Experiment 11: Measuring a human hair

The purpose of this lab was to accurately measure a human hair using the concepts of the interference of light though a slit.  To compare, we also used a micrometer to measure thickness as well.

Procedure: we taped a hair though a hole in an index card.  Then we passed a laser though it and measured the distance between the maxima and minima of the pattern produced.

Micrometer

The equation:

λ = dy/Lm

Solving for the diameter of hair d, we get d = λLm/y. 

The wavelength is given at 632nm. L is the distance to the projection, which was 1m.  Y is the distance between the maxima and minima.

Data

	λ(nm)	m	y(cm)	L(m)	d(mm)	micrometer(mm)
Tim	632	5.5	0.58±0.5	1	0.60±0.057	0.05±0.005
Erwin	632	3	2.44±0.5	1	0.078±.002	0.06±0.005

Analysis

When we did Tim’s hair we used the small slit spacing to calculate m.  This gave us an order of magnitude of error.  We calculated 0.60±0.057mm, when the micrometer measured 0.05±0.005mm.  When we used my hair we used the large spacing.  The hair had a calculated diameter of 0.078±.002mm.  The micrometer measured 0.06±0.005mm.  My hair had a much closer value, even though it does not fall within our margin of error. For comparison, the average diameter for black hair the professor provided was 0.05mm.   

The large slit spacing was the actual interference pattern that needed to be used.  The small slits were probably some internal interference.