Wave Reflection -- xmdemo 138
Why Physics Stack Exchange recommends it
Physics Stack Exchange users link it as a quick visual of how a wave pulse reflects at a fixed or free end, to back up answers about reflection, phase flips and boundary conditions.
Recommended by 1 people across 1 different months, 2020 to 2024.
What it is
A 72-second demonstration from the xmdemo channel showing a pulse travelling along a spring or string and reflecting from an end. It comes from a physics teacher's collection of short A-level demos.
What people said
Recommends Mark Allen · 2024-09 · Physics Stack Exchange · on “Can taut membranes and strings that are clamped at both ends propagate non-standing waves?”
@Edward perhaps you're asking too much of a guitar string. But here's a video using a spring designed for physics demonstration purposes: [this video]
Cites Kphysics · 2020-05 · Physics Stack Exchange · on “What force creates the reflected wave in a string attached to a rod?”
The normal explanation is that if the ring is massless then by Newton's law the slightest force acting on it along the rod will create infinite acceleration. What that really means is that the the string will always be normal (at 90') to the rod. In terms of the incoming and outgoing waves we usually write: $$ \frac{d}{dx} [ \sin(x-vt) + c \sin(x+vt+\phi) ] =0 , ~~at~~ x=0 ~~and ~~for ~~all ~~t$$ Using trigonometry try to find c and $\phi$ such that this is true. You should be able to see that this sum of incoming and outgoing waves is indeed at 90' to the rod. But to visualize the forces is hard. What really happens is that the furthermost piece of the string (say the last 1mm attached to t
Cites user171780 · 2021-03 · Physics Stack Exchange · on “A light problem: What happens when light completely destructively interferes?”
Think of an analogous but simpler problem. You have a string attached to a wall in one side and you are handling it on the other side (see this video [this video] , your hand is the spherical source of light and the wall is the spherical mirror). If you left your hand without moving when the reflection comes back (as in the video) the pulse will bounce again. If you move your hand with the same phase as the reflected pulse you will increase its energy by constructive interference. If you move your hand in anti-phase then the pulse will transmit its energy to your hand and it will disappear because you created the exact opposite wave and canceled it. You can think of this case as moving your hand
Cites agaminon · 2024-10 · Physics Stack Exchange · on “Should the sign of ω of a wave change upon reflection off a rigid surface?”
A rigid boundary (infinite impedance) also flips the sign of the wave, not only the direction of propagation. Check this experimental demonstration. [this video] . I should have used exponential notation or cosines as it clears this out. In exponential notation, $e^{-i(kx+\omega t)}e^{i\pi}=-e^{-i(kx+\omega t)} \to -\sin(kx+\omega t)$ or $-\cos(kx+\omega t)$.