Interactive Lecture Questions for Single Slit Diffraction
This activity has benefited from input through a review and suggestion process.
This activity has benefited from input from faculty educators beyond the author through a review and suggestion process as a part of an activity development workshop. Workshop participants were provided with a set of criteria against which they evaluated each others' activities. For information about the criteria used for this review, see http://serc.carleton.edu/sp/compadre/devactivities/reviewcriteria.html.
This page first made public: Jul 30, 2007
This material is replicated on a number of sites as part of the SERC Pedagogic Service Project
- Students will understand how slit width affects the width of the central part of the diffraction pattern.
- Students will understand how the wavelength affects the width of the central pattern.
Context for Use
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 simulation of the experiment can be found at: http://www.physics.uoguelph.ca/applets/Intro_physics/kisalev/java/slitdiffr/index.html
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.