Optics: Deriving thickness of a film

Replies

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  Ramani Aswath

  If the two waves are exactly 180 degrees out of phase there will be total destructive interference. This can happen if the film has a thickness of Lamda/4. However. it will work only for that wavelength that is four times the film thickness.
• 

  lal

  Non reflecting films, so, doesn't absorb any radiation? I actually thought they absorbed radiation without allowing it to reflect. I was thinking in that case non-reflective coatings on mobile phone displays and all should get heated up when exposed to sunlight.
  
  Well, destructive interference is interesting. The name of these coatings should have been something else as scientist said.
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  the scientist

  bioramani

  If the two waves are exactly 180 degrees out of phase there will be total destructive interference. This can happen if the film has a thickness of Lamda/4. However. it will work only for that wavelength that is four times the film thickness.

  thats impossible.......
  that will result into a contradiction to law of conservation of energy......
• 

  Ramani Aswath

  the scientist

  thats impossible.......
  that will result into a contradiction to law of conservation of energy......

  The First Law of Thermodynamics is inviolate. Either that energy appears as heat or (in the case of normal incidence) will enhance the transmitted light.
  There will be destructive interference if there are two waves exactly out of phase. I am sure that you are aware of the noise cancelling head phones. That is how they work, by destructive interference of the noise sound wave and the out of phase wave generated by the electronics. In this case the energy appears as heat. However, the calories are so few that it is unnoticed.
• 

  the scientist

  bioramani

  The First Law of Thermodynamics is
  inviolate. Either that energy appears as heat or (in the case of normal incidence) will enhance the transmitted light.
  There will be destructive interference if there are two waves exactly out of phase. I am sure that you are aware of the noise cancelling head phones. That is how they work, by destructive interference of the noise sound wave and the out of phase wave generated by the electronics. In this case the energy appears as heat. However, the calories are so few that it is unnoticed.

• 

  the scientist

  now how we can talk of two rays.......
  instead we a have two beams of light......in which we have no. of rays .if one pair satsfies the condition of minima at an angle then other pair will form minima at other angle..... thus redistribution of energy is there......
• 

  the scientist

  now how we can talk of two rays.......
  instead we a have two beams of light......in which we have no. of rays .if one pair satsfies the condition of minima at an angle then other pair will form minima at other angle..... thus redistribution of energy is there......
• 

  the scientist

  the scientist

  now how we can talk of two rays.......
  
  i meant to say that two light rays which are coming out after reflection as in thin films.... should behave as the two interfering light rays behave normally as in young's double slit experiment...
  instead we a have two beams of light......in which we have no. of rays .if one pair satsfies the condition of minima at an angle then other pair will form minima at other angle..... thus redistribution of energy is there......

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  Ramani Aswath

  the scientist

  now how we can talk of two rays.......
  instead we a have two beams of light......in which we have no. of rays .if one pair satsfies the condition of minima at an angle then other pair will form minima at other angle..... thus redistribution of energy is there......

  The argument does not consider how many rays there are, nor whether all are in phase. Each ray and its own reflection at a film which is quarter wave thick will be out of phase. If the beam has a mixed bunch of waves the reflected beam will also have the same mix. However. every wave will be out of phase with its own corresponding incident wave.
• 

  the scientist

  how can u say that?????
  when a pair forms maxima at some inclination..... then at some other inclination psth difference can't remain same. it will vary. hence the condition of maxima will not be satisfied!!!!!!!! again
• 

  Ramani Aswath

  If you take a sine wave and superimpose the same wave shifted by 180 degrees you get essentially a mirror image. Every point on one wave has an exact negative point, which is why you get destructive interference. There is no maxima or minima.
• 

  the scientist

  then why not this thing happens in case of young's double slit experiment......
• 

  Ramani Aswath

  the scientist

  then why not this thing happens in case of young's double slit experiment......

  That is exactly what happens in the double slit experiment. Each slit acts as a source of a spherical wave front. Since the same source of light illuminates both the waves are in phase. However, the phase at different points on the spherical front depends on the distance from the slit. Whichever wave is 180 degrees out of phase destructively interferes producing a dark line. Whichever wave is in phase constructively interferes producing a band brighter than the original ray. The pattern therefore is alternating bands of dark and bright bands. As you said earlier, the total energy remains the same, but redistributed.
  
  If you have the time watch this video from MIT, which not only explains the phenomenon, but also lets you experience it first hand making your own simple experiment.
  
  #-Link-Snipped-#