4 ms·

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 ma

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 2y ago
Yeah, that's a thing. 3Blue1Brown did a video on it a while back: https://www.3blue1brown.com/lessons/prism https://www.3blue1brown.com/lessons/prism It's part of a larger series that starts here: https://www.3blue1brown.com/lessons/barber-pole-1 https://www.3blue1brown.com/lessons/barber-pole-1
smegsicle 2y ago
why did you think it happened?
- jjtheblunt 2y ago
  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 2y ago
    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 λ = λ0/n, where λ0 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/λ) of the wave is not affected by the refractive index.
jbmchuck 2y ago
They travel c in vacuo but not through different media.
webstrand 2y ago
Yeah, the author absolutely confused this with transmission grating which works by transmission rather than reflection.
- kragen 2y ago
  transmission gratings also do not work by dispersion
yorwba 2y ago
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 2y ago
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 2y ago
Yes; a dispersive prism (https://en.wikipedia.org/wiki/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) 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 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 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%20Grating%20Handbook%20(8th%20edition).pdf https://www.edmundoptics.jp/ViewDocument/MKS%20Diffraction%2... and http://snl.mit.edu/pub/papers/WP/Nanoruler-White-Paper.pdf http://snl.mit.edu/pub/papers/WP/Nanoruler-White-Paper.pdf
- kragen 2y ago
  however, diffraction gratings do not use dispersion, so the page is confused this error is not contained in the tech ingredients video the page links; unlike, for example, nighthawkinlight, tech ingredients is careful to get the science correct
  - dekhn 2y ago
    Dispersion is the name for the process of separating the light into individual frequency components but there is more than one mechanism. The term 'dispersion' to refer to what diffraction gratings do is already well established.
    - kragen 2y ago
      usually the term 'dispersion' is contrasted to what diffraction gratings do. 'dispersion' normally refers to the frequency-dependency of phase velocity in a wave medium, which can result in separating waves into separate frequencies but does not always (for example, a pulse train traveling through a one-dimensional dispersive medium is a case of great practical interest). but it's true that from time to time people do use the term 'dispersion' to refer to separating light into individual frequency components with a diffraction grating. it just isn't the normal meaning, and it isn't one i'd seen before
DavidVoid 2y ago
> 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 2y ago
thanks all for the pointers: they led me to an explanation from Feynman, which I'd not found before. And hadn’t seen in high school. https://www.feynmanlectures.caltech.edu/I_31.html https://www.feynmanlectures.caltech.edu/I_31.html
aaron695 2y ago
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AdamH12113 2y ago

Yeah, that's a thing. 3Blue1Brown did a video on it a while back: https://www.3blue1brown.com/lessons/prism https://www.3blue1brown.com/lessons/prism It's part of a larger series that starts here: https://www.3blue1brown.com/lessons/barber-pole-1 https://www.3blue1brown.com/lessons/barber-pole-1

smegsicle 2y ago

why did you think it happened?

jjtheblunt 2y ago
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 2y ago
  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 λ = λ0/n, where λ0 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/λ) of the wave is not affected by the refractive index.

jbmchuck 2y ago

They travel c in vacuo but not through different media.

webstrand 2y ago

Yeah, the author absolutely confused this with transmission grating which works by transmission rather than reflection.

kragen 2y ago
transmission gratings also do not work by dispersion

yorwba 2y ago

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 2y ago

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 2y ago

Yes; a dispersive prism (https://en.wikipedia.org/wiki/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) 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 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 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%20Grating%20Handbook%20(8th%20edition).pdf https://www.edmundoptics.jp/ViewDocument/MKS%20Diffraction%2... and http://snl.mit.edu/pub/papers/WP/Nanoruler-White-Paper.pdf http://snl.mit.edu/pub/papers/WP/Nanoruler-White-Paper.pdf

kragen 2y ago
however, diffraction gratings do not use dispersion, so the page is confused this error is not contained in the tech ingredients video the page links; unlike, for example, nighthawkinlight, tech ingredients is careful to get the science correct
- dekhn 2y ago
  Dispersion is the name for the process of separating the light into individual frequency components but there is more than one mechanism. The term 'dispersion' to refer to what diffraction gratings do is already well established.
  - kragen 2y ago
    usually the term 'dispersion' is contrasted to what diffraction gratings do. 'dispersion' normally refers to the frequency-dependency of phase velocity in a wave medium, which can result in separating waves into separate frequencies but does not always (for example, a pulse train traveling through a one-dimensional dispersive medium is a case of great practical interest). but it's true that from time to time people do use the term 'dispersion' to refer to separating light into individual frequency components with a diffraction grating. it just isn't the normal meaning, and it isn't one i'd seen before

DavidVoid 2y ago

> 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 2y ago

thanks all for the pointers: they led me to an explanation from Feynman, which I'd not found before. And hadn’t seen in high school. https://www.feynmanlectures.caltech.edu/I_31.html https://www.feynmanlectures.caltech.edu/I_31.html

aaron695 2y ago

[dead]