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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
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This explanation may be incomplete or incorrect: Created by an ALARMED BOT THAT GOES NYOOOOM, updated by I AM NO SIREN EXPERT BUT... - 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 in the light from distant galaxies. She states that why this occurs is an interesting question, then follows this by talking about the Doppler effect of sirens. While this is commonly used as an analogy for the concept of the Doppler Shift, and is hence relevant to the preceding topic, Miss Lenhart appears to have raised it for a completely unrelated purpose - she simply has a special interest in sirens. This becomes apparent as her explanation quickly becomes extraneous to the preceding topic of astronomy, the field in which she teaches, similar to 1519: Venus.
Emergency vehicles usually have different siren tones, which they can switch between for different circumstances : long NYEEEOOOWWW to alert people in a distance and short PYEEW PYEEW when they are closer to drivers (for example when crossing an intersection).
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 (for details/explanation see wiki link above).
Redshift is the same concept applied to wavelength of light. Red has a longer wavelength than blue, so light-emitting objects get 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 relative to us, most other galaxies are moving away from us. The fact that more distant galaxies are moving away quicker the farther away they are shows that the universe is expanding.
Unlike the usual explanation of redshift as equivalent to the Doppler effect for sirens, a major component of the redshift of light from distant galaxies is due to the expansion of space in between us and the light source. This effect is not an important component of the Doppler shift for sirens. Redshift has been mentioned multiple times before, such as in 2764: Cosmological Nostalgia Content and 2853: Redshift.
The title text claims that the Doppler effect particularly affects sirens. This isn't actually true,[citation needed] but it may seem like it because sirens tend to employ a recognisable tone (or a given sequence of them), which most people who aren't totally tone deaf would have experienced a shift of from a passing vehicle's siren, whereas something less distinctinve like engine noise could change according to differences of speed and the gearbox. 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
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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 you 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 Ooooeeeeooooeeee...
- 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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