# Does intensity/amplitude of waves near coherent sources and near screen during interference experiment of light remain constant somehow?

Context of this question is based on high school level physics.

As far as I could notice in many highschool textbooks, amplitude of the waves during interference are not changed in the region between the sources and the screen. That is if the wave we are dealing with is supposed to be $$y=Asin(\phi)$$ then all the mathematical calculations regarding the interference is showed in terms of A instead of any other function which changes with distance. Even if all the conditions are thought to be ideal,shouldn't amplitude still change with distance since intuitively a particle participating in the vibration of the wave would influence its succeeding particles to osciallate? Am I wrong regarding this? I mean could it be that because of innumerous waves in that region, amplitude remain constant because of the adjacent waves? Or is that the main priority of high school level physics is just to show where there could be the formation of bright or dark fringes instead of the issues of intensity/amplitude for which the amplitudes are kept constant without complicating much?

Edit: I forgot to mention that this question is based on Young's Double Slit Experiment. Since it was carried out before the doscovery of electromagnetic theory I used the term "particles" to mean aether and waves=>mechanical.

• I'm not sure about your setup. "... a particle participating in the vibration ..." by this do you mean to draw a distinction between interference in a vacuum vs a material? If not, please clarify why you mention "particles". Nov 2, 2021 at 15:52
• I meant secondary waves spreading out.
– MSKB
Nov 2, 2021 at 16:33
• In the double split experiment the amplitude does change with distance from the slit. On the other hand, your description of light influencing "succeeding particles" sounds very much like the Huygen's construction where contributions from nearby virtual sources can indeed add up to produce a wave whose amplitude does not change ... a plane wave. Not sure if that helps. Nov 3, 2021 at 1:43
• Shouldn't we take Huygen's construction in account while explaining Young's Double slit experiment since both of them were explained before the discovery of electromagnetic theory?
– MSKB
Nov 3, 2021 at 4:46