Young's double slit experiment edges problem

[meru]

1. The photograph shows the interference pattern produced when monochromatic light falls on a pair of slits.

I cannot post links yet but the photo is of a typical fringe pattern produced by coherent light waves from a double slit.

Mark with an X on the photograph the fringe or fringes where light from one slit has traveled a distance of two wavelengths further than the light from the other slit.

Explain why the fringes near the centre of the photograph are clearer than those near the edges
of the photograph.

3. The answer was given to me. The X's were above two maximas which were 4 fringe spacings and 3 maximas apart. However, i don't understand why the X's are placed so far apart. I thought they would be 2 fringe spacings away from each other as one fringe spacing = one wavelength path difference. And I also don't know why the fringes are clearer at the centre, is it due to the waves being almost completely inphase there? Can someone please clear this up for me. thank you!

 

[Chi Meson]

There will be 3 maximas between the x's, right? The one in the middle is the brightest because that is where the light from each slit has traveled the same distance. The next maximum on each side will be at the location where the path difference is one wavelength.

So, the second maximum on each side of the central will be the locations where the path difference will be twice the wavelength.

And by the fringes being "clearer at the center," do you mean that they are dimmer at the edges? Or are they fuzzier/longer/less distinct? How many orders of fringes are there (count the center as "zero" and proceed to one edge).