Hair Diffraction Experiment
The hair diffraction experiment is a classic physics demonstration that uses a laser to measure hair diameter. When coherent laser light passes around a hair, it diffracts and creates a pattern of bright and dark fringes on a distant screen, identical to the pattern from a slit of the same width (Babinet's principle).
By measuring the distance between diffraction minima, you can calculate the hair diameter with remarkable accuracy. This experiment demonstrates wave optics principles and is commonly performed in introductory physics courses using an inexpensive laser pointer and a strand of hair.
Diffraction Formula
Where d is hair diameter, theta is the diffraction angle, m is the order number, lambda is wavelength, L is screen distance, and y is fringe spacing.
Typical Hair Diameters
| Hair Type | Diameter (μm) |
|---|---|
| Fine blonde | 50-70 |
| Medium brown | 60-90 |
| Thick black | 80-120 |
| Red hair | 70-100 |
Frequently Asked Questions
What is Babinet's principle?
Babinet's principle states that the diffraction pattern from an opaque obstacle is identical to the pattern from an aperture of the same shape, except for the central bright spot intensity. This means a hair produces the same fringe pattern as a slit of equal width, which is why the single-slit formula works for measuring hair diameter.
Why use a laser instead of regular light?
Lasers produce coherent, monochromatic light essential for clear diffraction patterns. Regular light sources have many wavelengths that produce overlapping patterns, blurring the fringes. The laser's narrow beam also concentrates light on the hair for a brighter, more visible pattern.