Help me with sensor

I want to separate plastic bottles and glass bottles. Which sensor is best for my project. And it must be suitable with arduino. Until now I haven't found the answer yet :( . This is my college project

throw them in water, the plastic bottles will float ...

How would you yourself separate them?

Post a list of the different properties you considered yet?

e.g. the plastic bottles will float and the glass bottles will sink (example above)

How about a microphone and a solenoid? Trigger the solenoid to "thump" the bottle in question - a glass bottle should have a marked difference in sound frequency than a plastic bottle.

You might try a light sensor - perhaps the reflection off a glass bottle is different to a plastic bottle? Or shine a laser or other light thru the bottle, and measure that?

How about a capacitive sensor? Two metal plates between which the bottles pass - perhaps a glass bottle has a different capacitance than a plastic bottle?

If the plastic bottle are PET - you might look at this thread:

http://forum.allaboutcircuits.com/threads/sensors-for-pet-bottles.59183/

Measure a weight.

Descartes is your friend.

Every material have his own indice of diffraction.