Measuring a Human Hair Lab

Objective: Determine the thickness of a human hair by observing the interference pattern created when a laser beam passes through a single strand of hair.

Lab Equipment: 


- index card
- strand of hair (at least one group member must not be bald)
- helium neon laser
- meter stick
- micrometer

Lab Procedure 


1) Punch a hole in the index card and tape the strand of hair across the hole. Make sure that the hair is straight.

2)  Clamp the card at least 1 meter away from a wall so that it is parallel to the wall.

3) Mount the laser so that the beam is perpendicular to the wall.

4) Adjust the card so that the beam falls directly on the hair. This should produce a diffraction pattern on the wall. Measure the distance between the card and the wall and the spacing between the 2 first order minima of the pattern.

5) Calculate the width of the hair using these measurements. Confirm this result using a micrometer.

Data:


distance between 2 first order maxima = 2y = 4.1 cm (error = 0.04 cm)
distance from card to wall = L = 180 cm (error = 0.5 cm)
wavelength of laser beam = lambda = 632.8 nm (error = 0.1 nm)

d*sin(theta) = m(lambda)

theta = arctan(2.05/180)

d = m(lambda)/sin(theta) ~ m(lambda)(L)/y = 55.5 micrometers (error = 0.73 micrometers)

Using the micrometer we found the width of the hair to be about 60 micrometers. This does not fall within our predicted interval. However, we had trouble figuring out how to use the micrometer and the discrepancy was most likely due to our inability to use the instrument.