You can measure the width of a hair. All you need is a dark room, a high power laser pointer, some cardboard, some tape, and some mathematics.
experiment procedure。

experiment procedure:

1. Make a frame for fixing the hair. I cut a piece of paper 15 cm (about 6 inches) wide and cut a small rectangle inside.I measured how they scatter the light from the Green laser pointer. This is how you do it:
2. The inner depression is approximately one centimeter (0.39 inches) wide and four centimeters (1.5 inches) high. Hair, it may come from your own hair, or from someone else. Make sure it is long enough so that there is tape on both ends of the inner rectangle. For me, the length of each hair should be at least 5 cm to make sure you can stick it on the ends.
3. Fix the hair as firmly as possible on the top and bottom of the frame, so that the hair passes through the middle part of the hair bundle. In a dark room, stand one meter or more (more than three feet) away from the wall.
4. Fix the frame with hair, place the laser pointer on the wall behind the hair, and make sure it hits the hair along the hairline.

calculation steps:

1. d=hair diameter, m=sort of dark bands from the center (±1, ±2… according to different directions, θ'=diffraction angle, λ=laser wavelength, θm=sin(y) function value is 0 The corresponding angle, D = the distance between the projection screen (white paper) and the hair, ym = the distance between the dark band and the center of the beam.
2. In order to reduce the error in measuring the distance between the dark spots of the diffraction pattern, we decided to use the second-order darkest point of the diffraction pattern. The distance from the hair strand holder to the paper screen where the diffraction pattern is projected is 1.14 meters.
3. As mentioned above, the main experiment participants are students, so by default there is no dye layer caused by hair dyeing or other hairdressing and hair care on the tested hair, so there will be no interference of thickening of the hair due to the thickness of the extra dye layer. The diameter of the hair itself is measured. In the end, we added a relative error of ±22% to the measurement of the hair diameter. Laser pointer, a common commercial laser pointer, about 13 yuan/piece, packaging specifications: wavelength 650 nanometers, power 5 milliwatts.

Experimental results:

1. Most of the data are distributed in a small interval near the average value of 92 microns
2. Some data points deviate greatly from the fitting results, and the ratio of the maximum value to the minimum value of the measurement data of the same age group can reach 3 times
3. Most of the results fall within the normal range of Asian hair diameter (80 micrometers to 120 micrometers, with an average of 99 micrometers)
4. There are two data points that deviate significantly from the trends of other measurement results, the largest of 165 microns and the smallest of 35 microns (two red triangle data points). The experiment did not record any abnormalities between the two testers, so it is believed that these two data points are caused by incorrect measurements, and they were excluded in further research.