Wave Interactions What happens when two waves collide

Wave Interactions • What happens when two waves collide with one another? • They will pass through each other unaffected by the collision. • Superposition- two waves can occupy the same space at the same time, when this is happening we say the two waves are interfering with one another.

Interference (superposition) • Constructive interference- if the displacement from equilibrium is in the same direction the amplitude of the resultant wave will be the addition of the original wave’s amplitude. • Destructive interference- if the displacement from equilibrium is in the opposite direction from each other the resultant will be the subtraction of the original wave’s amplitudes.

Constructive Interference Before Interference After

Destructive Interference Before Interference After

Reflections • When a wave comes into contact with a different medium it can be reflected. • Medium is what the wave is traveling through • *It can also pass through the object or be absorbed (converted to another form of energy) by the object. • Normally it does some measure of all 3 but we are just going to focus on the reflection for now

For a transverse wave traveling on a rope tied to something… • If the end is fixed (cannot move) the wave will be inverted (or flipped upside down) when it is reflected • If the end is loose (the rope can slide up and down) the wave will be reflected exactly as it came in.

Fixed end Newton’s 3 rd Law The rope pulls up on the end. The end pushes the rope down

Loose end Since the rope can slide, it never Pulls up on the end.

Standing Waves

Standing Wave • If we get a periodic wave on a string with a fixed end at the appropriate frequency we can create a Standing Wave Pluck! • ~A wave that looks life its standing still

Standing Wave • If we get a periodic wave on a string with a fixed end at the appropriate frequency we can create a Standing Wave • ~A wave that looks life its standing still

Where do you find a standing wave in real life? • Standing waves are extremely important in music. • Any stringed instrument has standing waves set up when they are plucked. • Any wind instrument has a standing wave (of air) set up when they are played.

Terminology • Nodes- points on the standing wave where there is no vibration. • There is maximum destructive interference at this point. • Antinodes- points on a standing wave with the most vibration. • There is maximum constructive interference at this point.

Wavelengths • Each loop on a standing wave is equal to ½ a wavelength. • So if you measure the length of the string and multiply by 2 and divide by the number of loops you have the wavelength. • So if a string has 5 loops on it, then the wavelength is 2/5 the length of the string

Standing Wave This string is 1 ½ wavelengths long Antinode Node 3 m The wave has a wavelength of 2 m Node