💇 Hair Diffraction Calculator

What This Calculator Does

A single strand of hair is thin enough to diffract laser light almost exactly like a single narrow slit does (a consequence of Babinet’s principle). By shining a laser through a taped-up strand of hair onto a screen and measuring the resulting pattern of bright and dark bands, you can calculate the hair’s thickness surprisingly precisely with simple tools.

The Formula

The single-slit diffraction minima occur where a · sin(θ) = mλ, where a is the obstruction (hair) width, θ is the angle to the m-th dark fringe, and λ is the laser’s wavelength. For small angles, sin(θ) ≈ y/L, where y is the measured distance from the central bright spot to the m-th dark fringe and L is the distance from the hair to the screen. Rearranging gives the hair diameter: a = mλL / y.

Note: This is a real, widely used physics lab demonstration formula, for general educational purposes. Accuracy depends on carefully measuring the fringe positions and keeping the hair straight and taut; results are typically within a reasonable range of a hair’s true diameter (commonly 40-100 micrometers for human hair) but are not a substitute for microscope measurement.