N-source versus N-slit interference

jdstokes

Hi all,

I don't understand why the intensity pattern for an arbitrary number of slits will decay with distance from the centre of the screen, whereas the intensity pattern due to N-point sources will vary periodically with no average decay. I can derive the electric field and intensity for both cases to show it is true, but I can't justify it physically. Any hints would be appreciated.

Thanks

James

jdstokes

I should also have mentioned that I'm working in the Fraunhofer regime.