# 4.4 Waves Flashcards

What is a transverse wave? (2)

Oscillations are perpendicular to the direction of energy transfer

The particles vibrate at 90° to the motion of the wave

What is a longitudinal wave? (2)

Oscillations are parallel to the direction of energy transfer

The particles vibrate in the same direction as the wave

What are examples of transverse waves? (4)

Water waves

EM waves

Strings

S-waves

What are examples of longitudinal waves? (2)

Sound waves

P-waves

What is displacement?

The distance of a particle from the equilibrium position (s), metres

What is amplitude?

Maximum displacement from equilibrium (A), metres

What is wavelength?

Distance between two identical points on adjacent waves (λ), metres

What is period?

Time taken for one wavelength to pass a given point (T), seconds

What is frequency?

Number of wavelengths passing a given point per unit time (f), Hz

What is wave speed?

Distance travelled by wave per unit time (v/c), ms^-1

How can an oscilloscope be used to determine frequency? (4)

An oscilloscope plots a graph of PD against time

Horizontal squares (timebase) represent a certain time interval

The height represents PD

From the timebase the period can be determined and used to find the frequency

What is the equation linking frequency and time period?

f = 1/T

What is the wave equation?

v = fλ

How is the wave equation derived? (2)

Since v = d/t, in the time for one cycle a wave travels one wavelength (v = λ/t)

As T = 1/f:

v = λ / (1/f) so v = λf

What is a history graph? (2)

Displacement-time

Shows how the position of a single particle series with time as a wave passes through (time period)

What is a snapshot graph? (2)

Displacement-distance

Shows entire wave at one moment (wavelength)

What is phase difference?

The difference in the relative positions of two or more particles in the same wave or the relative points in the cycle of two or more different waves

What is phase? (2)

Describes the current point in the cycle of something that changes periodically, expressed either in radians or degrees

One complete wave = 2π radians

How can the direction a particle is moving be determined?

Draw the wave as it would look a short amount of time later in order to see whether specific particles are moving up or down

What is antiphase?

A phase difference of 180°

What is refraction? (4)

Refraction occurs when a wave passes from one median to another

If the wave slows down it refracts towards the normal and vice versa

There is always partial reflection

Refraction affects wavelength but not frequency

What is reflection? (3)

Reflection occurs when a wave changes direction at a boundary between two media

The angle of incidence = the angle of reflection

The wavelength and frequency remains the same

What is diffraction? (3)

Diffraction occurs when a wave passes through a gap or around an obstacle

Speed, wavelength and frequency stay the same

Diffraction effects are most significant when the gap is similar in size to the wavelength

What is dispersion?

When white light enters a prism it is refracted, first as it enters the prism and again as it leaves Longer wavelengths (red) are refracted less than shorter wavelengths (purple)

What is polarisation? (2)

The particles oscillate along one direction only, confining the wave to a single plane

The plane of oscillation is both where the particles oscillate and the direction they travel in

Which waves can be polarised?

Transverse waves

Why can’t longitudinal waves be plane polarised?

The oscillations are already limited to one plane (the direction of energy transfer)

What is partial polarisation?

When transverse waves reflect, there are more waves oscillating in a particular plane but the wave is not completely polarised

What is the relationship between intensity and distance? (3)

Intensity (Wm^-2) = Power (W) / Area (m^2)

When a wave travels from a point source, they spread across the surface area of a sphere: I = P / 4πr^2

Intensity is proportional to 1/(r^2)

What is the relationship between intensity and amplitude? (3)

Intensity is proportional to amplitude^2

Further away from a source, intensity decreases, causing a decreasing amplitude which in turn leads to a reduced average speed.

Halving the amplitude, halves the speed which causes the kinetic energy to be four times as small

What is an electromagnetic wave? (3)

A transverse wave

They require no medium to travel through

Electric and magnetic waves oscillating at right angles to each other

Wavelengths - Radio Waves

> 10^6 metres to 10^-1 metres