Waves - Owen Shepherd

Two ways to graph a wave

Displacement time for a particle in a fixed point

Displacement distance to model the whole wave

Make sure you know which one tyo’re using!

Amplitude

The maximum displacement of the wave from the normal

Wavelength

The distance between two of the same points on the consecutive waves.

Time period

The time taken for a wave to complete a full cycle or pass through a point

Frequency

How many waves pass a point per second

Velocity

How fast a wave is travelling through a medium

Phase

The exact position or stage of a point within a wave cycle, measured as an angle in degrees or radians

Intensity

The amount of energy passing through a unit in a certain amount of time

Watts per metre squared

Mechanical vs EM waves

Mechanical needs a physical medium or substance to pass through

Whereas EM waves can travel through a vacuum

Transverse vs longitudinal waves

Transverse oscillate at 90 degrees to direction of energy transfer

Longitudinal oscillate parallel to direction of energy transfer

Polarisation

When a waves oscillation is restricted to one plane

Polarising filters

Reduce the oscillation in direction parallel to polymer chains

Polymer chain absorb the light - parallel to the chains

Partial polarisation

When reflected, partial plan polarisation occurs, where the majority of the oscillation is reflected only in one plane

Uses; Sunglasses to block out glare

Sunglasses

Block out horizontal light

Blocks out some glare from water

As light has been partially horizontally plane polarised

Liquid Crystal

When not electrified, rotates the plane of light 90 degrees

When electrified - does not do this - no plane rotation

LCD Layers

Mirror / Backlight

First polarising filter

Liquid Crystal

Second polarising filter

Glass front

How LCD work

Light enters back

polarised into a plane

Crystal rotates plane - if not electrified

passes through second and out

If crystal electrified - blocks light getting out by no rotation to pass through second filter

= white and black display

angle of incidence / refraction between

Normal to the plane and angle

Principle of superposition

When two or more waves in the same plane cross, the resultant displacement equals the vector sum of the individual displacements

Coherence

Waves are of the same amplitude, frequency and wavelength

And that they have constant phase difference

What do coherent waves lead to?

Regular interference patterns

How to determine if constructive or destructive?

Depending if in phase or not

How to determine if in phase or not?

integer wavelength = Constructive = 360 degrees

Half an integer out of wavelength = destructive = 180 degrees = antiphase

Stationary wave

The superposition of two progressive waves moving in opposite directions.

Forms a regular interference pattern = standing wave

How to form a standing wave?

Oscillator and fixed end

Node

Point where displacment is always 0

Between nodes is ½ wavelength

Always a node at a fixed end

point where waves are always 180 degrees out of phase

Antinode

¼ wavelength from every node

Points of maximum oscillation

Vary between Max and Min

Why is fixed point a node?

When reflected - change in polarity from + to _ or vice versa

So always 180 out there

Resonant Frequency

The frequency at which a standing wave forms

First Harmonic

Where two nodes are present at each end

Antinode in the middle

Lowest possible resonant frequency

Second, third … harmonic

All present at the harmonic number x frequency of the first harmonic

Stationary waves only form with

Superposing waves with equal frequency ampitude and wavelength

Travelling in opposite directions

usually after being reflected

Harmonics in tubes

Closed ends = nodes

open ends = antinodes

Patterns for each frequency can be derived from that