Waves

Question 1

Define a Transverse wave

Answer 1

A wave in which the direction of vibration is perpendicular to the direction in which the wave travels.

Question 2

State the condition required for a transverse wave to be polarised

Answer 2

It’s vibrations must stay in one plane only

Question 3

Explain how longitudinal waves become polarised

Answer 3

They can’t

Question 4

Define the plane of polarisation of an electromagnetic wave

Answer 4

The plane in which the electric field oscillates

Question 5

Define a longitudinal wave

Answer 5

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

Question 6

Define the displacement of a wave

Answer 6

A particles distance and direction from its equilibrium position

Question 7

State an equation to calculate speed (c) given f

Answer 7

C = fλ

Question 8

Define the phase difference of 2 vibrating particles

Answer 8

The fraction of a cycle between the vibrations of the two particles
Where 1 cycle = 360° = 2π

∴ phase difference (in radians) = 2πd / λ
(Where d = distance along the wave)

Question 9

State the principle of superposition

Answer 9

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

Question 10

What is the distance between adjacent nodes in a stationary wave?

Answer 10

λ/2

Question 11

State the 3 conditions required for the formation of a standing wave

Answer 11

1) 2 waves travelling towards each other
2) Same frequencies
3) Superpose

Question 12

On a standing wave, what are the names given to:

i) points of no amplitude
ii) points of maximum amplitude

Answer 12

i) nodes

ii) antinodes

Question 13

State a difference between energy transfer of a progressive wave and a stationary wave

Answer 13

Stationary waves do not transfer energy

Question 14

Describe the phase of all the particles between adjacent nodes

Answer 14

They are all in phase

Question 15

Describe the phase of the particles either side of a node

Answer 15

They are out of phase

Question 16

Define electromagnetic waves

Answer 16

Vibrating electric and magnetic fields that progress through space without need for a substance
The vibrating electric field creates a vibrating magnetic field, which generates a vibrating electric field further away, and so on

Question 17

State the types of electromagnetic waves in order of lowest to highest frequency

Answer 17

• Radiowaves
• Microwaves
• Infra Red
• Visible Light
• Ultra Violet
• X-Ray
• Gamma Ray

Question 18

Define amplitude

Answer 18

The maximum displacement of a vibrating particle

For a transverse wave it is the height of a wave crest or the depth of a wave trough from the middle

Question 19

Define wavelength

Answer 19

The least distance between two adjacent vibrating particles with the same displacement and velocity at the same time (e.g. distance between adjacent crests)

Question 20

Define complete cycle

Answer 20

From the maximum displacement to the next maximum displacement (e.g. from one wave peak to the next)

Question 21

Define wave period (Time period)

Answer 21

The time for one complete wave to pass a fixed point

Question 22

Define frequency and give its units

Answer 22

The number of cycles of vibration of a particle per second, or the number of complete waves passing a point per second
Units: Herts (Hz)

Question 23

Give the equation for the time period of a wave

Answer 23

T = 1 / f

Question 24

State and describe the set up of a piece of apparatus to view the interference of light by waves

Answer 24

A ripple tank
An electronic ‘dipper’ is set up just above a shallow, transparent tank of water with sloping sides. A stroboscope is used in accordance with a lamp so that the wavefronts are projected onto a surface below

Question 25

State the significance of the sloped sides of a ripple tank

Answer 25

It prevents the waves from reflecting off the sides of the tank, which would make it more difficult to see the wavefronts projected.

Question 26

Describe the reflection of a straight wave on a hard flat surface

Answer 26

Straight waves directed at a certain angle to a hard flat surface reflect off at the same angle. Therefore, the direction of the reflected wave is at the same angle to the reflector as the direction of the incident wave

Question 27

Define refraction

Answer 27

When waves pass across a boundary at which the wave speed changes, the wavelength also changes If the wavefronts are at a non-zero angle to the boundary, they change direction as well as speed

Question 28

State when the refraction of light is observed

Answer 28

Refraction of light is observed when a light ray is directed into a glass block non-normally The light ray changes direction when it crosses the glass boundary because light waves travel more slowly in glass than in air

Question 29

Define diffraction

Answer 29

Diffraction occurs when waves spread out after passing through a gap or round an obstacle

Question 30

State when diffraction is observed in water

Answer 30

When straight waves in a ripple tank are directed at a gap

Question 31

What 2 factors affect the amount of diffraction for waves passing through a gap

Answer 31

- The narrower the gap, the more the waves spread out | - The longer the wavelength. the more the waves spread out

Question 32

In which direction do satellite dishes in Europe need to point and why?

Answer 32

South, because the satellites orbit the Earth directly above the equator

Question 33

Explain the significance of the size of a satellite dish on the signal it receives

Answer 33

The larger the satellite dish, the stronger the signal it can receive, because more radiowaves are reflected by the dish onto the aerial.

Question 34

Define superposition

Answer 34

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

Question 35

Where waves interact, what happens when: i) A crest meets a crest? ii) A trough meets a trough? iii) A crest meets a trough?

Answer 35

i) A supercrest is created ii) A supertrough is created iii) The resultant displacement is 0; the two waves cancel each other out

Question 36

Give 2 examples of superposition

Answer 36

1) Stationary waves on a rope | 2) Water waves in a ripple tank

Question 37

What must the 2 sources of waves be to produce a interference between the 2 waves and why is this effect shown?

Answer 37

Coherent An interference pattern where they overlap is produced, because they vibrate at the same frequency with a constant phase difference

Question 38

Describe an experiment to demonstrate the reflection, refraction, interference and polarisation of microwaves

Answer 38

The transmitter produces microwaves of wavelength 3.0cm. The reciever can be connected to a suitable meter, which gives a measure of the intensity of microwaves at the reciever 1) Place the receiver in the path of the microwave beam from the transmitter. As you move it further away from the transmitter the signal decreases, showing that microwaves become weaker at further distances from the transmitter 2) Place a metal plate between the transmitter and receiver to show that microwaves cannot pass through metal 3) Use 2 metal plates to make a narrow slit and show that the receiver detects microwaves that have been diffracted as they pass through the slit. Show that if the slit is wider, less diffraction occurs 4) Use a narrow metal plate with 2 plates either side to make a pair of slits. Direct the transmitter at the slits and use the receiver to find points of cancellation and reinforcement, where the microwaves from the 2 slits overlap

Question 39

Describe how stationary waves are formed

Answer 39

A stationary wave is formed when 2 progressive waves pass through each other. This can be achieved on a string in tension by fixing both ends and making the middle part vibrate, so progressive waves travel towards each end, reflect at the ends and then pass through each other

Question 40

Define a node

Answer 40

A fixed point of no displacement

Question 41

State the distance between adjacent nodes

Answer 41

Distance between adjacent nodes = ½λ

Question 42

Define antinode

Answer 42

The point midway between 2 adjacent nodes which has maximum amplitude

Question 43

Define fundamental mode of vibration

Answer 43

The simplest stationary wave pattern on a string, consisting of a single loop that has a node at either end

Question 44

Describe the energy transfer for stationary waves vibrating freely

Answer 44

They do not transfer energy because the nodes and antinodes are at fixed positions

Question 45

Describe stationary waves in terms of the progressive waves that cause them

Answer 45

Considering the 2 progressive waves passing through each other: - When they are in phase, they reinforce each other to produce a large wave - A quarter of a cycle later, the 2 waves have each moved one-quarter of a wavelength in opposite directions, thus are now in antiphase, so cancel each other out - After a further quarter cycle, the 2 waves are back in phase. The resultant is again a large wave except reversed

Question 46

For 2 vibrating particles in a stationary wave, describe: i) The amplitude ii) The phase difference

Answer 46

i) The amplitude of a vibrating particle varies with position from 0 (at a node) to maximum (at an antinode) ii) The phase difference between 2 vibrating particles is: - 0 if the two particles are between adjacent nodes or separated by an even number of nodes - 180° (= λ radians) if the 2 particles are separated by an odd number of nodes

Question 47

Give the similarities between the frequency of stationary and progressive waves

Answer 47

Stationary: All particles except at the nodes vibrate at the same frequency Progressive: All particle vibrate at the same frequency

Question 48

Give the differences between the amplitude of particles of stationary and progressive waves

Answer 48

Stationary: The amplitude varies from 0 (at the nodes) to a maximum (at the antinodes) Progressive: All particles have the sane amplitude

Question 49

Describe the phase difference between 2 particles for stationary and progressive waves

Answer 49

Stationary: Equal to mπ, where m is the number of nodes between the two particles Progressive: Equal to 2πd/λ, where d is the distance apart and λ is the wavelength

Question 50

State the resonant frequency for the standing waves in an air-filled pipe closed at one end

Answer 50

In a pipe closed at one end, the resonant frequencies occur when there is an antinode at the open end and a node at the other end

Question 51

Describe how standing waves can be detected using microwaves

Answer 51

A microwave transmitter is directed at a metal plate which reflects them back towards the transmitter. When a detector is moved along the line between the transmitter and the metal plate, the detector signal is found to be 0 at equally spaced positions (half wavelengths apart) because the reflected waves interfere with the waves from the transmitter and form a stationary wave pattern

Question 52

Define fundamental pattern of vibration

Answer 52

The lowest possible frequency produced by a vibrating system. Has an antinode at the middle as well as a node at either end. Because the length L of the vibrating section of the string is between adjacent nodes and the distance between adjacent nodes is ½λ - the wavelength of the waves that form this pattern, the fundamental wavelength is: λ₀ = 2L

Question 53

Give the equation for the fundamental frequency from the fundamental wavelength

Answer 53

λ₀ = 2L f₀ = c / λ₀ = c / 2L where c is the speed of the progressive waves on the wire

Question 54

Define first overtone

Answer 54

The next stationary wave pattern from the fundamental pattern of vibration, where there is a node at the middle, so the string is in 2 loops Therefore the waves that form this pattern: λ₁ = L Because each loop has a length of half-a-wavelength

Question 55

Give the equation for the frequency of the first overtone, and its relation to the fundamental frequency

Answer 55

``` λ₁ = L f₁ = c / λ₁ = c / L = 2f₀ ```

Question 56

Define second overtone

Answer 56

The next stationary wave pattern from the fundamental pattern of vibration, where there is an antinode in the middle and the nodes are distance ⅓L from each other The wavelength of waves that form this pattern are: λ₂ = ⅔L

Question 57

State the condition for the formation of a stationary wave in terms of the time take for a wave to travel along the sting and back

Answer 57

The time taken for a wave to travel along the string and back must be equal to the time taken for a whole number of cycles of the vibrator f = mc /2L = mf₀ where m is a whole number

Question 58

For a string, how would you: i) raise the pitch? ii) lower the pitch?

Answer 58

i) Increasing the tension or shortening the length | ii) Decreasing the tension or increasing the length