Waves 1

Question 1

Define a progressive wave

Answer 1

A wave that transfers energy without transferring matter and is made up of particles in a medium/field oscillating

Question 2

Define frequency

Answer 2

The number of complete oscillations passing through a point per second, (measured in Hz)

Question 3

Define amplitude

Answer 3

The waves maximum displacement from the equilibrium position, (measured in metres)

Question 4

Define wavelength

Answer 4

The shortest distance between 2 points in phase, (measured in metres)

Question 5

Define wave speed

Answer 5

The distance travelled by the wave per second, (units are m/s)

Question 6

Define phase

Answer 6

The position of a certain point on a wave cycle (measured as an angle or fraction of a cycle)

Question 7

Define phase difference

Answer 7

How much 1 particle/wave lags behind another particle/wave (measured as an angle or fraction of a cycle)

Question 8

Define time period

Answer 8

The time taken for 1 full oscillation (measured in seconds)

Question 9

Describe what it means if 2 points are in phase

Answer 9

• They are both at the same point of the wave cycle
• They have the same displacement and velocity
• Their phase difference will be a multiple of 2pi or 360 degrees

Question 10

Describe what it means if 2 points are completely out of phase

Answer 10

• They are an odd number of half cycles apart ie (n+1/2) wavelengths apart
• They have the opposite displacement and velocity
• Also 1 half cycle is 180 degrees

Question 11

Describe the relationship between wave speed, wavelength and frequency

Answer 11

wave speed = frequency x wavelength

Question 12

Describe the relationship between frequency and time period

Answer 12

Frequency = 1/time period

Question 13

Explain the difference between longitudinal and transverse waves

Answer 13

In longitudinal waves, the oscillations are parallel to the direction of energy transfer whereas in transverse waves, the oscillations are perpendicular to the direction of energy transfer

Question 14

Describe the characteristics of EM waves

Answer 14

• Transverse
• travel at the same speed in a vacuum (3x10^8 m/s = speed of light)

Question 15

Explain whether longitudinal waves can travel through a vacuum.

Answer 15

• No, they are made up of compressions and rarefactions but they require matter to compress and rarefy.
• (This means they transfer mechanical energy whereas EM waves can travel in a vacuum because they transfer light and heat energy instead)

Question 16

Give 3 examples each of transverse and longitudinal waves

Answer 16

Transverse - EM waves, S waves and water waves

Longitudinal - Sound waves, P waves and ultrasound waves

Question 17

Define a polarised wave

Answer 17

A wave that has been restricted to one plane

Question 18

Explain why polarisation provides evidence for the nature of transverse waves

Answer 18

Polarisation can only occur if the waves oscillations are perpendicular to the direction of energy transfer and transverse waves oscillate in any plane perpendicular to the propagation direction whereas other waves don’t

Question 19

Explain 2 examples of the uses of polarisation

Answer 19

• Polaroid sunglasses block glare by filtering the light to one plane
• TV and radio signals are polarised due to the orientation of the transmitting aerial. Therefore, the receiving aerial must be aligned in the same plane of polarisation to receive the signals full strength

Question 20

Define superposition

Answer 20

When the displacements of 2 or more waves combine as they pass over each other. This can happen to all types of waves

Question 21

Explain how constructive interference occurs during superposition

Answer 21

During superposition, constructive interference occurs at points when the 2 waves have displacement in the same direction

Question 22

Explain how destructive interference occurs during superposition

Answer 22

During superposition, destructive interference occurs at points when 1 wave has a positive displacement and the other has a negative displacement

Question 23

Explain how a stationary wave is formed

Answer 23

Formed from the superposition of 2 progressive waves travelling in opposite directions, with the same frequency, wavelength and similar amplitude

Question 24

Explain the difference between a progressive and stationary wave in terms of amplitude

Answer 24

In a progressive wave, every particle has the same amplitude whereas each point on a stationary wave has a different amplitude, depending on the amount of superposition

Question 25

Explain the difference between progressive and stationary waves in terms of phase difference between points

Answer 25

- In a progressive wave, the phase difference of points is between 0 - 360 degrees with 360 being in phase - In a stationary wave points that have an even number of nodes between them are in phase whereas points with an odd number of nodes between them are out of phase

Question 26

Explain the difference between progressive and stationary waves in terms of energy transfer

Answer 26

- In a progressive wave energy is transferred along the wave - In a stationary wave, energy is stored, not transferred because there are 2 waves carrying equal energy in opposite directions

Question 27

Explain the difference between progressive and stationary waves in terms of wave speed

Answer 27

- In a progressive wave, wave speed is the speed at which the wave moves through a medium - In a stationary wave, each point on the wave oscillates at different speeds (max at antinodes, min at nodes) but the overall wave does not move

Question 28

Explain how nodes and antinodes are formed on a stationary wave

Answer 28

- Nodes are formed from the destructive interference of 2 points completely out of phase, no displacement - Antinodes are formed from the constructive interference of 2 points in phase, max displacement

Question 29

Define the first harmonic

Answer 29

The lowest frequency at which the stationary wave can be formed

Question 30

How would you calculate the wavelength of a stationary wave

Answer 30

Calculate the distance between 2 nodes/antinodes and double it. (Because the distance between 2 nodes/antinodes is half a wavelength)

Question 31

How would you calculate the frequency of the first harmonic

Answer 31

f=1/2l√t/u (√t/u is the speed of the wave and 2l is the wavelength of the first harmonic)

Question 32

How would you calculate the second harmonic frequency from the first harmonic frequency

Answer 32

x by 2

Question 33

Give 2 examples of how stationary waves can be formed in the real world

Answer 33

- Reflect microwaves off a metal plate, nodes and antinodes can be found using a microwave probe - Stationary sound waves are formed using a speaker on a closed glass tube, causing the sound waves to refelct of the tube. Speed of sound can be worked out from this using c=fλ, where λ can be found knowing that the distance between nodes is half a wavelength