Number of fringes in a 2-slit diffraction pattern

[nmsurobert]

When looking at a diffraction pattern made by two slits, and trying to calculate wavelength, what do we count as fringes? For example, in this picture are there five total fringes or, like, 25?
I've been trying to set up a lab for my high school physics 2 class and the math says that every bright point should be considered a fringe, but I am having a hard time finding anything in writing that confirms what my math is telling me. Thanks!

 

[hutchphd]

There are two diffraction effects here: the finite width of each slit and the spacing distance between each subsequent slit center. Obviously the slit spacing must be larger than the slit width. The smaller distance in real space produces larger diffraction spacings. I would guess the two slits are perhaps 8-10 widths apart
In x-ray crystalography these are known as the form factor (each scatterer) and the structure factor (from the lattice). The result is their product

 

[lodbrok]

They are all fringes. But in addition, you cannot calculate the wavelength from the pattern only. You need more details about the setup, like slit widths, slit separation, and distance of the screen from the slits.

Actually, for the pattern above the slit separation is about 4-5 widths, not 8-10. You have to look at the multiplicity of the first-order peaks, not the 0-th order.

 

[hutchphd]

Actually, for the pattern above the slit separation is about 4-5 widths, not 8-10. You have to look at the multiplicity of the first-order peaks, not the 0-th order.

Yes. I always get that wrong!Thanks

 

[nmsurobert]

They are all fringes. But in addition, you cannot calculate the wavelength from the pattern only. You need more details about the setup, like slit widths, slit separation, and distance of the screen from the slits.

Actually, for the pattern above the slit separation is about 4-5 widths, not 8-10. You have to look at the multiplicity of the first-order peaks, not the 0-th order.

Awesome. Thanks. I have all that information. I intend on using the equation λ = (yd)/(mR). The only value I was unsure of was "m".

 

[jtbell]

If you're using a two-slit interference formula, the $m$ that you want is the number of small spots from the center (0), including "missing" spots at or near the minima of the overall pattern. In your example photo, at the first overall minimum from the center, this would be $m=6$.

If you're using a single-slit diffraction formula, the $m$ that you want is the number of overall minima from the center. In your example photo, at the first overall minimum from the center, this would be $m=1$.

You should get consistent results for the two cases, provided you use the "slit spacing" in the two-slit formula, and the "slit width" in the single-slit formula.

(I did this lab many times, using a He-Ne laser and slides with single/double slits from Pasco Scientific.)

 

[nmsurobert]

If you're using a two-slit interference formula, the $m$ that you want is the number of small spots from the center (0), including "missing" spots at or near the minima of the overall pattern. In your example photo, at the first overall minimum from the center, this would be $m=6$.

If you're using a single-slit diffraction formula, the #m# that you want is the number of overall minima from the center. In your example photo, at the first overall minimum from the center, this would be $m=1$.

You should get consistent results for the two cases, provided you use the "slit spacing" in the two-slit formula, and the "slit width" in the single-slit formula.

(I did this lab many times, using a He-Ne laser and slides with single/double slits from Pasco Scientific.)

Perfect. Thank you. Like I said, my math was telling me what to do, but when I'm not sure you guys always point my in the right direction.