Section 3 - Waves

Question 1

What is the definition of frequency and what are the units?

Answer 1

The number of waves passing through a point per second (hertz) Hz

Question 2

What is the definition of wavelength?

Answer 2

The distance between 2 adjacent points on a wave

Question 3

What is the definition of amplitude?

Answer 3

The maximum displacement of the wave from its equilibrium position

Question 4

How can you find out the time period of a wave using its frequency?

Answer 4

T = 1/f

Question 5

What is phase difference and what is it measured in?

Answer 5

• How much a particle/wave lags behind another particle/wave
• Measured in radians, degrees or fractions of a cycle

Question 6

What is a longitudinal wave?
Give an example

Answer 6

• A wave in which the oscillation of the particles is parallel to the direction of energy transfer
• There are rarefractions (areas of low pressure) and compressions (areas of high pressure)
  Sound waves

Question 7

What is a transverse wave? Give 2 examples

Answer 7

• Waves where the particle oscillations are perpendicular to the direction of energy transfer
  Electromagnetic waves and waves on a string

Question 8

How fast do electromagentic waves travel in a vacuum?

Answer 8

All travel at the speed of light - same speed

Question 9

What is the relationship between the magnetic field and electric field?

Answer 9

Perpendicular

Question 10

What does a polarising filter do?

Answer 10

Only allows oscillations in one plane

Question 11

How is polarisation used as evidence of the nature of transverse waves?

Answer 11

Polarisation can only occur if a wave’s oscillations are perpendicular to its direction of travel (in transverse waves)

Question 12

How is polarisation used in antennas?

Answer 12

• TV and radio signals are usually plane-polarised by the orientation of the rods on the transmitting aerial
• The receiving aerial must be aligned in the same plane of polarisation to receive the signal at full strength

Question 13

What is a stationary wave?

Answer 13

A wave which transfers no energy and whose positions of maximum and minimum amplitude are constant

Question 14

What is a node?

Answer 14

A point on a stationary wave where the displacement is 0

Question 15

What is an antinode?

Answer 15

A point on a stationary wave with maximum displacement

Question 16

How are stationary waves produced?

Answer 16

A stationary wave is formed from the superposition of 2 progressive waves, travelling in opposite directions in the same plane, with the same frequency, wavelength and amplitude - often a wave reflected back on itself
* Where the waves meet in phase, constructive interference occurs so antinodes form
* Where the waves meet completely out of phase, destructive interfernece occurs and nodes form

Question 17

What does the first harmonic for a stationary wave with 2 closed end look like?

Answer 17

2 nodes at either end and an antinode in the middle

Question 18

What is the definition of coherence?

Answer 18

Coherent waves have a fixed phase difference and the same frequency and wavelength

Question 19

Why is a laser useful in showing interference and diffraction?

Answer 19

It produces monochromatic (same wavelength/colour) light so diffraction and interference patterns are more defined

Question 20

What was Young’s double-slit experiment?

Answer 20

• A single light source is directed towards 2 slits - which each act as a coherent light source
• The light interferes and superimposes constructively and destructively to create an interference pattern

Question 21

Describe the interference pattern created using white light

Answer 21

A bright white central maxima flanked by alternating spectral fringes of decreasing intensity with violet closest to the zero order and red furthest
The maxima is wider than the other maxima

Question 22

Why does an interfernce pattern form when light is passed through a single slit?

Answer 22

• Light diffracts as it passes through the slit
• Where the waves are in phase constructive interference occurs making bright fringes
• Where the waves are completely out of phase destructive interference occurs making a dark fringe

Question 23

How does increasing the slit width change the width of the central maximum?

Answer 23

The slit isn’t as close as it was to the wavelength in size so less diffraction occurs - the central maximum becomes narrower and more intense

Question 24

What does a single slit pattern on a graph look like?

Answer 24

Central peak and then tiny tiny peaks either side

Question 25

What is the approximate refractive index of air?

Answer 25

1

Question 26

When light enters a more optically dense medium does it bend towards or away from the normal?

Answer 26

Towards the normal

Question 27

When does total internal reflection occur?

Answer 27

When light is at a boundary to a less optically dense medium and the angle of incidence is greater than the critical angle

Question 28

What is the purpose of the cladding in a step index optical fibre?

Answer 28

* Protects core from scratches which would alow light to escape and degrade the signal * Allows TIR as it has a lower refractive index than the core

Question 29

How does signal degradation by absorption in an optical fibre affect the received signal?

Answer 29

Part of the signal's energy is absorbed by the fibre so its amplitude is reduced

Question 30

What is pulse broadening?

Answer 30

When the received signal is wider than the original, this can cause overlap of signals leading to information loss

Question 31

How does modal dispersion cause pulse broadening?

Answer 31

* Light rays enter the fibre at different angles so they take different paths along it * Some may travel down the middle while others are reflected repeatedly * The rays take different times to travel along the fibre, causing pulse broadening

Question 32

What is material dispersion?

Answer 32

When light with different wavelengths is used some wavelengths slow down more than others in the fibre so they arrive at different times, causing pulse broadening

Question 33

How can modal dispersion be reduced?

Answer 33

Use a single mode fibre (very narrow fibre) so the possible difference in path lengths is smaller

Question 34

How can material dispersion be reduced?

Answer 34

Use monochromatic light

Question 35

How can both absorption and dispersion be reduced?

Answer 35

Use an optical fibre repeater to regenerate the signal

Question 36

State the advantages of optical fibres over traditional copper wires

Answer 36

* Signal can carry more information as light has a high frequency * No energy lost as heat * No electrical interference * Cheaper * Very fast

Question 37

What path does a light ray take when the angle of incidence is equal to the critical angle?

Answer 37

It goes along the boundary ie the angle of refraction is 90°

Question 38

What is snells law?

Answer 38

n1sini = n2sinr

Question 39

What are 2 applications of diffraction gratings?

Answer 39

* Splitting up light from stars to make line absorption spectra - used to identify elements present in the star * X-ray crystallography to find the spacing between atoms - a crystal sheet acts as the diffraction grating the X-rays pass through

Question 40

Derive the formula dsinθ = nλ

Answer 40

1. Draw this https://commons.wikimedia.org/wiki/File:Youngs_slits.png 2. For first order maximum, the path difference between 2 adjacent rays of light is 1λ (as shown), the angle between the normal to the grating and the light ray is θ (the lower angle is 90-θ) 3. For the first oder maximum sinθ=λ/d (sinθ = opp/hyp) rearrange to dsin = λ 4. Other maxima occur when the path difference between the 2 rays of light is nλ, where n is an integer, replace λ with nλ to get dsin = nλ

Question 41

When light passing through a diffraction grating is changed from blue to red, do the orders get closer together?

Answer 41

The wavelength of light has increased so it will diffract more, the orders will become further apart

Question 42

What is diffraction?

Answer 42

The spreading out of waves when they pass through or around a gap

Question 43

How did Young's double slit experiment provide evidence for the wave nature of light?

Answer 43

Diffraction and interference are wave properties hence the interference pattern of light shows light has wave properties

Question 44

What are 4 safety precautions that must be followed when using a laser?

Answer 44

* Wear laser safety goggles * Don't shine the laser at reflective surfaces * Display a warning sign * Never shine the laser at a person

Question 45

How could you investigate stationary sound waves?

Answer 45

* Place a speaker at one end of a closed glass tube * Lay powder across the bottom of the tube * It will be shaken from the antinodes and settle at the nodes * The distance between each node is half a wavelength

Question 46

What is phase?

Answer 46

The position of a certain point on a wave cycle, (units are radians, degrees or fractions of a cycle)

Question 47

When a wave travels from air into a medium how do you ensure the light doesn't deviate?

Answer 47

Enters the medium at the normal

Question 48

When a wave travels between 2 mediums, how do you ensure there is no deviation?

Answer 48

* The 2 mediums have the same refractive index * The 2 mediums are fitted together without any gaps

Question 49

When a question says discuss what shoud you do?

Answer 49

* Describe what could happen * Comment on how likely that is to happen and why

Question 50

What is the combination of split spacing and wavelength that will give the greatest possible number of interference maxima?

Answer 50

* The biggest split spacing * Smallest wavelength of light emitted * By sinθ = nλ/d

Question 51

How do you calculate absolute percentage uncertainty when it's +/-?

Answer 51

1. Multiply value by 2 2. Divide by reading, 3. Times by one hundred

Question 52

What is a key property of microwaves?

Answer 52

They are polarised

Question 53

How can we demonstrate using an aerial and receiver that microwaves are vertically polarised?

Answer 53

* Rotate aerial in vertical plane * When the aerial is horizontal the signal is a minimum * When the aerial is vertical the signal is a maximum * Maximum occurs when the aerial is aligned with the plane of polarisation of the waves

Question 54

What happens when there is a direct signal, and a deflected signal that meet at a transmitter, but the thing the defelcted signal is deflected on moves down?

Answer 54

1. Recieved signal goes through a series of maximum and minimum 2. Refelcted and deflected waves interfere 3. Path difference increases as plate moves 4. Phase difference between deflected and direct waves changes

Question 55

When is Young's double slit equation valid?

Answer 55

If the slit distance is a lot greater than slit separation

Question 56

What happens to the ray within the optical fibre when it is bent?

Answer 56

1. Angle of incidence is decreased 2. Will be less than the critical angle 3. Some light will refract out

Question 57

What is a disadvantage of using a high order to determine N or d?

Answer 57

The maximum is wider so less defined