Why don't we see interference in thick films?

In summary: The reflected rays are laterally displaced from each other; in the wave model this is referred to as 'wavefront shear'.
  • #1
Joker93
504
36
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!
 
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  • #2
Adam Landos said:
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.
 
  • #3
tech99 said:
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"?
 
  • #4
Adam Landos said:
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.
 
  • #5
Andy Resnick said:
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
 
  • #6
Adam Landos said:
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:

thinfilmfullglory.jpg


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

Related to Why don't we see interference in thick films?

1. Why does interference occur in thin films but not in thick films?

Interference occurs when light waves reflected from the front and back surfaces of a thin film interfere with each other, resulting in a change in the color or intensity of the reflected light. This phenomenon only occurs when the thickness of the film is close to the wavelength of the light. In thick films, the light waves reflected from the front and back surfaces have traveled different distances and are no longer in phase, therefore, interference does not occur.

2. How does the thickness of a film affect interference?

The thickness of a film is directly related to the occurrence of interference. As mentioned before, interference only occurs when the thickness of the film is close to the wavelength of the light. If the film is too thick, the light waves reflected from the front and back surfaces will be out of phase and no interference will be observed.

3. Can interference occur in all types of light?

Interference can occur in all types of light, including visible light, UV light, and infrared light. The only requirement is that the thickness of the film is close to the wavelength of the light being used.

4. Why do we see interference patterns in soap bubbles and oil slicks?

In soap bubbles and oil slicks, the thin films are made up of layers of different substances with varying refractive indices. When light passes through these layers, it undergoes multiple reflections and refractions, causing interference patterns to be formed.

5. How is interference used in practical applications?

Interference is used in many practical applications, such as in anti-reflective coatings for glasses and camera lenses, in optical filters, and in interferometers used for precise measurements. It is also used in spectroscopy to study the properties of materials and in thin film technology for creating thin layers of materials for electronic devices.

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