Experiment 2: Interference and Diffraction

The main goals of the experiment are to:
* investigate patterns produced by single-slit diffraction and double-slit diffraction
* quantitatively relate the single-slit diffraction pattern to the slit width size
* quantitatively relate the double-slit diffraction pattern to the slit width size
* quantitatively relate slit separation pattern and double-slit diffraction pattern

The materials used were the following:

* Laser diode
* Optical bench
* Single slit disk
* Multiple slit disk
* White paper screen
* Pencil
* Ruler
* Desk lamp

The first part of the experiment involved single-slit diffraction. We placed the laser at one end of the optics, and the single slit disk with its holder about 3 cm in from of the laser. The white sheet of paper was also placed away from the laser assuring that the laser would hit it. The 0.04 mm width single slit was selected. It was made sure that the slit and the pattern are of the same level vertically. The horizontal distance from the slit disk and to the screen was determined. The boundaries of the mth intensity minimum (located at the center of a dark fringe was marked. The length of the minima divided by two. This figure was considered as y1 in the table below. The same procedures were repeated using a slit width of 0.02 mm.

The following results were gathered:

Hence, it was clearly showed that the relationship a=mXL/y was validated where X=wavelength of light used.

Below are photos for the diffraction patterns for single slit, 0.04 mm and 0.02 mm, respectively.

As shown in the photos, the lower slit width value produced a longer central minimum.

The second part involved the double slit interference.

The same procedures for setting-up the optics materials was done. This time a double slit was used with distance separation of 0.25 mm and slit width of 0.04 mm. The following results were gathered.

In double slit interference, one can see a single-slit diffraction pattern which outlines how the fringes are grouped. The fringes are not of equal brightness and the intensity peaks are contained in the single-slit diffraction envelope.

It was also showed from the data that the relationship d=mXL/y was validated where X=wavelength of light used, L=horizontal distance from slit to the screen.

Diffraction pattern for double-slit interference

The third part was changing the slit width and slit separation in double-slit interference.

The following results were gathered.

In double slit interference, the width of interference fringes are controlled by the slit separation d. The diffraction envelope is controlled by the slit width a. Higher a = lower number of fringes and lower value for width of central maximum.Higher d, same a = higher value for width of central maximum.

These are the photos for Table W3.

A=0.08mm D=0.25mm

A=0.08mm D=0.50mm

A=0.04mm D=0.25mm

A=0.04mm D=0.50mm

References:

(1) Young, H. D., Freedman, R. A., Ford, A. L., & Sears, F. W. (2004). Sears and Zemansky's university physics: With modern physics. San Francisco: Pearson Addison Wesley.