# Young's double slit experiment and intensity

I want to ask a question about double slit interference and the pattern that it produces on a screen (for example in Young's Experiment with a laser beam). I understand the reason that you see a series of light and dark spots - constructive and deconstructive interference due to different path differences. However, I do not understand why the central maximum is a lot brighter than all the subsidiary maxima. The intensity is higher in the centre, but why? Surely if complete constructive interference is occuring in all of these spots then the combined amplitudes will be the same and hence the brightness/intensity will also be the same? Is it to do with the number of rays that converge at each point? or the angle at which they hit?

• Can't it be that you make a confusion between the diffraction pattern of the double slit, and single slit? In the former I didn't see a specially enhanced central fringe, but in the latter yes, because the pattern goes in principle like $sinc^2(k\theta)$, function that has a very high central maximum and double in width that the subsidiary maxima. Mar 15, 2015 at 22:53
• In a single slit experiment the central maximum is definitely more enhanced than than the subsidiary ones. But from diagrams I have seen the same thing occurs in a double slit experiment. The rate of reduction of intensity is definitely lower, but as far as I'm aware it still exists Mar 15, 2015 at 22:58
• here is a totally random picture of the pattern bit.ly/1NYiyts IMO it seems to depend on the particular photo taken on the day but this one seems even all the way across to me, as Sofia says
– user74893
Mar 15, 2015 at 23:23
• @user75473 I looked at the pictures that you indicate. In pictures of double slit I didn't see a central maximum as in the single slit. Mar 16, 2015 at 1:15

Here, for example, is the squared product of a sinc and a cos function where slit width is 1/6th of the spacing: