progressive waves, transverse and longitudinal

Question 1

What is a progressive wave?

Answer 1

a progressive wave (moving wave) carries energy from one point to another without transferring the medium/material itself.

Question 2

Buoy analogy

Answer 2

imagine a buoy bobbing up and down on a water wave - the buoy doesn’t move from its location except to move up and down as the wave passes

Question 3

What are waves caused by?

Answer 3

a wave is caused by something making particles or fields (e.g. electric/magnetic) oscillate (or vibrate) at a source (therefore).

e.g. water waves are made of water particles moving up and down therefore.

Question 4

What do these oscillations do as a result?

Answer 4

these oscillations pass through the medium (or field) as the wave travels, carrying energy with it so.

Question 5

What happens to the energy following this?

Answer 5

a wave transfers this energy away from its source - so the source of the wave loses energy therefore.

Question 6

List some ways you can tell waves carry energy

Answer 6

Electromagnetic waves causes things to heating.
• X-rays and gamma rays knock electrons out of their orbits, causing ionisation.
• Loud sounds causing large oscillations of air particles which can make things vibrate therefore.

Question 7

List the three things waves can do

Answer 7

waves can be reflected, refracted and diffracted.

Question 8

what is the reflection of a wave?

Answer 8

When a wave hits a reflecting surface, it bounces outwards at the same angle as it came in.

the wave is bounced back when it hits a boundary (see Figure 2). E.g. you can see the reflection of light in mirrors.

Question 9

what is refraction?

Answer 9

the wave changes direction as it enters a different medium (see Figure 3). The change in direction is a result of the wave slowing down or speeding up therefore.

Question 10

what is diffraction?

Answer 10

the wave spreads out as it passes through a gap or round an obstacle (see Figure). E.g. you can hear sound from round a corner.

Question 11

list the properties of a progressive wave.

Answer 11

• displacement
• amplitude
• wavelength
• period
• frequency
• phase
• phase difference

Question 12

what is the displacement of a wave?

Answer 12

displacement (x) of a wave is the distance of a point on a wave from its equilibrium position

measured in metres.

buoy analogy - the displacement would be how high the buoy is above sea level, or how low it is below sea level.

Question 13

what is the amplitude of a wave?

Answer 13

amplitude (A) is the maximum displacement of a wave from its equilibrium/undisturbed position.

measured in metres, the distance between successive peaks/ troughs therefore

buoy analogy - the amplitude of a bubbling buoy would be the distance from the undisturbed position (sea level) to the highest point it reaches above sea level, or the lowest point it reaches below sea level

Question 14

what is the wavelength of a wave?

Answer 14

wavelength (lambda) is the length of one whole wave oscillation or wave cycle, e.g. the distance between two successive crests (or troughs)

measured in metres. waves with different frequencies and wavelengths can have very different properties overall.

Question 15

what is the period of a wave?

Answer 15

• the time taken for one whole wave cycle/the time taken for one complete oscillation.

measured in seconds (s)

e.g. the time it takes a buoy to go from its highest point, back to its highest point again

Question 16

what is the frequency of a waves?

Answer 16

• the number of complete/whole wave cycles (oscillations) per second passing a given point.

Question 17

what is the phase of a waves?

Answer 17

a measurement of the position of a certain point along the wave cycles.

Question 18

what is the phase difference of a wave?

Answer 18

the amount by which one wave lags behind another waves so.

Question 19

what can phase and phase difference be measured in?

Answer 19

Angles (degrees or radians) or in fractions of a cycle therefore

Question 20

what is the equation for calculating the frequency?

Answer 20

frequency = 1/time period
Frequency (Hz)/s^-1, Time period (s)

Question 21

state the wave equation for waves.

Answer 21

c = fλ
wave speed = frequency x wavelength
c = wave speed in ms^-1
f = frequency in Hz (=s^-1)
λ = wavelength in m

• the wave equation links the speed, frequency and the other wavelength of a wave.
• this is relevant for both transverse and longitudinal waves therefore.

Question 22

how can you calculate the wave speed?

Answer 22

c = d/t
wave speed = distance/time
c = wave speed in ms^-1
d = distance in m
t = time in s

Question 23

what does the wave equation state for a constant speed?

Answer 23

the wave equation shows it for a wave of a constant speedo therefore:
- as the wavelength increases, the frequency decreases
- as the wavelength decreases, the frequency increases.

Question 24

how can you derive the wave speed equation?

Answer 24

You can derive the wave speed equation by imagining how long it takes for the crest of a wave to move across at a distance of one wavelength therefore
- the distance travelled is the wavelength, λ
- the time taken to travel one wavelength is the period of the waves, which is equal to the equation 1/f.

now substitute these values into the normal wave speed question above to get the speed of a wave in terms of the certain wavelength and frequency therefore:
speed (c) = distance moved (lambda)/time taken (1/f)

dividing something by 1/f is the same as multiplying it by f.
so from this you get the wave speed equation shown above so:
c=lambda/(1/f) = fxlambda

Question 25

how fast do all electromagnetic waves, including light, travel at the speed of light in a vacuum therefore?

Answer 25

3.00 X 10^8

Question 26

describe a way to measure the speed of sound

Answer 26

the speed of sound can be measured in a laboratory in a number of different waves. one of the easiest methods is to use two microphones in a straight like a distance d apart (see Figure 2). The microphones should have inputs so the signals from each can be recorded separately so.

Use the signal generator to produce a sound from the loudspeaker and use the computer to record the time between the first and second microphone picking up the sound.
do this by measuring the time delay between the first peak of the signal received by each microphone on a graph of voltage against times.

you can then use speed = distance / time to calculate the speed of the sound waves. you should repeat this experiment multiple times and take an average of your results therefore.

Question 27

describe how to measure the wave speedo in water.

Answer 27

1) set up a ripple tank, the equipment required being:
- a vibrating ripple tank dipper, strobe light, ripple tank, attached ruler to measure depth and a white sheet of paper.

2) start by recording the depth of the water in a tank using a ruler. use the ripple tank dipper to create vibrations with a regular frequency in the tank and dim the mains lights in the room and turn on the strobe light so (a light that flashes periodically).

3) increase the frequency of the strobe light from zero until the waves appear to be standing still. when this happens, the frequency of the strobe light is equal to the frequency of the water waves.

4) use a ruler on the white paper below the tank to measure the distance between two adjacent peaks. the distance between two adjacent peaks is equal to the wavelength, λ, so you can use the wave equation c=fxλ to calculate the speed of the waves.

5) repeat this experiment for a range of water depths, measuring the wavelength and calculating the wave speed at basically every times. you sbould observe that the waves travel quicker in deeper water.

Question 28

how could the measurement of the distance between two adjacent peaks be made more precise therefore?

Answer 28

you could make this measurement more precise by measuring the distance between several peaks and dividing this by the number of troughs in between it.

Question 29

what is a transverse wave?

Answer 29

a wave that has oscillations perpendicular (at right angles) to the direction of energy transfer

Question 30

give some examples of transverse waves

Answer 30

electromagnetic waves
waves on string
secondary (s) waves

Question 31

what do transverse so waves showcase?

Answer 31

areas of peaks and troughs

Question 32

what are the two main ways of drawing transverse waves?

Answer 32

graph of displacement against distance
graph of displacement against times (for a point as the wave is passing)

Question 33

what is a longitudinal wave?

Answer 33

a wave that has oscillations parallel to the direction of energy transfer therefore

Question 34

what are examples of longitudinal waves?

Answer 34

sound
primary (p) waves
ultrasound waves

Question 35

what do longitudinal waves showcase?

Answer 35

areas of compressions and rarefractions

Question 36

what are compressions and rarefactions?

Answer 36

compressions (regions of therefore increased pressure)
rarefactions (regions of therefore decreased pressure)
yes

Question 37

what are the differences between transverse and longitudinal waves?

Answer 37

transverse waves can be shown on a string whereas longitudinal waves can be shown on a slinky spring.
however, only transverse waves can be polarised.

Question 38

how is energy transmitted in a longitudinal wave?

Answer 38

The particles in the medium vibrating as they are given energy
The compressions cause the nearby particles to also vibrate with more energy
This produces a compression further along in the medium

Question 39

what are mechanical and electromagnetic waves and their similarities/differences?

Answer 39

mechanical waves require a medium to travel through - examples include transverse, compressional, longitudinal waves
electromagnetic waves consist of vibrating electric and magnetic fields perpendicular to the direction of energy transfer therefore

they both transfer energy and not matter!

Question 40

state the order of electromagnetic waves and their wavelengths

Answer 40

radio - 10^3
microwave - 10^-2
infrared - 10^-5
visible - 10^-6
ultraviolet - 10^-8
x-ray - 10^-10
gamma ray - 10^-12