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Suppose you have some jelly balls with similar refractive index as that of water, like in this video.

You put them in water, making it invisible. So, the optical behavior of the balls should be similar to that of water.

laser

I've seen powerful lasers cutting glass, so you can cut transparent objects with lasers.

So if you direct a laser beam into the water, can you burn the jelly balls inside?

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  • $\begingroup$ NOTE: If anybody has started to answer, please be informed that the question is under reconstruction. $\endgroup$ – Krishnanand J Jul 27 '18 at 10:30
  • $\begingroup$ Ideally, you shouldn't make such drastic edits on a question. I think DarkKnight's comment was rendered obsolete, and even the votes will now be inaccurate. $\endgroup$ – user191954 Jul 27 '18 at 10:48
  • $\begingroup$ I know, but it was this point I always wanted to illustrate. Can laser be a solution to the question before edit? $\endgroup$ – Krishnanand J Jul 27 '18 at 10:58
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    $\begingroup$ Re, "powerful lasers cutting glass." If you were asked to design a laser glass cutting system, you would do well to choose a laser wavelength that is strongly absorbed by the glass. Just because glass is mostly transparent at wavelengths you can see, that does not mean it is transparent at every wavelength for which a powerful laser can be built. $\endgroup$ – Solomon Slow Jul 27 '18 at 14:52
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It appears that the questioner wants to know if a laser can burn the jelly balls in water even though they are invisible in the water due to having the same refractive index.

The answer is "yes". Having the same refractive index simply means that light will not be reflected at an interface between the jelly and the water. If the jelly can be burned by a laser when when it's not in the water, then it can be burned slightly more easily when it is in the water, because no light will be lost by reflection.

Also, the jelly will have a somewhat different absorption spectrum than water, so by selecting a laser wavelength that is more strongly absorbed by jelly than water, the jelly can be burned without heating the water much.

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    $\begingroup$ Although 'burn' might not be the right term if the water excludes the atmosphere. You may be able to turn the jelly into some other compound - or you need a jelly with a built in oxidiser (gelignite?) $\endgroup$ – Martin Beckett Jul 27 '18 at 16:11
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There are transparent things at any wavelength, and everything is intransparent at some wavelength. Cutting of 'clear' acrylic or glass happens at wavelengths that are strongly absorbed in those materials. Could your eye see those (and only those) the mazerial would appear black.

To cut the jelly in water, you need a wavelength that water is mostly transparent to, and that the jelly absorbs. A camera operating at this wavelength would show black balls in water.

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