Double Slit Diffraction

DOUBLE SLIT DIFFRACTION

Objective

To use the diffraction pattern produced by a double slit to calculate the wavelength of the light source being used.

Equipment

Helium-neon laser, glass slide with optical double slits with different spacing, meter stick, tape and paper.

Outline

1. When light passes through two closely spaced slits an interference pattern can be produced on the wall. The laser beam must be incident on both slits and perpendicular to the glass face. Set up the lab as shown in the diagram. Your data will consist of measuring the locations of the various maxima (bright spots) produced. The locations of these maxima are referenced from the location of the zeroth order maxima. The distance you will be projecting the beam after it has passed through the double slits is approximately the width of the lab (measure it).

Caution: Laser light can damage your eyes! DO NOT look into the beam or at any intense reflections from the beam.

2. Shine the laser through the double slits making sure that both slits are covered by the beam. Observe the interference pattern on the wall. You may have to rotate the slits slightly to get the sharpest pattern. Tape a piece of paper to the wall so that you can draw lines at the locations of the bright maxima. DO NOT make any marks on the wall!

3. Draw lines down the center of each bright spot. Use the first 3 or 4 maxima for your calculations. Measure and record the distance from the slits to the wall and the distance between the bright spots.

4. After you have completed the data collection, notice that the glass slide gives spacing dimensions for each of the double slits you will use in your experiment.

5. Repeat all measurements for the other slit spacings.

6. Rotate the slits, keeping the beam perpendicular to the glass face. Record your observations.

Graphs and Diagrams

1. Draw the interference pattern, labeling each of the maxima with their order number (calculation 1) and the angle (calculation 2) for each set of data.

2. Plot d*sin( ), where d is the distance between the slits, versus the order number (calculations 1 and 2 below). Put the data from all your measurements on this graph.

Questions and Calculations

1. For each set of data, assign each bright spot an "order" by labeling the central bright spot "zero" and counting outward in both directions. Use only positive numbers.

2. Use your data for the location of each bright spot and the distance from the slits to the wall to compute the angle between the central maximum and other maxima, q. Compute sin(q) also.

3. From graph 2, find the wavelength of the laser light.

4. What happened when you rotated the slits? Explain.

5. Why do maxima (bright spots) and minima (dark areas) appear when light is passed through the slits? Why don't we observe just two bright spots on the wall separated by the same distance as the slits.

6. If you were to pass the laser beam through only one slit, would you still get a interference pattern?

7. What is the difference between interference and diffraction?

8. Thomas Young used a single slit for the light to pass through before it hit the double slits, why don't we use a single slit before the double in our experiment?