spectrum is at around 800 nm. I'm not sure what to make from the peak, but infrared light is heat radiation, so it could be the representation of sharing warmth and love. 15:15, 25 December 2013 (UTC)

The top spectrum closely matches that of a cool-white LED. The strong peak in the blue, the broad peak from red to green, and the dip in the cyan range is a real give-away. Exactly like LEDs.ExternalMonolog (talk) 01:00, 26 December 2013 (UTC)ExternalMonolog

Here is a "filled-in" version of the comic.Guru-45 (talk) 17:06, 25 December 2013 (UTC)

Very nice, Guru!ExternalMonolog (talk) 01:35, 26 December 2013 (UTC)ExternalMonolog

I strongly disagree with the explanation's assertion that the peak in the large spectrum, the fire, is the 4.3µm CO2 emission listed in the source given. The peak in the source's spectrum is clearly outside of the near-infrared spectrum and it is just as clearly on the long wave side of the black-body peak in the chart. The comic doesn't even show these wavelengths at all. If, as it appears, that the chart in the comic is a semi-log plot, then the peak is roughly in the 1,000~1,200nm range.ExternalMonolog (talk) 01:20, 26 December 2013 (UTC)ExternalMonolog

The picture of Guru-45 a few lines above is great and explaining the situation very well. It should be included in the explain. --Dgbrt (talk) 16:04, 27 December 2013 (UTC)
As a clueless non-chemist, I'm wondering if the spike represents something *in* the fire, like roasting chestnuts? 14:34, 15 May 2014 (UTC)
Misleading picture in link
The picture :
found in the link of the explanation is strongly misleading. Visible radiation ( red to violet ) is 700 to 400 nm. The example spectrum in the middle does not fit to this. Maybe some guesswork here results from that link. After some "metering" with a ruler on the screen my guess for the spike in the fireplace spectum now is ca. 1.5 or 2.1 µm , being the first harmonic of either O-H or CO2 respectively. 23:23, 26 December 2013 (UTC)
I agree that the color band representing the spectrum of visible light given is not directly related to the (human) visible spectrum in the source (Flame Detection). The visible spectrum ranges from about 770nm to about 395nm, or about one octave. ExternalMonog. -- ExternalMonolog (talk) (please sign your comments with ~~~~)

"an angel, but still usually lit" ... aren't angels glowing in general? Did you ever see an angel who wasn't glowing? :-) -- Hkmaly (talk) 10:55, 27 December 2013 (UTC)

Yes. -Anonymous 12:29, 2 March 2015 (UTC)

Can someone please explain to me why my entries are constantly interrupting the flow of the room? When I place an entry, it is sometimes placed inside a dotted line block and sometimes interrupts a previous entry? ExternalMonolog -- ExternalMonolog (talk) (please sign your comments with ~~~~)

For example, my current post does not have a dotted-line block around it at all and it still interrupts previous entries. I'm using an old version of AOL and a non-current version of OSX 10.6 (Stuck here in dial-up hell, I can download no more than 2KBytes per hour so updates are extremely impractical. -- ExternalMonolog (talk) (please sign your comments with ~~~~)

See your talk page. Davidy²²[talk] 10:52, 29 December 2013 (UTC)
As far asI know, I have no "talk page" so I'm still confused about where to go for answers. Please advise. TY. ExternalMonolog -- ExternalMonolog (talk) (please sign your comments with ~~~~)
Here. There should also be a link to it at the top of the page, next to your username. Davidy²²[talk] 11:51, 29 December 2013 (UTC)