Waves

Question 1

Explain how a stationary wave is produced when a stretched string is plucked?

[3 mark]

Answer 1

Waves travel to the ends of the string and are reflected at the fixed points.
The waves travel in opposite directions and as they encounter each other they interfere meaning their displacements sum at each point.
Destructive interference occurs at the nodes - the fixed ends of the string.
For the first harmonic, constructive interference occurs in the middle this is where the amplitude is at a maximum.
A stationary wave is formed!

Question 2

Define wavelength

Answer 2

Wavelength is the distance between adjacent points on a wave

Question 3

Define amplitude

Answer 3

the maximum displacement of a particle in the wave from its equilibrium position

Question 4

Define wave displacement

Answer 4

the distance of a point on the wave from its equilibrium position

Question 5

Define period

Answer 5

time period is the time taken for one complete oscillation or cycle of the wave

Question 6

Define frequency

Answer 6

the number of complete oscillations per second.

Question 7

Define phase difference

Answer 7

The phase difference between two waves is a measure of how much a point or a wave is in front or behind another.

Question 8

What is a mechanical wave

Answer 8

A mechanical wave is a wave that is not capable of transmitting its energy through a vacuum. Mechanical waves require a medium in order to transport their energy from one location to another.

Question 9

What are the two types of mechanical waves

Answer 9

Transverse and longitudinal waves

Question 10

What are transverse waves?

Answer 10

A wave in which the particles oscillate perpendicular to the direction of the wave travel (and energy transfer)

Question 11

What are longitudinal waves?

Answer 11

A wave in which the particles oscillate parallel to the direction of the wave travel (and energy transfer)

Question 12

Longitudinal waves show areas of __________ and ________

Answer 12

compressions and rarefactions

Question 13

What are some examples of longitudinal waves?

Answer 13

• sound waves
• ultrasound waves
• seismic (P) waves

Question 14

What are some examples of transverse waves?

Answer 14

• electromagnetic waves – e.g light waves, microwaves, radio waves
• ripples on the surface of water
• seismic (S) waves

Question 15

How do you find the wavelength on a longitudinal wave

Answer 15

A wavelength on a longitudinal wave is the distance between two compressions or two rarefactions

Question 16

Which waves out of transverse and longitudinal waves can be polarised

Answer 16

Transverse waves can be polarised.
Longitudinal waves cannot be polarised.

Question 17

What is polarisation

Answer 17

Polarisation is the production of waves oscillating in one plane from a source of randomly oscillating waves.

Question 18

Define standing waves (stationary waves)

Answer 18

Standing waves are produced by the superposition of two waves of the same frequency and amplitude travelling in opposite directions

Question 19

What are the differences between standing waves and progressive waves

Answer 19

• progressive waves don’t have nodes and antinodes whereas standing waves do
• In a progressive wave all points have the same maximum amplitude whereas in a standing wave each point has a different amplitude
• energy is transferred along a progressive wave but stored in a standing wave

Question 20

What is the principle of superposition

Answer 20

The principle of superposition states:
When two or more waves with the same frequency arrive at a point, the resultant displacement is the sum of the displacements of each wave

Question 21

What does it mean if waves are said to be coherent?

Answer 21

Waves are said to be coherent if they have the same frequency and a constant phase difference

Question 22

Define path difference

Answer 22

Path difference is defined as the difference in distance travelled by two waves from their sources to the point where they meet

Question 23

What is meant by a monochromatic light source?

Answer 23

a light source that has one single wavelength

Question 24

What are the independent variable and dependent variable

Answer 24

Independent variable - the variable you change

Dependent variable - the variable you measure

Question 25

What is diffraction?

Answer 25

Diffraction is the spreading out of waves when they pass an obstruction

Question 26

In what ways can diffraction be increased?

Answer 26

• decreasing the slit separation
• increasing wavelength

Question 27

What would happen if the slit was made narrower?

Answer 27

If the slit was made narrower:
Diffraction would increase
Light intensity would decrease
The fringe spacing would be wider

Question 28

When light travels from a less dense to more dense material how does the light respond

Answer 28

from less dense to more dense e.g from air to glass light bends towards the normal

Question 29

Define refractive index

Answer 29

The refractive index is a property of a material which measures how much light slows down when passing through it

Question 30

What is an optically dense material?

Answer 30

A material with a high refractive index is called optically dense, such material causes light to travel slower

Question 31

What are the three main components that make up optical fibres?

Answer 31

• An optically dense core, such as plastic or glass
• A lower optical density cladding surrounding the core
• An outer sheath

Question 32

Why does total internal reflection occur in an optical fibre?

Answer 32

total internal reflection happens because the refractive index of the core is more than the refractive index of the cladding.

n cladding < n core

Question 33

What is the purpose of the outer sheaf in an optical fibre

Answer 33

The outer sheath:
- Prevents physical damage to the fibre
- Strengthens the fibre
- Protects the fibre from the outside from scratches

Question 34

What is the purpose of the cladding in an optical fibre

Answer 34

The cladding is required because:
- It protects the core from damage
- It prevents light escaping the core, which can cause information from the signal to be lost
- It keeps signals secure and maintains the quality of the original signal
- It keeps the core away from adjacent fibre cores hence preventing crossover of information to other fibres
- It provides the fibre with strength and prevents breakage

Question 35

Define material dispersion

Answer 35

Material dispersion occurs when white light is used instead of monochromatic light.

This is because different wavelengths of light travel at different speeds.

Question 36

Define modal dispersion

Answer 36

Modal dispersion occurs when the light pulses in the optical fibre spread out due to the different angles of incidence in the original pulse

To prevent modal dispersion, the core needs to be very narrow

Question 37

What are the advantages of using a narrower core in an optical fibre?

Answer 37

• Less light is lost by refraction out of the core
• The signal will be transferred quicker leading to improved data and information transfer
• There is a smaller change in angle between each reflection, so the angle of incidence is less likely to fall below the critical angle
• Less overlapping pulses hence reduction of modal dispersion