Width of a Hair Lab
Name_________________________________ Date ________ Hr. _______
Introduction
Waves diffract around obstacles or openings producing a characteristic interference pattern. In
this lab you will determine the width of a human hair using your knowledge of single slit diffraction
and interference of waves.
Theory
Together with the small angle approximation, Huygen’s principle can be used to show
Y= mX
d
Where
Y is the distance between the central bright fringe and
the mth dark fringe
m is the order number (central bright fringe is the 0th order)
X is the distance from the slide to the screen
d is the thickness of the hair
 is the wavelength of the laser light
Equipment
Laser Pointer (known )
Piece of clay
Meter stick
Stand and Clamp
Ruler
A Human Hair
Screen
Slide
Micrometer
Tape
Procedure
1. Set up the equipment as shown below
Hair mounted
vertically on slide
screen
laser
2. Mount a human hair on the slide in a vertical position.
3. Set the slide on the clay near the end of the laser and position the screen at the end of
your lab bench or against the wall. Your laser should be clamped in a horizontal position the
same height as the middle of the slide. Target the laser light at the hair.
4. Record the average wavelength of your laser in the data table.
5. Record X in the data table.
6. Shine the laser at the hair on the screen. Observe the interference pattern on
the screen.
7. Measure Y for 5 different dark fringes of order number, m, on one side of the
bright central fringe. Record m and Y in the data table for each dark fringe.
Data Table
 = ___________________
X = __________________
m
1
2
3
4
5
Y (m)
d (m)
Average
Analysis
1. Solve for d for each value of m in the data table. Record in the data table.
Show an example calculation (full work) for m = 1 below.
Example Calc:
2. Average the five values for d and record in the table.
3. Determine the absolute uncertainty in your measurement of the thickness of the hair.
Work:
d =  _____________
4. Try to measure the thickness of your hair with a micrometer. How does this compare with
your diffraction measurement?
Micrometer measurement __________________
5. Which is more precise, the diffraction measurement or the micrometer measurement?
Support your answer by describing sources of uncertainty in each measurement