The purpose of this experiment is to examine the interference and diffraction of visible laser light as it passes through narrow single and double slits.

The wave properties of light are most easily demonstrated by the interference and diffraction of a beam of light as it passes through one or more small slits. The wave nature of the light results in a pattern with a series of bright and dark regions related to the wavelength of the light and the number and size of the slits.

Double Slit Interference

In 1801 Thomas Young demonstrated this wave nature of light by observing the pattern formed when a narrow beam of light was sent through a pair of narrow slits. In a similar way, if a laser beam is passed through two vertical narrow slits onto a distant screen, a horizontal pattern of equally spaced dots is observed. The bright and dark regions are due to the constructive and destructive interference of the light waves from the two slits.

The geometry of the double slit setup is shown in Figure 1 where d is the slit separation, L is the distance from the slits to the screen, and y is the distance measured from the center of the pattern.

Figure 1. The intensity pattern on a screen due to diffraction through a double slit.

When the distance which the light beams travel from the two slits to the screen differs by one-half wavelength, the two beams will cancel, and a dark region (minimum) will be observed. When the screen is far from the slits, the positions of these minima are given by

(1)

where n is an integer, and is the wavelength of the light. It can easily be shown from Equation 1 that the minima in the light intensity are equally spaced with a spacing W given by

(2)

This distance W will be referred to as the width of the interference peak. It is constant for all peaks in this interference pattern.

Single Slit Interference

When a beam of light passes through a narrow vertical single slit, the beam is observed to spread out horizontally. This bending of the beam as it passes through a narrow opening is known as diffraction.

This diffraction pattern is also observed to have a series of maxima and minima due to the interference effects of the light passing through different portions of the single slit. The geometry of this single slit setup is shown in Figure 2, where b is the width of the slit.

Figure 2. The intensity pattern on a screen due to diffraction through a single slit.

The central maximum is by far the brightest part of this pattern. The positions of the minima in this single slit pattern are given by

(3)

The distance W₀ between the minima on either side of the central peak is found from Equation 3 to be twice the distance W between the other adjacent minima.

(4)

The effect of the single slit diffraction is also observed in the double slit interference pattern. When the light beam passes through the slits it spreads horizontally due to diffraction. The reason that the double slit pattern gradually decreases in intensity is due to the decrease in intensity of the light coming from the individual slits.

1. Use double slit A to determine the wavelength of the red laser. The slit separation is 0.054 mm. Perform a propagation of uncertainty for this objective.
2. Use the red laser to determine the slit separation of double slit B.
3. Use the red laser to determine the slit width of single slit C.
4. Use double slit A to determine the wavelength of the green laser.
5. Use the green laser to determine the slit separation of double slit B.
6. Use the green laser to determine the slit width of single slit C.
7. Use the red or green laser to determine whether slits D and E are single or double slits. If it is a single slit, determine the slit width. If it is a double slit, find its slit separation.

(Figure 3.) Red laser with slide. (Figure 4.) The green laser pointer mounted on a ring stand. (Figure 5.) The slide with slits A through E. (Figure 6.) Double slit pattern with the red laser. (Figure 7.) Single slit pattern with the green laser. Meter stick.

[Click on images to enlarge.] 3 4 5 6 7

1. Caution!!! The lasers can cause damage to your eye. Do not look directly at the beam, or point it toward another person!

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Each lab group should download the Lab Report Template and fill in the relevant information as you perform the experiment. Each person in the group should print-out the Questions section and answer them individually. Since each lab group will turn in an electronic copy of the lab report, be sure to rename the lab report template file. The naming convention is as follows:

For example the group at lab table #5 working on the Ideal Gas Law experiment would rename their template file as "5 Gas Law.doc".

Each student should download the questions. Each person in the group should print out the questions and answer them individually. Discussing the questions as a group is acceptable, but each student is responsible for turning in answers that represent their own work, not the work of others.

These Nudge Questions are to be answered by your group and checked by your TA as you do the lab. They should be answered in your lab notebook.

General Nudges

1. All objectives use the same (or very similar) procedures. Make sure your planned procedure is appropriate before beginning.
2. Think carefully about the geometry of your setup.
3. It is helpful to mark clearly which pattern you record goes with which objective. It is also helpful to mark the center of each pattern.

1. What is your raw data for these objectives? What is the uncertainty in this data?
2. How many data points will you take?
3. How will you assign n-values to your data? Where will your length measurements start?
4. What will you graph? Will it be linear? If so, what should the slope be?
5. When using a single slit, how will your n-values need to be modified to account for the large central maximum?

1. Which laser (red or green) is better to use in this objective and why?

• Get all of the groups to work out the procedure for all objectives, then turn out the lights and close the doors. Have the groups take data for all seven objectives at the same time, then turn on the lights and have them measure y-values all at once.

As of now, there are no CUPOL experiments associated with this experiment.