# Number of fringes in a 2-slit diffraction pattern

• B
• nmsurobert
In summary, the fringes in this diffraction pattern are due to the finite width of each slit and the spacing distance between each subsequent slit center.

#### 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!

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

vanhees71 and Dale
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.

lodbrok said:
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

lodbrok said:
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".

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.)

Last edited:
berkeman
jtbell said:
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.

berkeman

## What is a 2-slit diffraction pattern?

A 2-slit diffraction pattern is a phenomenon that occurs when light or a wave passes through two narrow slits and produces a series of bright and dark bands on a screen or surface behind the slits. This pattern is a result of interference between the waves that pass through the two slits.

## What is the significance of counting the number of fringes in a 2-slit diffraction pattern?

The number of fringes in a 2-slit diffraction pattern is directly related to the wavelength of the light or wave passing through the slits. By counting the number of fringes, scientists can calculate the wavelength of the light or wave and gain important information about its properties.

## How can the number of fringes in a 2-slit diffraction pattern be calculated?

The number of fringes in a 2-slit diffraction pattern can be calculated using the equation N = (dsinθ)/λ, where N is the number of fringes, d is the distance between the slits, θ is the angle between the central bright fringe and the first order bright fringe, and λ is the wavelength of the light or wave.

## What factors can affect the number of fringes in a 2-slit diffraction pattern?

The number of fringes in a 2-slit diffraction pattern can be affected by the distance between the slits, the wavelength of the light or wave, and the angle at which the pattern is observed. Additionally, the presence of obstructions or imperfections in the slits can also affect the number of fringes.

## What is the practical application of studying the number of fringes in a 2-slit diffraction pattern?

The study of the number of fringes in a 2-slit diffraction pattern has practical applications in various fields such as optics, engineering, and materials science. It can be used to determine the properties of light and waves, measure small distances, and analyze the structure of materials.