Experiment 11: measuring a human hair

Objective: The purpose of this experiment is to study the concept of interference of light with a double slit. The thickness of a human hair will be measured using d=λL/y. The hair will be taped on a punch hole of a card. A laser beam is pointed to the hair. Interference will appear and form on a white board. A human hair will be used as the separation for the slits (d), its thickness will be measured by micrometer. Also, the length from the card and the white board will be measured (L). The distance between the fringes on the interference pattern will be measured (y). The wavelength of the laser beam is 661nm. With all this data, the thickness of the hair can be calculated and be compared to the measured one.

Figure1: A hair is taped on the white card with a laser beam pointing at it. 



Figure 2: A laser beam is point directly through the hole with the hair.


Figure 3:The distance between the fringes is measured.



 Figure 4: the hair is measured using a micrometer.
Data:

Wavelength λ	632.8 (nm)
L	109±(cm)
y	0.8±0.05(cm)
D( calculated)	90±3 (mm)
D(measured)	86.2 ± 1.25
Percent error	4.41%

Conclusion

Since a micrometer is used to measure the hair, the result is more accurate. The percent error is small and acceptable. In this experiment, from the data, it is shown that L is much greater than d. one condition for using the equation d=λL/y is that L has to be much larger than d. so, in this case, the equation can be used. The laser beam has to be point directly onto the hair in order to do the interference effect. On the white board, a line with bright fringe of dots and dark fringe can be seen. The bright finge is where the construction interference occurs and the two waves are in phase. The dark fringe is where the destructive interference occurs and the two waves are out of phase for 180^o.