Naked Science Forum

Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: Curious Cat on 27/08/2021 17:34:04

Title: How does a diffraction grating really work?
Post by: Curious Cat on 27/08/2021 17:34:04

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Title: Re: How does a diffraction grating really work?
Post by: evan_au on 27/08/2021 22:44:07

If you have a beam of light falling on a Diffraction Grating (DG), you can treat every ridge on the diffraction grating as a source of light, with aligned phases.

So you could model a 1-dimensional diffraction grating, with 1,000 ridges (light emitters) every 1 um.
- Then you calculate the distance to the screen (in micrometers)
- And sum the phase and intensity of the light striking the screen from each ridge
- Repeat for every position on your screen
- Repeat for about 7 colors in the rainbow.

This computer model uses Huygens wave model of light. There are no photons to "go through" any ridge of the diffraction grating; the wave is emitted from all of the ridges.

With the terminology: There are reflective diffraction gratings, and transparent ones...
https://en.wikipedia.org/wiki/Diffraction_grating

Title: Re: How does a diffraction grating really work?
Post by: alancalverd on 27/08/2021 23:51:26

You can't model diffraction with particulate photons.

At some point, kids have to accept that we need two very different mathematical models for the behavior of electromagnetic radiation. I think it is important not to talk about "wave-particle duality"  as if the term were meaningful. Fact is that em radiation does what it does, and we don't have one model that predicts all of it.

If it were up to me, I'd demonstrate with a real grating and real light. Computer simulations do not inspire awe or inquisitiveness.

Title: Re: How does a diffraction grating really work?
Post by: Bored chemist on 27/08/2021 23:56:32

Quote from: alancalverd on 27/08/2021 23:51:26

If it were up to me, I'd demonstrate with a real grating and real light.

I'd start with a CD unless you have a "proper" grating handy.

Title: Re: How does a diffraction grating really work?
Post by: evan_au on 28/08/2021 11:34:19

Quote from: bored chemist

I'd start with a CD unless you have a "proper" grating handy.

I'd start by explaining that before we had USB sticks or the cloud, we had CDs...

Title: Re: How does a diffraction grating really work?
Post by: alancalverd on 28/08/2021 12:38:36

The good news is that we have cheap lasers so you can demonstrate the difference between red and green diffraction and a whole bunch more besides.

Title: Re: How does a diffraction grating really work?
Post by: TommyJ on 28/08/2021 13:42:14

- Explain that in this case a beam of light is a wave. For the 2 slit demonstration it could be done with 2 wave sources of water (e. g. in a bath). So it means each slit is an identical source.
- Let them know the scale: length for each colour of light-wave.
- CD, laser or holographic sticker sheets might be somewhere near.