### Notes:

The Two-Point Source Interference Patterns Interactive is an adjustable-size file that displays nicely on smart phones, on tablets such as the iPad, on Chromebooks, and on laptops and desktops. The size of the Interactive can be scaled to fit the device that it is displayed on. The compatibility with smart phones, iPads, other tablets, and Chromebooks make it a perfect tool for use in a 1:1 classroom.

### Teaching Ideas and Suggestions:

The Two-Point Source Interference Patterns Interactive was designed with the intention that it would be most useful in a unit that address Thomas Young's double-slit experiment. We view this as a tool to be used early on in the learning cycle addressing Young's experiment. The main outcome of the simulation is an understanding of the equations for the relationship between path difference and wavelength for points on nodal and antinodal lines. The hope is that students would use data provided by the simulation to discover the path difference equations.

The simulation consists of three activities. The first activity is a bit of a departure from the norm for us; it is an interactive tutorial on the topic of two-point source interference patterns. The tutorial includes 9 screens of information. Students navigate from one screen to the next or previous screen using the arrow buttons; there is no ability to skip screens. If a student exits the tutorial before completion, they will be returned to the last screen they were on. The tutorial reviews the topic of constructive and destructive interference in the context of a two-dimensional medium (e.g., a ripple tank). The concept of nodes and antinodes and nodal lines and antinodal lines is also introduced. Names for the nodal and antinodal lines are presented. Considerable attention is given to how the number of waves from the sources to a point on the pattern can be counted. The path difference is explained and three examples are provided.

The second activity - DIY Patterns - provides students an opportunity to alter the wavelength and the source separation distance and to view the effect that it has upon the pattern. The pattern is redrawn each time the wavelength or the distance is incremented upward or downward.

In the third activity - Path Difference - students select four nodal and antinodal points on a pattern ... one at a time. The simulation counts (animates) the number of waves from the two sources to each selected point, displays the S1P and S2P distances (in terms of wavelength), and calculates the path difference (in terms of wavelength). Students can gather the data, analyze it, and brainstorm an equation that relates the wavelength to the path difference for points on the nodal and the antinodal lines.

We recommend our student activity sheet for this simulation. It guides students through each of the three activities, with an emphasis on the last activity and the goal of determining the PD-wavelength relationship.

Our Two-Point Source Interference Patterns simulation is now available with a Concept Checker. Do the simulation. Then follow it up with the Concept Checker.

### Related Resources

There are several resources at The Physics Classroom website that serve as very complementary supports for the Two-Point Source Interference Patterns Interactive. These include:
• Physics Video Tutorial:
We have developed a rather thorough collection of video tutorials on physics topics. Our Light Waves section consists of 13 videos. Three videos align well with this simulation and the broader topic of Young's experiment.

Anatomy of a Two-Point Source Interference Pattern

Path Difference and Wavelength

Young's Equation and Young's Experiment

Visit the Physics Video Tutorial

• The Calculator Pad:
The Calculator Pad includes a collection of interactive problem sets. Numerical information in the problems is randomized such that two side-by-side students are likely to get the same problem with difference numbers. Students can submit their answers and receive immediate feedback and opportunities to correct their mistakes. The resource is free-for-use. Teachers with a Task Tracker subscription have the added advantage of customizing the problem sets, assigning them to their classes, saving student progress, and accessing student scores. Problem Sets LW6 - LW10 in the Light Waves section coordinates well with this interactive and the broader topic of Young's experiment.

• Labwork:
Simulations should always support (never supplant) hands-on learning. The Laboratory section of The Physics Classroom website includes several hands-on ideas that complement this Interactive. Three notable lab ideas include ...

Ripple Tank Lab
Two-Point Source Analysis Lab
Young's Experiment Lab

You will find these in the Light and Color section of The Laboratory. Visit The Laboratory. Teacher Guides are available.

Additional resources and ideas for incorporating the Two-Point Source Interference Patterns Interactive into an instructional unit on Light and Color can be found at the Teacher Toolkits section of The Physics Classroom website. Visit Teacher Toolkits.