When the light beam hits the water stream, the laser light gets reflected inside the stream. Remember the surface of water is reflective both on its surface and below the surface. When light leaves a denser material, it will change directions. However when the critical angle is reached it will reflect back. Critical angle is the smallest angle of incidence for which light is totally reflected. Total internal reflection happens when a wave strikes a medium boundary at an angle larger than a critical angle, with respect to the normal to the surface. Critical angle of water is 48.8 degrees. When the angle of the beam crosses the critical angle the light does not go out of the stream and total internal reflection takes place. the water is a denser medium than air, and when the laser light is shone into the water, the light then creates total internal reflection. when the water picks up speed and flows out of the hole in the water bottle, and the light stays trapped in the stream. the beam of light then reflects on the under and top sides of the water stream.
constructive interference: when a wave is created on two sides of a medium, and then once it meets in the middle the wave becomes double the size of the two that met. they pass through each other, and then reduce down to their original sizes.
destructive interference: when two waves pass through each other but one is a trough and one is a crest, and once they meet, the briefly become flat in one spot. then they pass through each other and continue down the medium.
standing wave: a standing wave is when a medium is destructive but progressively gets faster and smaller, creating still spots called “nodes”. ours has 4 nodes which you can briefly see in the last few seconds of the video. this is also where the peaks do not move spatially.
pulse wave: a wave done only once, which creates a ripple effect in the medium, shown by the one time movement of the spring which then moved the spring all the way to the other end