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Doppler Effect

Title text: The Doppler effect is a mysterious wavelength-shifting phenomenon which seems to primarily affect sirens, which is why the 🚨 emoji is red.

Explanation

This explanation may be incomplete or incorrect: Created by an ALARMED BOT THAT GOES NYOOOOM - Please change this comment when editing this page. Do NOT delete this tag too soon.

Miss Lenhart is teaching a class about the concept of redshift and using the Doppler effect as a demonstration. The comparison begins as relevant, but then quickly becomes extraneous when she brings up the "BUTTON THAT MAKES IT GO PYEEW! PYEEW!". In fact, it seems she cares more about her special interest in various emergency vehicle sirens than about astronomy, the field in which she teaches, similar to 1519: Venus.

In the second and third panels, Miss Lenhart talks about the strange noise sirens (and cars) make when the pass you. The usual explanation of Doppler effect is that the source of the sound waves is moving. Consider a sound being generated at a frequency of 1000 cycles per second (Hz); each wave will propagate at a fixed speed in the air, the speed of sound. The car generates peak A, which begins moving away at the speed of sound, and 1/1000th of a second later, generates peak B, which also begins moving away at the speed of sound. In that 1/1000th of a second between peaks being generated, the car has traveled slightly forward, so peak B is produced slightly farther along the car's direction of travel than the previous peak. For an observer in front of the car, because peak B was generated a bit closer to the observer, they would measure (hear) a shorter wavelength (higher pitch) than if the peaks were generated from a stationary car. Similarly, for an observer behind the car, because peak B was generated a bit further away, they would hear a lower pitch. However, Miss Lenhart doesn't make this usual explanation and instead starts talking about how cool emergency sirens are.

Redshift is the same concept applied to wavelength of light and stellar objects. Red has longer wavelength than blue, and stars gets red when they move away from us and blue when they move towards us. We usually talk about redshift and not blueshift because while stars in our galaxy can move in any direction, the other galaxies are all moving away from us, and in fact they are moving quicker the father away they are, due to universe expanding. Note that unlike the usual explanation of redshift for sirens, a major component the redshift of light from distant galaxies is due to the expansion of space. This effect is not an important component of the Doppler shift for sirens. Redshift has been mentionned multiple times before, such as in 2764: Cosmological Nostalgia Content and 2853: Redshift.

The title text explains that the Doppler effect particularly affects sirens. This isn't actually true,^{[citation needed]} but it may seem like it because sirens are almost always the analogy given, as in Miss Lenhart's lecture. Then it claims that the emoji for sirens is red because they're associated with redshift. Actually, the emoji is a picture of the rotating light on top of emergency vehicles; these tend to be used in conjunction with sirens, and they're red because this color typically signifies danger or warning.

Transcript

This transcript is incomplete. Please help editing it! Thanks.

[Miss Lenhart is pointing with a stick to a whiteboard with various scientific drawings and words, including but not only a graph.]

Miss Lenhart: The more distant a galaxy is, the redder its light.

Miss Lenhart: Why? Well, that's an interesting question.

[Zoom in on Miss Lenhart.]

Miss Lenhart: Ever notice how, when a siren is approaching, it sounds like Bweeeeeeeeee...

[Zoom in on Miss Lenhart with her arms raised.]

Miss Lenhart: ...but then it zooms past and goes Nyeeeeooooowww?

Miss Lenhart: And sometimes they hit a button that makes it go Pyeew! Pyeew! really loud?

[Miss Lenhart with her finger raised is standing in front of the whiteboard and holding the stick down.]

Miss Lenhart: And in Europe they go Oooo^eeeeoooo^eeee...

Off-panel voice: So why are galaxies red?

Miss Lenhart: Oh, no idea.

Miss Lenhart: Anyway, another siren I like is...

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