Editing 2926: Doppler Effect

{{comic
| number    = 2926
| date      = April 29, 2024
| title     = Doppler Effect
| image     = doppler_effect_2x.png
| imagesize = 671x317px
| noexpand  = true
| titletext = The Doppler effect is a mysterious wavelength-shifting phenomenon which seems to primarily affect sirens, which is why the 🚨 emoji is red.
}}

==Explanation==
[[Miss Lenhart]] is teaching an astronomy class about the concept of {{w|redshift}} in the light from distant galaxies. She states that why this occurs is an interesting question, then follows this by talking about the {{w|Doppler effect}} of sirens. While sirens are commonly used as an example of 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 veers away from the preceding topic, similarly to [[1519: Venus]], or due to a form of topical monomania similar to that which [[Hairbun]] exhibited in [[1610: Fire Ants]].
 
Emergency vehicles usually have different siren tones, which they can switch between for different circumstances: long NYEEEOOOWWW to alert people at a distance and short PYEEW PYEEW when they are closer to drivers, as for example when crossing an intersection. 

In the second and third panels, Miss Lenhart talks about the strange change in perceived noise sirens (and cars) make when they pass you. The usual explanation of Doppler effect is that the source of the sound waves is moving and the wave can sound different depending on whether the source is coming towards you or away from you (for details/explanation see the {{w|Doppler effect}} in Wikipedia).

Redshift is the same concept applied to wavelengths of light. Red has a longer wavelength than blue, so light-emitting objects get redder when they move away from us and bluer when they move toward 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.{{Citation needed}} Redshift has been mentioned multiple times before, including in [[2764: Cosmological Nostalgia Content]] and [[2853: Redshift]].

The title text claims that the Doppler effect particularly affects sirens. This isn't actually true, but it may seem like it because people hear Doppler shifts for sirens more than for other sounds. Sirens tend to employ predictable tone(s), which people who aren't {{w|Amusia|totally tone deaf}} would have experienced as a shift in pitch from a passing vehicle's siren, whereas something equally subject to Doppler shift like engine noise could also change pitch according to differences of speed and gearing. Then the text 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 (though, in fact, blue lights used with sirens are also common).

==Transcript==
:[Miss Lenhart is pointing with a stick to a whiteboard. There is an unreadable heading and two lines of unreadable text above a drawing of a spiral galaxy, this is what she points at. Below that there is a graph with a curve that looks like it is increasing exponentially. The line is going through a cloud of points, scattered on either side of the curve. Beneath the graph there is another unreadable line of text.]
: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...'''''

:[Same setting but Miss Lenhart has raised her arms.]
: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?

:[Back to the original view with Miss Lenhart in front of the whiteboard. She is now raising a finger in the air while holding the stick down with her other hand. A student ask a question from off-panel.]
:Miss Lenhart: And in Europe they go '''''Oooo<sup>eeee</sup>oooo<sup>eeee...</sup>'''''
:Off-panel voice: So '''''why''''' are galaxies red?
:Miss Lenhart: Oh, no idea.
:Miss Lenhart: Anyway, another siren I like is...

{{comic discussion}}

[[Category:Comics featuring Miss Lenhart]]
[[Category:Physics]]
[[Category:Astronomy]]
[[Category:Emoji]]