Tutorial details

Students begin the tutorial by considering the pattern produced by a single narrow slit. Next, they are told that a second identical slit is cut in the mask, and asked to predict what they would see on the screen if the original slit were covered and the second slit were uncovered. They are then guided to determine how the pattern would change if both slits were uncovered.

To check their understanding, students are asked to identify which minima would be affected if (1) the slit width were decreased or (2) the slit spacing were decreased. These questions provide a review of concepts in single-slit diffraction and two-source interference. Students also are asked to identify features of the graph such as the locations of the first interference and diffraction minima and are led to recognize that the diffraction envelope is the graph of relative intensity that would result if one of the slits were covered.

Next students consider how a relative intensity graph would change as the slits are made narrower and narrower. Many students fail to make connections between the general case of combined interference and diffraction and the limiting case of an extremely narrow slit. Some students must be guided to do so.

In the last part of the tutorial, students are given a quantitative problem. They are shown a relative intensity graph and asked to determine the width of the slits and the distance between them.

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Prerequisite tutorials

The Two-source interference, Wave properties of light, Multiple-slit interference, and Single-slit diffraction tutorials are prerequisites to Combined interference and diffraction.

Other prerequisites

Students should have worked through the prior tutorials on physical optics: Two-source interference, Wave properties of light, Multiple-slit interference, and A model for single-slit diffraction.

Equipment

List

• white board or large sheet of paper with markers

Discussion