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First of all, I am in high school[to tell you that I am a newbie and lack knowledge]

What I want to achieve for now is a thing that can differentiate between poly bags[polyethylene] and other stuffs. Or a thing that could detect polyethylene.

I have to built a robot and therefore we have only a few method accessible.

Anyway any knowledge or suggestion or external links provided by you, about this topic would be welcomed by me.

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    $\begingroup$ Some more information maybe needed. Is the sensor going to touch the sample material or should it detect from a distance and what constitutes "stuff" There is a post on chemistry on how to detect polyethylene using infrared spectral imaging chemistry.stackexchange.com/questions/28457/detect-polyethylene. This company manufactures a sensor that can detect PET bottles using light polarization levels ia.omron.com/products/family/1897 digikey.com/Web%20Export/Supplier%20Content/… $\endgroup$ – Ali Apr 10 '15 at 3:36
  • $\begingroup$ Sensor should not touch the sample material. $\endgroup$ – sedflix Apr 10 '15 at 10:07
  • $\begingroup$ And the link you provided "Detects 'transparent' objects made by PET, resin, and glass", only... It's great except it only works for transparent one. $\endgroup$ – sedflix Apr 10 '15 at 10:12
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    $\begingroup$ Since this question is related to a sensor for a Robot @Ugo I think it's fine here. $\endgroup$ – Mark Booth♦ Jun 22 '15 at 13:51
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    $\begingroup$ I've seen this question posted by the same author on the Chemistry channel: chemistry.stackexchange.com/questions/28457/detect-polyethylene. I don't like when the same question gets posted multiple times on different channels; but it's a personal opinion. The author should have then the care to share the results among them. $\endgroup$ – Ugo Pattacini Jun 22 '15 at 14:09
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You might be able to do something with hyperspectral imaging.

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Mentioned this as a comment, but getting no response I'm turning it into an answer:

This is going to depend very heavily on what the "other stuffs" are. You could probably differentiate between polyethylene and water using a humidity sensor. You could differentiate between polyethylene and iron by using a magnet.

In other words, if you are comparing polyethylene against other materials for which there is an existing sensor, then it might be reasonable to just set up the sensor for each of those other materials and assume that a negative detection for all those sensors is a positive detection of polyethylene.

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You could measure the change of capacitance when you put a sheet of plastic between 2 metal plates but it then does depend on the thickness of the sheet of plastic as well as its type.

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