4 Waves

Question 1

What are electromagnetic waves?

Answer 1

oscillating magnetic and electric fields that progress through space without needing a medium

Question 2

What is a longitudinal wave?

Answer 2

A wave in which the direction of vibration of the particles is parallel to the direction of travel

Question 3

What is a transverse wave?

Answer 3

a wave in which the direction of vibration is perpendicular to the direction of travel

Question 4

What is plane-polarisation?

Answer 4

when the vibrations of a wave stay in one plane only

Question 5

what does unpolarised mean?

Answer 5

when the vibrations of a wave change from one plane to another

Question 6

what type of waves can be polarised?

Answer 6

transverse

Question 7

what does displacement mean?

Answer 7

the distance and direction of a vibrating particle from its equilibrium position

Question 8

what is the amplitude of a wave?

Answer 8

the maximum displacement of a vibrating particle

Question 9

what is the wavelength of a wave?

Answer 9

the least distance between two adjacent vibrating particles with the same displacement and velocity at the same time

Question 10

what is the cycle of a wave?

Answer 10

fro maximum displacement to the next maximum displacement

Question 11

what is the period of a wave?

Answer 11

the time for one complete wave to pass a fixed point

Question 12

what is the frequency of a wave?

Answer 12

the number of cycles of vibration of a particle per second, or the number of complete waves passing a point per second

Question 13

what equation links frequency and period?

Answer 13

T = 1/f

Question 14

what is the phase of a vibrating particle?

Answer 14

the fraction of a cycle it has completed since the start of the cycle

Question 15

what is phase difference?

Answer 15

the fraction of a cycle between the vibrations of the two particles vibrating at the same frequency

Question 16

what is the formula for phase difference?

Answer 16

phase difference (in radians) = 2 π d / λ (lambda)

Question 17

what are wavefronts?

Answer 17

lines of constant phase (lines which connect particles in phase)

Question 18

what is reflection?

Answer 18

when a wave hits a hard, flat surface the wave is reflected at the same angle

Question 19

what is the rule for reflection?

Answer 19

angle of incidence = angle of reflection

Question 20

what is refraction?

Answer 20

the bending of light due to a change in wave speed. this happens as the wavelength changes. this causes the wave to change direction

Question 21

when does refraction occur?

Answer 21

when the wave is aimed at the boundary not along the normal

Question 22

What is diffraction?

Answer 22

When waves spread out after passing through a gap or round an obstacle

Question 23

what increases diffraction?

Answer 23

• a narrower gap
• a longer wavelength

Question 24

how are satellite dishes designed to use reflection and diffraction?

Answer 24

larger dishes:
- reflect more radio waves to the aerial

Question 25

what is the downside of larger satellite dishes?

Answer 25

a bigger dish reflects waves into a smaller focus as they diffract less. this means they have to be aligned more carefully

Question 26

what is superposition?

Answer 26

when waves meet, pass through each other, combine and then move apart

Question 27

what does the principle of superposition state?

Answer 27

when two waves meet, the total displacement at a point is equal to the sum of the individual displacements at that point

Question 28

what is a supercrest or supertrough?

Answer 28

supercrest - when a crest meets a crest and the two waves reinforce each other
supertrough - when a trough meets a trough and the two waves reinforce each other

Question 29

what happens when a crest meets a trough of the same amplitude?

Answer 29

the resultant displacement is 0

Question 30

what are progressive waves?

Answer 30

a wave that transfers energy from one point to another without transferring the medium itself

Question 31

what are stationary waves?

Answer 31

when two progressive travel through each other and combine. they then form fixed points with no displacement called nodes

Question 32

what is an antinode?

Answer 32

the point on a stationary wave with maximum displacement

Question 33

what is interference?

Answer 33

when two waves interact and they have identical phase, frequency and amplitude

Question 34

what is constructive interference?

Answer 34

when two waves have displacement in the same direction (eg. two crests or two troughs)

Question 35

what is destructive interference?

Answer 35

when one wave has positive displacement and the other has negative displacement

Question 36

when does total destructive interference occur?

Answer 36

when the two waves have equal but opposite displacements

Question 37

what does coherent mean?

Answer 37

when the waves the same phase or constant phase difference

Question 38

what are stationary waves?

Answer 38

wave pattern with nodes and antinodes formed when two or more progressive waves of the same frequency and amplitude pass through each other.

Question 39

what are some properties of stationary waves?

Answer 39

• stationary waves that vibrate freely do not transfer energy to their surroundings
• distance between adjacent nodes = 1/2 wavelength
• the phase difference between two vibrating particles:
  ➢ zero if the two particles are between adjacent nodes or separated by an even number of nodes
  ➢ 180 degrees or 𝜋 radians if the particles are separated by an odd number of nodes
• all particles except those at nodes vibrate at the same frequency
• the amplitude varies from zero at the nodes to a maximum at the antinodes

Question 40

How are stationary waves formed on a string?

Answer 40

• A progressive wave is sent out by the vibrator.
• the crest reverses its phase when it is reflected at the fixed end of the string.
• it then travels back along the string as a trough
• It is reflected and reverses phase again once it reaches the vibrator
• this wave can be reinforced by crests from the vibrator. it can also deconstructively interfere with a trough that has been reflected

Question 41

what is the first harmonic pattern of vibration?

Answer 41

the pattern seen at the lowest possible frequency that gives a pattern.

Question 42

what is an osciloscope?

Answer 42

a specially made electron tube and associated control circuits.
(an electron gun ad glass tube emits an electron beam towards the fluorescent screen)

Question 43

what are oscilloscopes used for?

Answer 43

displaying the current/voltage of what it is connected to so it can be measured

Question 44

in an oscilloscope, what is the displacement of a spot proportional to?

Answer 44

the applied pd

Question 45

what are the X-plates connected to and what do they adjust?

Answer 45

the oscilloscope’s time base circuit
they make the spot move at a constant speed left to right across the screen

Question 46

what is connected to the Y-plates and what do they adjust?

Answer 46

the pd to be displayed
they make the spot move up and down as it moves left to right across the screen

Question 47

how do you work out the frequency of a wave from an oscilloscope?

Answer 47

• look at the time base
• count how many squares it takes for however many complete waves are displayed on the screen.
• multiply the number of squares by the time base
• divide this measurement by the number of waves measured
• use f = 1/T to find the frequency