Young's double slit: why fringes gets dimmer and inconsistent?

[Simon George]

Hi all!

In Young's double slit experiment, there are two things I cannot explain. Any help is appreciated!

The first one is why the bright fringes get dimmer as you get further from the central/brightest spot. My theory, after looking in the two books I have, is that each single slit decreases the amplitude the further it goes from the center. I find this explanation kind of "too easy" and simple. Is that just it? and does that relate to any equation of intensity for double slit?

The second one is why the bright fringes get inconsistent with the formula for double slit (dsin(theta)=m*walength) as we get further form the center. For that, I have absolutely no idea and can't seem to find the answer anywhere.

Thank you!

 

[harambe]

Search about double slit interference in khan academy.They have explained it well very clearly

 

[kuruman]

These are good questions.
For your first question, see the discussion here.
https://www.physicsforums.com/threads/youngs-fringes-experiment-interference-fringes.674616/
The answer to your second question is that the formula is as good as the small angle approximation used to derive it. The larger the angle, the larger the discrepancy.

 

[Simon George]

Search about double slit interference in khan academy.They have explained it well very clearly

Thank you! Still don't get it why it gets dimmer. It seems vague

 

[jtbell]

My theory, after looking in the two books I have, is that each single slit decreases the amplitude the further it goes from the center.

That's basically it. Do a Google search for "single slit diffraction" and you'll find many graphs of the intensity pattern for a single slit. For double slits (try searching for "double slit diffraction") the basic two-slit interference pattern "modulates" the single-slit pattern by "chopping it up" into narrower maxima. The "interference maxima" are narrower than the "diffraction maxima" because the distance between the slits is (normally) much larger than the width of each slit.