Interference and Diffraction: Optics Experiment 4

“The nature of light is a subject of no material importance to the concerns of life or to the practice of the arts, but it is in many other respects extremely interesting.” (Young 1845)

This is a quote from “On the Nature of Light and Colors”, a lecture by Thomas Young, which originally proposed the dual nature of light through the double slit experiment. To build on this, an experiment based on these concepts was also performed by Physics 103.1 students. The main goal of the experiment was to analyze the effects of slit width and the number of slits on a light source.

A laser diode that emitted red light, which has a wavelength of approximately 650nm, was used as the light source. To ensure precision, specialized single slit and multiple slit disks were used to observe the effects of different types of slits on the behavior of the light. The entire set-up was perched on an optical bench, and a white paper screen was placed a distance away to provide a clear backdrop for the light. Lastly, a 12-inch ruler was used for measurement purposes.

For the first test, a single slit was used to calculate the wavelength of the diode. Two slits were used, the first with slit width (a) of 0.02mm and order (m) of 1. For the second, a=0.04mm and m=2. The length of the central maximum was measured by marking its ends on the white paper using a pencil, then measuring the distance between each mark.


The experimental wavelength was then calculated using the formula below:

Screen Shot 2016-02-21 at 6.00.21 PM

The table below shows the results of the experiment:


It is obvious that the trial where a=0.04mm and m=1 is the far more accurate and precise trial. The high error from the other trial may be attributed to experimenter’s error in measuring the length of the central maximum. As the experimenter who measured this herself, this is very frustrating. The relief, however, is that the later parts of the experiment do not reflect this at all! If anything, These results confirm that the equation seen above does hold.

Another angle was also used to test this relationship. The experimental value for these tests is now the wavelength of the diode. The results may be seen below.


These results were extremely accurate and precise considering that our equipment consisted of a pencil and a ruler! This further confirms the equation above.

An interesting observation is that there clearly are smaller and dimmer fringes, multiple diffraction envelopes, on either side of the central maximum. This phenomenon seems to be counterintuitive; most would assume that the resulting pattern would be a single strip of light. It hints at the fact that light’s behavior as a wave is an inherent part of its nature.

Next, we moved on to the multiple slit disk to test the behavior of double slit interference. Aside from the disk, the same set-up and method of measurement was used.


Within the diffraction envelope, there are peaks in brightness, as well as dark areas. The bright spots indicate constructive interference, while the dark spots indicate destructive interference. This is demonstrates double slit interference, and is the exact phenomenon that Thomas Young used to demonstrate light as a wave.

For this initial trial, the slit width was calculated. The equation seen above still holds for double-slit interference. The results may be seen below.


The results are extremely accurate and precise, as with the previous results.

Another aspect of this experiment that was analyzed was the relationship between the slit width and the distance in between the slits and the resulting interference.

W4This experiment demonstrates several phenomenon. If the slit width is held constant, then the number of fringes within the central maximum is dependent on the distance between the slits, where increasing the distance increases the number of fringes. However, if slit width is manipulated, is appears that the number of fringes decreases as the slit width increases. The central maximum also appears to be dependent only on the slit width. Lastly, the fringe width seems to depend only on the distance between the slits, where an increase in slit distance means a decrease in fringe width.

In conclusion, the results of the experiment confirm the fact that nature behaves as a wave, as manifested through diffraction, and constructive and destructive interference patterns.


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