# 6: Wave Behaviour Flashcards

What does the principle of superposition state?

When two or more waves overlap (superimpose), the **resultant** displacement equals the **vector** sum of the individual displacements at that position and time

Define these words

- wavelength
- displacement (and amplitude)
- frequency
- time period

**Wavelength** is the distance between equivalent points on 2 consecutive waves e.g. crest to crest or trough to trough

**Displacement** is the distance from the equilibrium position (amplitude is the maximum displacement from the equilibrium position)

**Frequency** is the number of complete waves passing a given point in one second.

**Time period** is the time taken for a wave to completely pass through any given point. This is the reciprocal of the frequency: 1/time period = frequency & 1/frequency = time period

Interference can be [] or destructive

Constructive

What is total destructive interference?

When a crest and a trough of equal size (amplitude), combine to give zero amplitude

What is an example of constructive interference?

When two crests combine to create a bigger crest (a crest with a larger amplitude)

What is phase?

What does a phasor represent?

Phase describes the stage in a wave cycle

The phase of each point on the wave

Which way does a phasor rotate?

Anticlockwise

How do you work out the rate of phasor rotation

Phasor rate of rotation is the same as wave frequency

What is the phase difference of waves exactly out of phase (antiphase)?

What about their phasors?

The phase difference is an odd-numbers of π radians.

Their phasors point in opposite directions

What does ‘in phase’ mean for 2 points on a wave?

Two points on a wave are in phase if they are both at the same point in the wave cycle

Points that have a phase difference of zero or a multiple of 2π are in phase - their phasors point in the same direction

What is the phase difference of two waves emitted from an oscillator?

They are in phase so their phase difference is a multiple of 2π

To get clear interference patterns the two sources must be []

coherent

What does it mean if two sources are coherent?

They have the same wavelength and frequency and a fixed phase difference between them

What affects whether you get constructive or destructive interference at a point?

Phase difference - depends on how much further one wave has travelled than the other wave to get to that point (assuming the sources are coherent and in phase)

What is path difference?

The amount by which the path travelled by one wave is longer than the path travelled by the other wave is called the path difference

Describe constructive interference

At any point an equal distance from both sources (that are coherent and in phase), or where the path difference is a whole number of wavelengths

What is the path difference for constructive interference?

nλ where n is an integer

What is the path difference for total destructive interference?

(2n+1) λ/2

n is an integer.

i.e. a whole number of half wavelengths

Describe total destructive interference

At any point where the path difference is an odd number of half wavelengths

Define standing waves

transmitted wave reflected at boundary with a 180 degrees phase change. transmitted and reflected waves superpose creating nodes and antinodes. at certain frequencies positions of nodes and antinodes are constant, resulting in a standing wave

What is a standing wave? (simple)

The superposition of two progressive waves with the same wavelength, moving in opposite directions

When do you get a standing wave?

When a progressive wave is reflected at a boundary and superposes with another wave of the same wavelength

Is energy transmitted by a standing wave?

No

What are resonant frequencies?

Frequencies where the oscillator happens to produce an exact number of waves in the time it takes for a wave to get to the end and back again, then the original and reflected waves reinforce each other

What is a node?

A position, on a standing wave, of zero amplitude

What is an anti-node?

A position, on a standing wave, of maximum amplitude

Describe the fundamental frequency

The standing wave is vibrating at the lowest possible frequency, the fundamental frequency This is the first harmonic. It has one loop with the node at each end

What is another name for the second harmonic?

The first overtone

Describe briefly standing waves on stringed instruments

They are transverse standing waves. Your finger or the bow sets the string vibrating at the point of contact. Waves are sent out in both directions and reflected back at both ends

What does a cathode ray oscilloscopes measure? What does it display?

Voltage It displays waves from an oscillator as a function of voltage over time

On a cathode ray oscilloscope, what does the vertical axis show? What does the horizontal axis show?

Vertical: voltage Horizontal: time

Describe standing waves and the wind instrument or other air column.

They are longitudinal standing waves If a source of sound is placed at the open end of a wind instrument, there will be some frequencies for which resonance occurs and a standing wave is set up Nodes form at close ends. Antinodes form at the open ends

What is refraction?

The way a wave changes direction as it enters a different medium

When does refraction occur?

When the medium a wave is travelling in changes

When a wave is refracted does the speed, wavelength, and frequency change?

The speed changes, the frequency stays constant, so the wavelength changes too

What happens if light meets a boundary at an angle to the normal? (refraction)

The transmitted ray is bent or refracted as it travels at a different speed in each medium