Tutorial details

The tutorial begins by having students make observations of water waves passing through slits of various widths. They come to recognize that when a slit is sufficiently narrow, it results in waves that are nearly those from a point source and that when the slit is sufficiently wide it does not significantly affect the shape of the wavefronts. Students are led to realize that they can apply ideas developed in the preceding tutorial, Two-source interference, to the case of water waves incident on two very narrow slits.

In the next section of the tutorial, students consider a double-slit interference pattern from light. They recognize that the pattern is different from that predicted by geometrical optics. Students compare this situation to that of periodic water waves incident on two very narrow slits. They are guided to make an analogy between water waves and light. They are led to recognize that if one of the slits were covered, the screen would be (essentially) uniformly bright; there would be no points of zero intensity.

The students then make predictions about how a variety of changes (e.g., decreasing the distance between the slits or moving the screen closer to the mask) would affect the interference pattern. The final problem is quantitative.

Note: The tutorial makes the approximation that the slits are sufficiently narrow that they may be treated as point sources of light. (In the tutorial Diffraction students consider how the width of the slit affects the diffraction pattern.)

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Prerequisite tutorials

The Two-source interference tutorial is a prerequisite to Wave properties of light.

Other prerequisites

Students should have worked through the tutorial Two-source interference and the related homework.

Equipment

List

• paper towels
• water
• white board or large sheet of paper with markers

Discussion