This scene is critical. After a string of successes using the simple model underlying geometric optics we encounter a phenomenon that cannot be explained by geometric optics.
When we use the flashlight app on a smartphone to cast the shadow of our hand on a wall and keep our hand still at a constant distance from the wall and then move the flashlight toward and away from the hand the size of the shadow changes. When the flashlight is further from the hand the shadow is smaller. This is exactly what is predicted by geometric optics.
But, when we use a a diffraction grating or diffraction glasses as the target and a laser pointer in place of the flashlight app the size of the “shadow” stays the same regardless of the distance between the laser pointer and the diffraction grating. We need a new model!!
Some experiments with the “ripple tank” in your SWAG or some observations about waves in puddles can suggest that waves might be a possible model.
This concludes Act I — with questions. In a course studying trigonometric functions students might use interference, which is related to diffraction, as a nice application of adding sine functions of different phases.
Just Add Math 