Hair Diffraction Calculator

Your details

Choose which quantity you want to calculate. The other three fields become the inputs.
The wavelength of your laser pointer. Red ~650 nm, green ~532 nm, blue ~405 nm.
nm
Distance from the hair to the wall or screen where you see the diffraction pattern.
cm
Distance from the center of the bright central spot to the dark fringe you measured (use the m-th dark band matching your order below).
mm
Diameter of the hair strand. Human hair is typically 17-180 micrometers wide.
μm
Which dark band from the center you measured. Using a higher order reduces relative measurement error but requires the pattern to be visible far enough from center.
Quick preset for common laser pointer colors. Overrides the wavelength field unless you choose Custom.
Calculated valueMedium
Hair diameter: 65.0 μm

The unknown you are solving for

Hair diameter65μm
Diffraction angle (θ)0.573°
Small-angle approximationValid (θ = 0.57° < 10°)
Max observable order99
Central maximum width20mm
Hair typeMedium hair
65 μm
Very fine<40Fine40-60Medium60-90Thick90-120Very thick120+

Hair diameter: 65.0 μm (Medium hair)

  • A diameter of 65.0 μm places this hair in the "Medium hair" range (human hair spans roughly 17 to 180 μm).
  • The diffraction angle of 0.57° is well within the small-angle limit, so the approximation d ≈ mλD/x is accurate to better than 0.5%.
  • With this wavelength and diameter, up to 99 dark fringes are theoretically visible. You measured the 1st - measuring a higher order reduces the percentage error in x.

Next stepFor best accuracy: measure the 2nd or 3rd dark fringe instead of the 1st (larger x means smaller relative ruler error), and repeat the measurement 5-10 times on different hairs to get a mean and standard deviation.

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