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Interactive Lecture Questions for Single Slit Diffraction

Terry Bradfield, Department of Natural Science, Northeastern State University, Tahlequah OK
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This material is replicated on a number of sites as part of the SERC Pedagogic Service Project


Single Slit Simulator
This is a set of interactive lecture demonstration questions designed to probe student understanding of single-slit diffraction. Students are asked to predict the result when the slit width or wavelength of the light change. They can be used in association with an actual demo or by using a simulation of the experiment (simulation link unavailable).

Learning Goals

  1. Students will understand how slit width affects the width of the central part of the diffraction pattern.
  2. Students will understand how the wavelength affects the width of the central pattern.

Context for Use

These questions are to be used for in-class peer instruction. Students should make, share and discuss their predictions prior to the demonstration. There should also be a general class discussion/presentation afterward to clarify the material. Students should have had an introduction to Huygen's principle as the model for predicting future behavior of a wave.

Description and Teaching Materials

The demonstration can be done either as an actual experiment or using a simulation. For a real experiment, two monochromatic light sources of different wavelengths (e.g. red and green laser pointers) are needed. In addition, a variable-width slit or multiple slits of different widths will be required.

The link for the simulation of the experiment is currently unavailable.

Diffraction Question 1: Effect of Changing the Slit Width (Microsoft Word 28kB Jul30 07)
Diffraction Question 2: Effect of Changing the Wavelength (Microsoft Word 28kB Jul30 07)

Teaching Notes and Tips

About the Demo

The demonstration can be performed either with the simulation at the link given above, or with a real source and slit. A laser pointer and a couple of slits of different widths are the minimum equipment requirements. A variable slit and holders of some kind to keep the pointer stationary will make things much easier. To do both demonstrations in this activity will require a second source with a different wavelength (e.g. red and green laser pointers).

Answer Choices for Question 1

B is the correct choice. The width of the central bright zone is bounded by the points at which destructive interference occurs. These are given (approximately by the equation x = Lλ/W where x is the distance from the center of the pattern, L is the distance from the slit to the projection screen and W is the slit width. Note that in this relation sinθ = tan θ = x/L has been used.

Students who choose A may be using a ray model of the process whereby a larger opening means a larger angle of acceptance for incoming rays and hence a wider illuminated zone on the screen. Those choosing C may realize that the aperture size determines the amount of light that passes through the opening, but have no mental picture of the process by which the light moves from opening to screen.

Answer choices for Question 2

A is the correct choice. The width of the central bright zone is directly portional to the wavelength(see above)

Students who choose B may simply be assuming that inverse relationship found in the first question also applies here. Those choosing C in this case still have no mental picture of the process by which the light moves from opening to screen.


Qualitative questions of a similar nature can be included on exams.

References and Resources

For additional information about student's difficulties with the photoelectric effect, see Five Easy Lessons, Strategies for Successful Physics Teaching by Randall D. Knight, Addison-Wesley ISBN 0-8053-8702-1