How to Use a Laser to Measure Thickness of Human Hair

Use a laser to measure the thickness of a human hair.

While a human hair is by no means the thinnest object on earth, it is one of the thinnest seen on a daily basis. A laser can be used to determine the thickness of a human hair, as well as other very thin objects. Similar methods are used to find the diameter of thin slits, holes and even the size of the pits on a CD track.

Things You'll Need

• Cardboard, 3 inches square
• Single hole punch
• Human hair
• Tape
• 2 binder clips
• White, smooth wall
• Laser
• Ruler
• Paper
• Pencil
• Calculator

Instructions

1. Punch a hole in the center of the cardboard with the single hole punch.

2. Position the hair over the center of the hole and tape it to the cardboard to hold it in place. Make sure the hair is stretched taut.

3. Clip the binder clips to the bottom of the cardboard, one on either side, so the clips act as “feet” to hold the cardboard upright.

4. Place the cardboard parallel to the wall. Measure the distance from the cardboard to the wall. Record the value, and call it "D."

5. Find the wavelength of your laser. This is displayed on the laser. You may find that the wavelength is given as a range of values. In this case, average the two values together, and use that for the wavelength. Call the wavelength "λ."

6. Turn the laser on and position it, so the beam hits the hair and goes through the hole in the cardboard before shining on the wall. The laser should make a dashed pattern on the wall, with a bright dot in the center of the middle dash.

7. Turn the lights out, so you can see all the dashes.

8. Measure from the center of the middle dash, using the dot as a reference point, to the right end of the first dash on the left. Record this measurement as "y_1i."

9. Measure from the center of the middle dash to the left end of the first dash on the left. Record this measurement as "y_1f."

10. Calculate the average of y_1i and y_1f. Record this as "y_1avg."

11. Measure from the center of the middle dash to the right end of the second dash on the left. Record this measurement as "y_2i."

12. Measure from the center of the middle dash to the left end of the second dash on the left. Record this measurement as "y_2f."

13. Calculate the average of the last two measurements. Record this as "y_2avg."

14. Repeat those two measurements and the average calculation for each of the dashes on the left. Label each measurement and calculation following the pattern described above, where you add 1 to the subscript for each dash.

15. Measure from the center of the middle dash, using the dot as a reference point, to the left end of the first dash on the right. Record this measurement as "y_xi," where x is one more than the final number from the left side.

16. Measure from the center of the middle dash to the left end of the first dash on the left. Record this measurement as "y_xf."

17. Calculate the average of y_xi and y_xf. Record this as "y_xavg."

18. Repeat those two measurements and the average calculation for each of the dashes on the right. Label each measurement and calculation following the pattern described above, where you add 1 to the subscript for each dash. Make sure you have the same number of average values from the right side as you did on the left. You should have an even quantity of averages.

19. Calculate the value d_m=(λ*m*D)/y_mavg for each average, where m is the number in the subscript.

20. Average all the values of d_m. This is the approximate diameter or thickness of the hair.

Tips & Warnings

• Tape a large piece of white paper to a colored or textured wall or use a white board screen.

• Ken Johnson of Muscatine Community College in Iowa has a worksheet to help with these calculations available online.

References

• Ken Johnson: Measuring the Diameter of a Human Hair by Laser Diffraction
• Ohio State University: Measuring the Large and Small