Diffraction grating paradox


Registered Member
During 2nd year Experimental Physics i remember doing an interesting experiment that at the time ( about 15 years ago ) defied explanation.

Shoot a laser into a diffraction grating and about a metre away from the grating insert a detector - ( i think we used a Photo Transistor )
and you get a sparkle at the diffraction grating.

Any explanations ? something to do with entangled photons ?
Reflection or transmission grating?

I'd say schizophrenic photon particle-wave duality, multiple slit effect.
Could you please explain in more detail exactly what you did, where and when you saw the "sparkle", and so on. Thanks.
It was a transmission grating with low number of slits placed about a metre from the laser - photo transistor placed just outside of the beam about a metre from the grating.

When the detector was inserted into any part of the beam after the grating there was a noticable sparkle on the laser side of the diffraction grating.

schizophrenic photon particle-wave duality, multiple slit effect ??

Cripes it must have been someone schizophrenic that named it :p but if thats what its called these days then so be it.

I think this experiment was conducted specifically to prove light acts as both a particle and a wave - so the name is apt in that respect.

And here are the questions i have atm

Is the effect only present or detectable on a small scale ?

So has any sucessful scaling up been conducted ?

Has any timing been taken to see if the sparkle is traveling at or slower than the speed of light ?

Is the sparkle the result of anihilating a photon ( and converting it into electrical energy ) that has been entangled with a friend at the grating ?
My guess is that you were getting reflections from the photodetector back onto the grating.
James R :

that seems unlikely considering it was a smokey lens detector and angle of insertion into the beam did not make any difference , and if that was the case surely the sparkle would have been on the far side of the diffraction grating , not the laser side.
Even a smokey lens is made in part of electrons capable of absorbtion and random, back-scattering emission -- reflection: yes, at reduced rate.

Entanglement strikes me as being a currently special type of back-scatter -- from the perspective of the laser-side observer.
I think this experiment was conducted specifically to prove light acts as both a particle and a wave
You are exactly right. If I read it correctly, you shine a beam of lazer through a small slit and you could see a serious of dark and light lines, instead of just one spot. This shows that light can act as both partical and waves. This infact raised some intereting thoughts, the photon have thoughts? How does it know which slit to go through (if we have more slits)? So that everything we do is infact uncertain eg. computer operation.
As far as I know the photon passes through both slits at the same time! The same has been shown with electrons. Even if they are passed through one at a time you still get interference patterns which only happens with waves passing through two slits.
With electrons though, and I think photons (correct me), the interference patterns are probability patterens. There are regions where the electron is likely to hit, corresponding to the bright dots when using a laser, and regions where the electron will never hit, corresponding to the dark bits.
So the Photon doesn't need to "decide" which slit to got through.
cheers guys.:D