Why don't we see interference in thick films?

[Joker93]

Bear in mind that I am talking about monochromatic light.
I looked into this subject and the thing that many say is that when we have thick films, the two reflected waves are not considered to be coherent any more(and something about coherence length). I can't see why this is the case(and I have to say that I don't fully understand coherence in this case). If the above is not the case and we can indeed have interference effects on thick films, why does every book only consider thin film interference? Is there a problem with thick films? I am not talking about the surface not being a plane, but if there is something about the interference itself that can't happen in thick films.
Thank you!

 

[tech99]

Bear in mind that I am talking about monochromatic light.
I looked into this subject and the thing that many say is that when we have thick films, the two reflected waves are not considered to be coherent any more(and something about coherence length). I can't see why this is the case(and I have to say that I don't fully understand coherence in this case). If the above is not the case and we can indeed have interference effects on thick films, why does every book only consider thin film interference? Is there a problem with thick films? I am not talking about the surface not being a plane, but if there is something about the interference itself that can't happen in thick films.
Thank you!

The fringes are smaller as the film becomes thicker, so they are more difficult or impossible to see.

 

[Joker93]

The fringes are smaller as the film becomes thicker, so they are more difficult or impossible to see.

But theoretically speaking, if we could make a thin with perfectly uniform thickness, would there be any problem with light interfering? I mean, most books treat the subject theoretically and just name it "thin film interference". So why not just say "film interference"?

 

[Andy Resnick]

Bear in mind that I am talking about monochromatic light.
I looked into this subject and the thing that many say is that when we have thick films, the two reflected waves are not considered to be coherent any more(and something about coherence length). I can't see why this is the case(and I have to say that I don't fully understand coherence in this case). If the above is not the case and we can indeed have interference effects on thick films, why does every book only consider thin film interference? Is there a problem with thick films? I am not talking about the surface not being a plane, but if there is something about the interference itself that can't happen in thick films.
Thank you!

You can see interference in thick films under certain conditions. Recall that this form of interference involves shear- the field is laterally displaced. Thus, the source light must be highly spatially coherent in order to see fringes when the lateral displacement is large.

In the lab, we use thick film interference to align a spatial filter- substitute the word "optical flat" for "thick film". In addition, many types of optical testing involve interference fringes created by thick objects to check surface figure, spacing, tip/tilt, etc. I think the device is officially called a "lateral shearing interferometer", but Twyman-Green interferometers also seem to use this principle.

 

[Joker93]

Recall that this form of interference involves shear- the field is laterally displaced. Thus, the source light must be highly spatially coherent in order to see fringes when the lateral displacement is large.

I don't understand what do you mean with shear and why does it need to be coherent. My level of understanding is that of an introductory physics book(Serway) so I am kind of uninitiated as far as these concepts are concerned. Could you please explain it to me with a more plain language

 

[Andy Resnick]

I don't understand what do you mean with shear and why does it need to be coherent. My level of understanding is that of an introductory physics book(Serway) so I am kind of uninitiated as far as these concepts are concerned. Could you please explain it to me with a more plain language

Look at the 'standard' diagram of film interference:

The reflected rays are laterally displaced from each other; in the wave model this is referred to as 'wavefront shear'. Does this help?