Diffraction Pattern Spacing?

Subject: Diffraction Equation vs Diffraction Pattern ---------------------------------------------------- I am confused between the diffraction equation and a corresponding diffraction pattern. Refer to the following web site: http://dev.physicslab.org/DocumentPrint.aspx?doctype=3&filename=PhysicalOptics_InterferenceDiffraction.xml At the bottom of that site's page are some excellent pics of interference, diffraction, and combination of the two. What I do not understand is that if I use the diffraction equation sin(theta) = (m*lambda)/a, where "a" is the appeture size, this formula will indicate that the destructive(dark) areas are equally spaced because the only thing that would change in the formula would be the m. However, when I look at the diffraction pattern shown at the bottom of that site, my interpretation of the dark destructive areas would be that they are not equally spaced in the pattern I see. So the question is "why"? Why does the formula for dark spacing and the diffraction pattern showing the destructive not agree? I must be missing something or else I am observing an optical illusion.
- Ken Baratko (age 65)
Houston, Tx, USA

Hello Ken,

I visited the web site you mentioned, printed out the diffraction pattern at the bottom, and measured the positions of the dark fringes.  As far as I could tell, within the accuracy of my ruler, they were equally spaced just like the formula.

LeeH

(published on 05/02/2011)