Measuring a Human Hair

Interference is a property of wave interaction. The presence of interference in the propagation of light is a strong argument for it being a wave.

Using a laser, it is possible to observe interference when it is projected past a human hair. A picture of the experimental set up follows.

The note card in the picture has had a hole punched in it, and a human hair is suspended across the hole and held in place by tape. A laser is aimed past the hair so that the beam is bisected by it. On the recieving surface, it is possible to observe interference effects, though this photo did not capture them. This is due to the low intensity of the constructive interference bands.

By measuring the distance from the notecard to the flat surface, the distance from the center of the laser beam to the center of one of the intensity peaks, and using the wavelength of the laser, it is possible to measure the diameter of the hair suspended in the notecard.

Our experiment generated the following results

Length from notecard to board : 2 meters
distance from laser center to 3rd anti-node (Ym): 6.5 cm

wavelength of light emitted: 630 - 680 nanometers

using the constructive interference approximation equation
Ym = Length * wavelength * anti-node number / diameter of hair

Solving this equation for diameter results in a range of diameters from 5.85*10^-5 meters to 6.27*10^-5 meters. Upon further investigation, a microscope determined that the diameter of the hair was approximately 6*10^-5 meters which supported the laser measurement.