Name:___________________________________________________________ PHYS141
Partner’s Names:__________________________________________________ November 18,
2002
Lab #12: Light as a Wave: Diffraction and Interference
1. Introduction
Waves are distinguished by the fact that they diffract (spread out) when they pass by obstacles or through
openings ("slits") and that they display interference behavior when two or more waves come together at a point. In the
18th and 19th centuries, experiments similar to the ones you will perform in this lab demonstrated that visible light shows
diffraction and interference behavior. The conclusions drawn from these experiments were profound: visible light is a
wave of some sort. In the late 1800s, it was shown convincingly by Maxwell that light (as well as many other forms of
radiation) is an electromagnetic wave. As we have seen, however, Einstein provided a curious twist in our understanding
of the nature of light. In 1905, Einstein explained that light must be a particle. It is this wave-particle duality that forms
much of the basis of quantum mechanics.
Diffraction and interference phenomena are not just a curious side-show in our discussion of waves. Rather,
these effects are prevalent and important in numerous systems involving waves, such as concert halls, microwave ovens,
anti-reflective coatings, and any sort of wireless communications. Diffraction effects limit our ability to image objects of
very small size. Interference effects can be used to make incredibly precise measurements of distance, an application
used for very precise scientific experiments and for fabrication of devices with very tight tolerances. Interference can be
used to measure the spacing between energy levels in atoms (as you will see in the lab on Emission Spectra). An
understanding of interference will also help you appreciate the colorful reflections of a compact disk or a soap bubble.
Furthermore, an understanding of interference is critical for understanding many of the implications of quantum
mechanics.
In this lab, you will become familiar with the concepts of diffraction and interference, and use these ideas to
demonstrate that light acts as a wave. You will use a laser, which is a source of light of one particular frequency (and
wavelength), and several diffraction gratings, which are simply slits in some material (in this case, a metal foil) which
light could pass through.
There are several goals of this lab: in particular, after finishing this lab, you should (a) gain some practical
experience with lasers and diffraction gratings; (b) get some exposure to the idea of diffraction and interference; (c)
understand how diffraction effects limit our ability to see small objects; and (d) be able to quantify the relationship
between the diffraction size and the effect on light.
2. Diffraction
When a wave passes through a small opening (a "slit"), it spreads out. This effect -- referred to as diffraction --
is apparent for sound waves (you can hear someone talking to you from the next room, even if you are not in a direct
line-of-sight). Diffraction is not as noticeable for light passing through an opening, due to the much smaller wavelength
for visible light (red light has a wavelength of 6 x 10 -7 m, as opposed to 1 m for typical sound waves). Here, you will be
able to observe the diffraction of light by passing it through a slit that is on the order of the wavelength of light.
Procedure
Warning: Be careful not to look directly into the laser or to look directly into the reflections of the beams
off the metal foils.
(a) Set up a laser at the end of the lab table pointing toward the wall. Place the white screen close to the wall.
Look for "Slide 1" which contains four slits. Carefully place the slide in the holder so it is tilted downwards on the
side of the laser . This will direct all reflections towards the floor instead of your eyes. Position the slide in front of the
laser beam such that the beam falls squarely on the 0.08 mm wide slit (the second widest slit). With the room lights off,
observe the light falling on the screen (the "diffraction pattern"). If the slit formed a sharp shadow, you would expect to
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