LAB 12: Human hair width
Introduction
The lab aim is to determine the thickness of the hair using a laser pointer. First, we will
experiment with optical grating diffraction behavior when using light of a certain wavelength
pointers. Then we will lead the laser beam through the hair. The distance of the detected
maxima determine the hair thickness. This is caused by the diffraction phenomenon too.
Procedure
1. On some wall we choose the screen and one meter in front of it we place the
diffraction grating.
2. We fix the pointer in front of the grating so that the beam passed through the grating
on the screen.
3. We measure the distance of the first maximum from the central maximum.
4. The distance from 3. we substitute into the equation 1 and calculate the wavelength
of the laser pointer.
5. We fasten the human hair into the index card by masking tape and place to the same
position like the grating.
6. We let the beam pass through the hair. Some short lines appears on the screen.
7. We measure the distance from the central maximum to the center of any of the
peaks. (I chose the third one)
8. We calculate thickness of the hair by the help equation 2. and calculated
wavelength.
Results
Diffraction by the grating:
Diffraction by the human hair:
Discussion
Measured thickness of hair correspond with standard size. For example:
http://hypertextbook.com/facts/1999/BrianLey.shtml
Factors affecting accuracy of measurement:
1. Mistake given by measuring the distance between the diffraction grating (or hair) and
the central maximum. When this measured distance will be greater, calculated
wavelenght will be greater too.
2. Mistake given by measuring the distance l1. Eventhough diffraction pattern are set of
points, there could be mistake given by inaccurate measurement of distance. When
this measured distance will be greater, calculated wavelenght will be greater too and
vice versa.
3. Mistake given by measuring the distance l2. Diffraction pattern are set of segments
with blurred edges, this imply problems connected with finding centers of the
segments.
4. Mistake given by the fact that beam direction was not perfectly perpendicular to the
screen.
5. Problem given by limited size of my “laboratory”. Bigger ratio l/ ( small distance
between diffraction grating and screen) would imply bigger mistake given by
measuring the angle. This suggest smaller ratio l/r, so bigger r. The lab room is limited
hovewer. Next, the diffraction pattern is going to be poorly measurable in the case
of big r.