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Revision as of 16:59, 20 November 2012


Welcome to the explain xkcd wiki! We already have 4 comic explanations!

(But there are still 1940 to go. Come and add yours!)

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The End of the Rainbow
The retina is the exposed surface of the brain, so if you think about a pot of gold while looking at a rainbow, then there's one at BOTH ends.
Title text: The retina is the exposed surface of the brain, so if you think about a pot of gold while looking at a rainbow, then there's one at BOTH ends.

Explanation

Ambox notice.png This explanation may be incomplete or incorrect: Edited by a leprechaun. We are here. We are watching. The bit about percentages of the mass of the Sun should be made more readable. Also need to explain title text.

Cueball and Megan are having a discussion. Megan brings up the myth that at the end of every rainbow lies a leprechaun's pot of gold. Instead of claiming that leprechauns and their gold don't exist, Cueball refutes that technically, rainbows are circles, so they do not have an end.

However, Megan then expands on the scientific explanation Cueball is stating. She states that if one considers the path light takes to form a rainbow, then it forms a two-cone structure, where the Sun (the vertex of the outer cone) emits light rays that move towards the Earth (forming the faces of the outer cone), then reflect off of water droplets located at just the right angle (the circular base) to reach our eyes (the vertex of the inner cone ). Thus, such a rainbow structure can be said to have "ends", represented by the vertices of the two cones: one at the eye of the viewer, and another at the light source.

Megan then says that the Sun is indeed a pot of gold. The Sun is approximately 2x10^30 kg (two billion billion trillion kilograms), and is approximately 1 ppb gold (1 part gold per billion parts of mass); therefore the Sun contains approximately 2x10^21 kg (two billion trillion kilograms) of gold, far more than a hypothetical leprechaun's pot of gold. Using that same percentage, you can compute the ratio of gold in the Sun against sea water on Earth and find a ratio of around 1.5. While the two values are pretty similar, this confirms Megan's claim that there is more gold in the Sun than water in the oceans mass-wise. Gold being far denser than water, if you compare the two volumes (using the volume the gold would have on the surface of Earth, the temperature and pressure in the Sun could make it vary greatly) then the oceans' waters beat the Sun's gold by a factor 13.

Cueball then asks about leprechauns. Megan replies that the leprechauns all died when the Sun formed, implying all the gold in the Sun is actually the remnants of leprechaun gold. However, she does not consider the explanation where the leprechaun is on the other end of the rainbow...

This part is suggested by the title text. Leprechauns possessing gold exist (only) in the brains of people[citation needed]. The retina is the foremost part of the brain for the light perception and so for the other end of the rainbow.

Transcript

[Megan and Cueball are walking.]
Megan: There's a pot of gold at the end of the rainbow.
Cueball: Rainbows are circles. They have no end.
Megan: Not quite!
[In a borderless panel, a multi-part graphic is shown depicting what Megan is describing off-panel: a short cone inside a longer cone, with the longer cone having its point starting at the Sun, the shorter cone having its point at a miniature Cueball's head, and both cones sharing the same circular base. The diagram is repeated from 3 different perspectives to make the structure easier to grasp.]
Megan (off-panel): A rainbow is light leaving the Sun, bouncing off the clouds, and converging on your eye. It's an inside-out two-ended cone.
[Megan and Cueball are still walking.]
Megan: One end of that cone is your retina.
[A wider view of the same scene, with Megan and Cueball walking on a dark ground.]
Megan: The other end is the Sun—which contains quintillions of tons of gold. There's more gold in the Sun than water in the oceans.
Cueball: So there is a pot of gold!
Cueball: What about leprechauns?
Megan: All incinerated as the sun formed. Very sad.


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