 So another way that we can use the lightboard is to annotate on top of images. Such as this one here, this is a schematic of sound waves coming out of two loudspeakers. And if I was using this with my students, I'd be getting them to think about the interference pattern that an observer who's standing somewhere around here would be perceiving. I'd get my students to think about the initial phase of the two waves that are being emitted from the loudspeaker. And also to think about this quantity here, delta x, this is the path length difference. And in this particular situation, the two sources are in phase, as we can see from the initial heights of the waves coming out of the speaker. But the path length difference is a half number of wavelengths. So the observer standing here, in this case, is going to experience destructive interference.