Double-slit interference is a difficult phenomenon for students to grasp in an introductory physics course. A thorough understanding of constructive and destructive interference is critical, but even with such an understanding, visualizing exactly how an interference pattern is formed by light passing through two slits can still be a challenge for students. Here, we describe a simple demonstration that seems to have improved our students’ understanding of this phenomenon. The required materials are readily available in nearly any physics classroom or lab, and the demonstration can easily be constructed in 5 to 10 minutes.

1.
The size of the aperture should be similar to the wavelength.
2.
Here we ignore the envelope due to the finite width of the individual slits, which would modulate the intensity of the double-slit interference fringes.
3.
Bradley S.
Ambrose
,
Peter S.
Shaffer
,
Richard N.
Steinberg
, and
Lillian C.
McDermott
, “
An investigation of student understanding of single-slit diffraction and double-slit interference
,”
Am. J. Phys.
67
,
146
155
(
Feb.
1999
).
4.
Karen
Wosilait
,
Paula R. L.
Heron
,
Peter S.
Shaffer
, and
Lillian C.
McDermott
, “
Addressing student difficulties in applying a wave model to the interference and diffraction of light
,”
Phys. Educ. Res., Am. J. Phys. Suppl.
67
,
S5
S15
(
July
1999
).
5.
In case two colors of tape are not readily available, the demonstration can also work well with a single color of tape, in which case the tape is used to represent crests, and troughs are understood to lie halfway between the crests.
6.
For shorter wavelengths, it may be necessary to use only a single color of tape to avoid clutter on the strings.
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