[HN Gopher] Diffractive Chocolate
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Diffractive Chocolate
Author : joebig
Score : 50 points
Date : 2024-06-12 17:27 UTC (5 hours ago)
(HTM) web link (wp.optics.arizona.edu)
(TXT) w3m dump (wp.optics.arizona.edu)
| ctenb wrote:
| https://archive.is/Lphsl
| jjtheblunt wrote:
| what the heck are they talking about in the linked article? each
| color of light travels at a different speed?
|
| > White light can be separated into all seven major colors of the
| complete spectrum or rainbow by using a diffraction grating. The
| grating separates light into colors as the light passes through
| the many fine slits of the grating. Each color travels at a
| different speed and therefore has a different angle of refraction
| when it hits the grating.
| AdamH12113 wrote:
| Yeah, that's a thing. 3Blue1Brown did a video on it a while
| back:
|
| https://www.3blue1brown.com/lessons/prism
|
| It's part of a larger series that starts here:
|
| https://www.3blue1brown.com/lessons/barber-pole-1
| smegsicle wrote:
| why did you think it happened?
| jjtheblunt wrote:
| refraction angles are wavelength dependent, i thought...not
| the fraction of c by wavelength of photon.
|
| from the wikipedia article on dispersive prisms, for example,
| though i asked above because i remember this from physics at
| Fermilab in high school.
|
| > The refractive index of many materials (such as glass)
| varies with the wavelength or color of the light used, a
| phenomenon known as dispersion. This causes light of
| different colors to be refracted differently and to leave the
| prism at different angles
| muti wrote:
| Frequency is constant, wavelength changes and so does
| velocity, from wikipedia refractive index
|
| > The refractive index, n, can be seen as the factor by
| which the speed and the wavelength of the radiation are
| reduced with respect to their vacuum values: the speed of
| light in a medium is v = c/n, and similarly the wavelength
| in that medium is l = l0/n, where l0 is the wavelength of
| that light in vacuum. This implies that vacuum has a
| refractive index of 1, and assumes that the frequency (f =
| v/l) of the wave is not affected by the refractive index.
| jbmchuck wrote:
| They travel c in vacuo but not through different media.
| webstrand wrote:
| Yeah, the author absolutely confused this with transmission
| grating which works by transmission rather than reflection.
| yorwba wrote:
| With a diffraction grating, all colors travel at the same
| speed, but some light waves reflected off/transmitted through
| the grating end up taking a longer path to reach the same point
| in your eye, and depending on the wavelength, that length
| difference causes constructive or destructive interference,
| amplifying some colors and suppressing others.
|
| Colors traveling at different speeds through a medium is the
| usual explanation for color separation through refraction, but
| it doesn't really help for understanding diffraction.
| xutopia wrote:
| It's indeed not correct... but it diffracts so some of the
| light lands on your eye later because it's angled differently
| hence the colour change... just guessing. I'm not a physics
| major.
| dekhn wrote:
| Yes; a dispersive prism
| (https://en.wikipedia.org/wiki/Dispersive_prism) is a simple
| example of this (both prisms and diffraction gratings are used
| to break light up into its spectrum). The fundamental physical
| mechanism is known as "dispersion":
| https://en.wikipedia.org/wiki/Dispersion_(optics)
|
| The backstory of diffraction gratings is part of the ongoing
| story of precision. The first ones were created shortly after
| Newton's use of the prism to demonstrate dispersion, and by the
| late 1800s,
| https://en.wikipedia.org/wiki/Henry_Augustus_Rowland was quite
| good at making high quality gratings that were used in
| astronomy to figure out some of the most fundamental details.
| They were highly sought after- effectively he was the only
| person who could make high quality gratings for a while (and he
| shared them widely).
|
| If you really want to go down the rabbithole, see
| https://en.wikipedia.org/wiki/Dividing_engine (fans of the
| screw cutting lathe will appreciate that the same technology is
| used) and
| https://www.edmundoptics.jp/ViewDocument/MKS%20Diffraction%2...
| and http://snl.mit.edu/pub/papers/WP/Nanoruler-White-Paper.pdf
| DavidVoid wrote:
| > each color of light travels at a different speed?
|
| That is indeed correct when light is not traveling through a
| vacuum. Refraction occurs because of a change in velocity
| between mediums, and the refractive index is wavelength-
| dependant (dispersive) in many mediums.
| jjtheblunt wrote:
| thanks all for the pointers: they led me to an explanation from
| Feynman, which I'd not found before.
|
| https://www.feynmanlectures.caltech.edu/I_31.html
| zifk wrote:
| https://www.youtube.com/watch?v=UsDnkrDvkBo
|
| Ben from Applied Science has a good video going over the same
| topic.
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