Interference and Diffraction

Source: Yong P. Chen, PhD, Department of Physics & Astronomy, College of Science, Purdue University, West Lafayette, IN

Interference and diffraction are characteristic phenomena of waves, ranging from water waves to electromagnetic waves such as light. Interference refers to the phenomenon of when two waves of the same kind overlap to give an alternating spatial variation of large and small wave amplitude. Diffraction refers to the phenomenon of when a wave passes through an aperture or goes around an object, different parts of the wave can interfere and also give rise to a spatial alternation of large and small amplitude.

This experiment will demonstrate the wave nature of the light by observing diffraction and interference of a laser light passing through a single slit and double slits, respectively. The slits are simply cut using razor blades in an aluminum foil and the characteristic diffraction and interference patterns manifest as patterns of alternating light and dark fringes on a screen placed after the foil, when the light is shone through the slit(s) on the foil. Historically, the observation of diffraction and interference of light played important roles in establishing that light is an electromagnetic wave.

1. Acquiring the Required Components for the Experiment

Obtain laser safety goggles and wear during this experiment before turning on the laser.
Obtain a piece of aluminum foil, and use scissors to cut it into two (approximately) 2 x 2 in square pieces.
Obtain two cardboards, each about 3 x 3 in, with a hole (diameter about 1 in) cut in the middle.
Obtain a device that can clamp the cardboard or a block to which the cardboard can be taped.
Obtain a few thin razor blades

For step 2.3, a representative light pattern that may be observed on the wall is shown in Figure 3b, exhibiting the characteristic diffraction fringes. Note that the central bright fringe is approximately twice as wide (in the y-direction) as the other bright fringes (which are about the same in width), and the over intensity of the bright fringes decay away from the center along the y-axis, as expected for the single slit diffraction pattern.

In this experiment, we have demonstrated the single slit diffraction pattern and double slit interference pattern of light, using a laser beam. Observing these characteristic wave phenomena demonstrates the wave nature of the light.

The diffraction and interference of light played essential roles in the development of optics as they helped establish that light is an electromagnetic wave. These effects are also important in many technologies based on optics and photonics. For example, diffracti

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