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Old Jun 22nd 2019, 11:13 AM   #1
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Color fringes on heated plate

I've recently noticed that the ceramic plates I have home show an interesting light pattern whenever light is reflected on a slightly heated area of them.
The setup is the following: a ceramic plate, a lightbulb a meter above it, and any means of heating such as blowing hot air or putting hot food on the plate.
The areas of the surface that get in contact with the hot "vapor" and are propperly illuminated by the light producte the following patterns:



I assume this is related to a change of the refractive index due to a temperature gradient but I have no idea what is really going on behind it. Any help is more than welcome!
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Old Jun 22nd 2019, 05:57 PM   #2
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Nice post ... this is how you become a great physicist , always observing , always asking questions ...

Notice the colours are not that of a rainbow , but a dirty sort of rainbow we get from oil on water , caused by interference , and not the refraction that creates a rainbow ...



You need a transparent surface on top of a reflective surface , .... Many ceramics are 'glazed ' , first fired to cook the clay , then a layer of glass powder applied in a wash , cooked again , this glass melts and forms a thin layer of smooth glass ...So your plate has been glazed , but apparently also has another reflective surface beneath ...double glazed??

Heating must expand the top glass layer just of the order of the wave width of a light wave , in the pattern shown .... this causes interference pattern to change over the surface and be noticed .... the effect we see ...

Or it could be a greater effect results form heating changing the refractive index of the glass , and that the change in thickness is not significant ... either way interference changes and we see the pattern

I guess the pattern disappears when the plate has cooled ???

EDIT ... perhaps there is not a need for two layers of glass , with just one layer , the light could be bouncing back off the lower surface after passing through the glass , but this bottom surface must be perfectly smooth ...
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Last edited by oz93666; Jun 22nd 2019 at 06:15 PM.
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Old Jun 23rd 2019, 01:46 AM   #3
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That must be it! Knowing how these plates are made makes it clear. Thanks a lot.
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Old Jun 23rd 2019, 02:18 AM   #4
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I've kept looking at it and trying new things. I was wrong when I said that these fringes appear when hot temperature is applied to the plate's surface. After heating the plate in the microwave there were no colors at all. In fact, I've only been able to see them when water vapor (either from hot food or blowing hot air) gets in contact with it, and they quickly disappear as this moisture is evaporated again.

My new hypothesis would be using oz93666's answer in a system of air - condensed vapor - glass. Any thoughts?
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Old Jun 23rd 2019, 03:27 AM   #5
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Two options occur to me.
1) The condensed water film you suggest is reasonable.
2) It (perhaps) requires a temperature gradient to see the fringes.
A temperature gradient would mean that the thickness of the glaze would vary over the area of interest.
It is (perhaps) these changes in thickness that produce the interference patterns.
The microwave would (perhaps) heat the plate too evenly to generate these gradients.

In your original post you stated that blowing hot air over the plate could produce the fringes.
If the hot air is dry, this would favor the second explanation.
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