Chapter 12 - Wave 2

Question 1

What happens when two waves superpose?

Answer 1

The resultant displacement can be greater or lesser than initial displacement

Question 2

What dictates the magnitude of the final displacement of the superposed wave?

Answer 2

The phase difference between them

Question 3

What happens to the magnitude of the final displacement if they are in phase?

Answer 3

There will be maximum positive displacement
Constructive interference (Even multiples of pi)

Question 4

What happens to the displacement if the waves are antiphase?

Answer 4

The waves cancel out
Destructive Interference (Odd multiples of pi)

Question 5

What is interference?

Answer 5

Interference is the pattern that is observed when superposition takes place

Question 6

Practically, when can interference effect be observed?

Answer 6

In practice, interference effects are only observed when two coherent wave sources superpose

Question 7

What are coherent wave sources?

Answer 7

Coherent wave sources are those that maintain a constant phase difference, at a point in space, with respect to time

Question 8

What properties must a coherent wave source have?

Answer 8

Constant phase difference
Same Frequency

Question 9

Why are sound wave interference patterns easily observed?

Answer 9

Large wavelengths
Slow wave speed

Question 10

When are interference patterns of light observed?

Answer 10

When two coherent sources of light waves superpose

Question 11

What is the path length difference?

Answer 11

The path length difference between two waves is the difference in length in the paths travelled by each wave

Question 12

What must PLD and phase difference be for coherent wave sources?

Answer 12

PLD must be a fraction of the wavelength of the waves
PLD must directly correlate to the phase difference between two waves

Question 13

What must PLD and phase difference be for constructive interference?

Answer 13

PLD of even multiple of wavelength///2
Phase difference of multiple of 2pi
This applies to young’s double slit experiment

Question 14

What must PLD and phase difference be for destructive interference?

Answer 14

PLD of odd multiple of wavelength///2
Phase difference of odd multiple of pi
This applies to young’s double slit experiment

Question 15

What is coherency?

Answer 15

Coherency is the potential for two waves to interact with them having a constant phase difference and same frequency

Question 16

What is the set up for the young double split experiment?

Answer 16

Lamp
Colour Filter
Narrow Single Slit
Double Slits
Screen
Interference Fringe

Question 17

Why is light put through the colour filter?

Answer 17

Colour filter produces a monochromatic source of light which limits the light to the same frequencies

Question 18

Why is a single slit used?

Answer 18

A single slit diffracts to illuminate the double slit. The narrow slit ensures the light is of constant phase difference

Question 19

Why are double slits used?

Answer 19

The double slit produces two sources of coherent waves

Question 20

What happens after the double slits?

Answer 20

The light from each slit moves forward, they overlap and superpose, producing dark and bright fringes

Question 21

What are dark and bright spots produced by?

Answer 21

Light waves in phase make bright fringes as they constructively interfere
Antiphase light waves make dark fringes they destructively interfere

Question 22

What are the variables in this practical?

Answer 22

Distance between slits and screens (D)
Slit separation (a)
Wavelength of incident light (y)
Fringe separation (x)

Question 23

What is the formula for the relationships between the young double slit experiment?

Answer 23

y = ax / d

Wavelength = (Slit separation x Fringe separation) /// Distance between slits and screen

Question 24

In what situation does this formula work?

Answer 24

The formula only works when slit separation is significantly smaller than distance between slits and screen

Question 25

What is a stationary wave?

Answer 25

A stationary wave is a wave that oscillates in time, but does not transfer any net energy

Question 26

How is a stationary wave formed?

Answer 26

A stationary wave is formed when two progressive waves of the same frequency, propagate in opposite directs, and superpose

Question 27

What is a node?

Answer 27

A node is a point of zero displacement in a stationary wave

Question 28

When does a node occur?

Answer 28

It is formed when the waves are in antiphase and destructively interfere

Question 29

What is an antinode?

Answer 29

An antinode is a point of maximum displacement in a stationary wave

Question 30

When does an antinode occur?

Answer 30

It is formed when the waves are in phase and constructively interfere

Question 31

What is the distance between each node/antinode?

Answer 31

The distance between each node/antinode is y/2 metres

Question 32

What is the phase difference between each node/antinode?

Answer 32

Pi radians

Question 33

What happens to points between adjacent nodes?

Answer 33

All points between adjacent nodes oscillate in phase (when one hits max displacement, so do others in their cycle)

Question 34

What is the difference in energy transfer in progressive and stationary waves?

Answer 34

In progressive waves, energy is transferred in the direction of the wave Stationary waves have no net energy transfer

Question 35

What is the difference in wavelength between progressive and stationary waves?

Answer 35

For progressive waves, wavelength is minimum distance for between adjacent points in phase For stationary waves, wavelength is twice distance between adjacent nodes

Question 36

What is the difference in phase difference between progressive and stationary waves?

Answer 36

For progressive waves, phase changes across one cycle of wave For stationary waves, all points between two nodes are in phase, different sides of node are anti phase

Question 37

What is the difference in amplitude between progressive and stationary waves?

Answer 37

For progressive waves, all parts of the wave have same amplitude For stationary wave, maximum amplitude at antinode and 0 at node

Question 38

What is the difference between nodes closer to the barrier and further away?

Answer 38

Nodes furthest away from the barrier are composed of waves with less energy, and so they are more wobbly as the waves have a lower amplitude and cannot completely destructively interfere with

Question 39

What is the fundamental frequency?

Answer 39

The lowest frequency sound that can be produced on a string of length, mass, and tension

Question 40

What are the determinants of a strings harmonics?

Answer 40

Length Mass Tension

Question 41

What is the relationship between each harmonic?

Answer 41

Every harmonic is a multiple of the first harmonic (f₀)

Question 42

Where are the nodes in a pipe closed at one end?

Answer 42

The pipe is open at one end (antinode), closed at the other end (node)

Question 43

Where are the nodes in a pipe open at both ends?

Answer 43

The pipe is open at both ends so they must have an antinode at each end

Question 44

What is the fundamental frequency dictated by for a pipe?

Answer 44

For a pipe of given length, the density and temperature dictate the fundamental frequency as they dictate the speed of the wave

Question 45

What frequencies occur on both pipes?

Answer 45

Odd harmonics of the fundamental frequency occur on the one ended pipe. All harmonics of the fundamental frequency occur on the open pipe

Question 46

What is the difference between nodes and maxima?

Answer 46

Nodes and antinodes only appear in standing waves Points of interference are called minima or maxima

Question 47

Why do maxima and minima occur?

Answer 47

Two waves have travelled different distances from their sources because of the path length difference

Question 48

What is superposition?

Answer 48

Superposition is when two waves overlap and produce a singular wave

Question 49

Why does the fringe pattern become less obvious further from the centre?

Answer 49

The intensity of light further from the centre decreases as the path length difference increases leading to more destructive interference

Question 50

How do the properties of a harmonic change on a string?

Answer 50

The frequencies are multiples of first harmonic The wavelengths of string follow trend: 2L / n Therefore wavelength compared to the fundamental is 1/n

Question 51

How do the properties of a harmonic change in an open ended air column?

Answer 51

The frequencies are multiples of first harmonic The length of column starts at (1/2)y and increases by (1/2)y

Question 52

How do the properties of a harmonic change in an air column with one open end?

Answer 52

Frequencies are odd multiples of first harmonic Length of column starts at (1/4)y and increases by (1/2)y