Cheatography

# A-Level Physics - Waves Cheat Sheet (DRAFT) by amstoffel

AQA A-Level Physics Topic 3 - Waves; made directly in accordance with the AQA 7408 specification

This is a draft cheat sheet. It is a work in progress and is not finished yet.

### Waves

 A wave is a means of transf­erring energy and momentum from one point to another without there being any transfer of matter between the two points. They can be mechan­ica­l/e­lec­tro­mag­netic, progre­ssi­ve/­sta­tionary and longit­udi­nal­/tr­ans­verse

### Types of waves

 Mechanical waves are made up of vibrating particles. They require a substance for transm­ission, so can't travel through a vacuum.eg sound (air molecu­les), water (water molecules) Electr­oma­gnetic waves are made up of oscill­ating electric and magnetic fields. They don't require a substance for transm­ission, so can travel through a vacuum.eg light and radio Progre­ssive waves are when there is a net transfer of energy and momentum from one point to anothereg sound from a person speaking, light from a lamp Stationary waves are when there is NO net transfer of energy and momentum from one point to another.eg the wave on a guitar string Longit­udinal waves are when the direction of vibration of the particles is parallel to the direction of traveleg sound Transverse waves are when the direction of vibration of the particles is perpen­dicular to the direction of traveleg water and electr­oma­gnetic waves

### Measuring Waves

 Displa­cement, x: the distance of an oscill­ating particle from its undist­urb­ed/­equ­ili­brium position Amplitude, a: the maximum displa­cement from an oscill­ating particle from its equili­brium positionIt is equal to the height of a peak or depth of a trough Wavele­ngth, λ: the distance between two consec­utive particles at the same phase, measured in metres/mExample: peak to peak Period, T: the time taken for one complete oscill­ation of a particle in a waves, measured in seconds/s Frequency, f: the number of complete oscill­ations in one second performed by a particle in a wave note: f =1/ T Phase, φ: the point that a particle is at within an oscillation It can be expressed in terms of an angle up to 360° Phase differ­ence, Δφ: the fraction of a cycle between two particles within one or two wavesExample:the top of a peak has a phase difference of half of one cycle compared with the bottom of a trough. Phase difference is often expressed as an angle differ­ence. So in the above case the phase difference is 180°. Also with phase differ­ence, angles are usually measured in radians.

### Wave equation

 For all waves: c = f λ OR speed = frequency x wavelength (where speed is in ms^-1 provided frequency is in hertz and wavelength in metres)

### Polari­sation

 Only TRANSVERSE waves undergo polari­sation (where they all travel at the same speed in a vacuum) The oscill­ations within a transverse wave and the direction of travel of the wave define a plane. If the wave only occupies one plane the wave is said to be plane polarised. Light from a lamp is unpola­rised. However, with a polarising filter it can be plane polarised. If two ‘crossed’ filters are used then no light will be transm­itted.

### Applic­ations of polari­sation

 Polarised sungla­sses: these contain lenses with polarising filters that only transmit vertically polarised light.When light is reflected from a reflective surface with water, it undergoes partial plane polari­sation (a proportion of the reflected light will oscillate more in the horizontal plane than the vertical plane). Polaroid photog­raphy: work in a similar way to polarised sunglasses.They are useful for capturing intens­ified colour and reducing glare on bright days. They also enable photog­raphers to take photos of objects underw­ater. Polari­sation of Radio & Microwaves Signals: Radio and television services are broadcast either horizo­ntally- or vertically-polarised.Therefore, the reception aerial needs to be mounted horizontal or vertical, where its orient­ation will depend on the transm­itter it is pointing towards and the polarity of the services being broadcast.