Waves and Optics

Question 1

Define monochromatic (1)

Answer 1

Single wavelength

Question 2

Define coherent (1)

Answer 2

Constant phase difference

Question 3

State a safety precaution when using a laser (1)

Answer 3

Do not look into laser

Question 4

State and explain the effect of using a laser with a shorter wavelength on the maxima spacing in Young’s double slit (2)

Answer 4

Maxima closer together

use w=𝜆D/s

Question 5

Use wave theory to explain how the fringe pattern is formed in Youngs double slit experiment (4)

Answer 5

Slits act as coherent sources
Waves diffract at slits
Waves superpose - 
Bright patches: constructive - in phase 
Dark patches: destructive - 180 degrees out of phase

Question 6

State two requirements for two light sources to be coherent (2)

Answer 6

Same wavelength

Same phase

Question 7

Explain how Young’s double slit arrangement produces interference fringes, refer to width of first slit and coherence of second slits (6)

Answer 7

Narrow single slit gives wide diffraction to ensure that both second slits are illuminated
Paths to second slits are of constant length giving constant phase difference
Light diffracted at slits and overlap and interfere
Where path lengths differ by whole number of wavelengths, constructive interference occurs producing a bright fringe
Where path lengths differ by 0.5 x n wavelengths, destructive interference occurs producing a dark fringe

Question 8

If Young’s double slit was carried out with red light and then with white light, how would the two differ? (3)

Answer 8

Central fringe would be white
Dark fringes would be narrower together
Side fringes are spectra

Question 9

State two ways a diffraction pattern on single slit would change if the slit became narrower (2)

Answer 9

Increased separation - see single slit equation

Lower intensity - less light could get through

Question 10

State and explain what happens to angle θ in λ = d sin θ when wavelength decreases? (2)

Answer 10

Angle θ gets smaller

As path difference gets smaller

Question 11

Why will total internal reflection occur when light ray travels from water into glass? (refractive index) (1)

Answer 11

TIR only occurs when ray travels from higher n to lower n

as long as the incident angle is greater than the critical angle

Question 12

What is the name for the part of an optical fibre that is around the core? (1)

Answer 12

Cladding

Question 13

State and explain an advantage of a smaller diameter core (2)

Answer 13

Reduce multipath dispersion

Which would cause poor resolution

Question 14

State one application of optical fibres and its benefit to society (2)

Answer 14

Communications

Improved transmission of data

Question 15

Explain why optical fibres used for communication need to have cladding (2)

Answer 15

Keeps signal secure

Keeps most light rays in

Question 16

State and explain two physical properties of the light produced by a laser which makes it different to that produced by a lamp (4)

Answer 16

Monochromatic
Waves of single wavelength
Coherent
Waves produced in constant phase

Question 17

Explain how glass cladding around the optical fibre’s core improves the security of data being transmitted through it and give a reason (3)

Answer 17

Light doesn’t enter cladding so can’t pass across from one fibre to a neighbouring fibre
Fibres without cladding can allow light to pass between fibres when scratched or linked by moisture
Personal data must be transmitted along fibres where there is no danger of light leakage

Question 18

Define longitudinal wave (1)

Answer 18

A wave with a direction of vibration that is parallel to the direction of propagation of the wave

Question 19

Define transverse wave (1)

Answer 19

A wave with a direction of vibration that is perpendicular to the direction of propagation of the wave

Question 20

Explain why it is important to correctly align the aerial of a TV to receive the strongest signal (2)

Answer 20

Transmitted radio waves are often polarised

Aerial rods must be aligned in the same plane of the wave

Question 21

Define amplitude of a wave (1)

Answer 21

The maximum displacement of the wave from the equilibrium position

Question 22

If there are two polarising filters inbetween an observer and light, if one is turned about 360 degrees, what would the observer see? (2)

Answer 22

Variation in intensity between max and min

Two maxima in 360 degree rotation

Question 23

Give three examples of waves that are transverse (2)

Answer 23

Electromagnetic radiation
Surface of water
Rope

Question 24

State one application of a polarising filter and a reason for its use (2)

Answer 24

Camera/sunglasses

Reduce glare

Question 25

A microwave transmitter directs waves towards a metal plate. A detector is placed between and moved along a line, what causes the maxima and minima of waves detected? (3)

Answer 25

Superposition
reflection from metal plate
two waves of same frequency
travelling in opposite directions

Question 26

How can a guitarist raise the fundamental frequency of vibration in their string? (1)

Answer 26

Tighten the string
shorter length of string
Use a string of lower mass per unit length

Question 27

Describe the structure of a step-index optical fibre outlining the purpose of the core and cladding (3)

Answer 27

Core is transmission medium for electromagnetic waves to progress by total internal reflection
Cladding has a lower refractive index to allow total internal reflection to occur
Cladding offers protection from scratching that could lead to light loss

Question 28

Blue light has a higher refractive index fibre than red light, explain how this difference results in a change in a pulse of white light by the time it leaves the fibre (2)

Answer 28

Blue travels slower than red due to greater refractive index

Red reaches the end of the fibre before blue leading to material pulse broadening

Question 29

Discuss two changes to reduce material pulse broadening of white light(2)

Answer 29

Monochromatic source so speed is constant

Shorter distance between repeaters so pulse is reformed before significant broadening occurs

Question 30

Suggest an experiment to demonstrate the wave nature of sound (1)

Answer 30

Diffraction through a door

Question 31

Which quantities are changed when monochromatic light passes from air into glass? (1)

Answer 31

Speed and wavelength only, frequency remains constant

Question 32

Of the electromagnetic waves, which has the greatest energy? (1)

Answer 32

Gamma rays

Question 33

How do we know waves transfer energy?

Answer 33

Electromagnetic waves heats things up when they are absorbed
X-rays and gamma rays knock electrons out of their orbits (ionisation)
Loud sounds cause oscillations of air particles which make things vibrate.
Waves power can be used to generate electricity.

Question 34

Define wavelength

Answer 34

The distance from a point on the wave to the same point on the adjacent wave. It is measured in meters

Question 35

Define frequency

Answer 35

Is the number of waves that pass a point every second. It is measured in hertz (Hz)

Question 36

Define period

Answer 36

It is the time taken for each one whole wave to pass a fixed point. It is measured in seconds (s)

Question 37

Define phase difference

Answer 37

the amount by which one wave lags behind another wave.

Question 38

What units can phase difference be measured in?

Answer 38

Phase and phase difference can be measured in degrees, radians or fractions of a cycle.

Question 39

Describe an experiment to measure the speed of sound

Answer 39

Start with signal generator + microphones in phase.
Move microphone and measure the distance for 1 wavelength.
Measure the time delay using the picoscope and collect a range of distance and time measurements.
Draw a graph of distance against time.
Use the gradient to find the speed of sound.

Question 40

Define polarisation

Answer 40

Restriction of a wave to a single plane

Question 41

What waves can be polarised?

Answer 41

Transverse

Question 42

Define interference

Answer 42

Interference – When two waves arrive at the same point and at the same time, the resultant displacement is given by the algebraic sum of the two individual displacements.

Question 43

Define constructive interference

Answer 43

If two waves meet exactly in phase the amplitudes add up to produce large crests and troughs

Question 44

Define destructive interference

Answer 44

If two waves meet exactly out of phase the amplitudes cancel to produce no crests and troughs

Question 45

State the difference between a progressive wave and a standing wave.

Answer 45

Progressive (travelling wave) – transfer energy from one region to another

Standing (stationary waves) – Are fixed in space and do not travel. They are caused by the superposition of two identical waves.

Question 46

How are stationary waves formed?

Answer 46

Vibrations cause waves to travel along the string towards the pulley.
The waves reflect from the pulley and travel back to the vibration generator.
The two waves meet and constructively and destructively interfere.

Question 47

What are the conditions required for standing waves to be formed?

Answer 47

Same frequency and wavelength. Similar amplitude in the medium. Same speed

Question 48

Define a node

Answer 48

NO DisplacEment – Positive displacement from one wave is cancelled by an equal negative displacement from the other wave. (Destructive interference)

Question 49

Define an antinode

Answer 49

A point of maximum displacement (constructive interference)

Question 50

For a string that is fixed at both ends what are the rules for nodes and antinodes?

Answer 50

There must be a node at each end.

Question 51

State 3 ways to increase the fundamental frequency of a guitar string

Answer 51

Shorter string
Higher tension
Lower mass per unit length

Question 52

For a closed pipe what are the rules for nodes and antinodes

Answer 52

The closed end of the pipe is always a node, the open end of the pipe is always an antinode

Question 53

For an open pipe what are the rules for nodes and antinodes

Answer 53

There must be an antinode at each end

Question 54

Define diffraction

Answer 54

The spreading of waves on passing through a gap or near an edge

Question 55

When is diffraction noticeable?

Answer 55

When the wavelength is similar in size to the gap/obstacle

Question 56

Using the single slit equation state 2 ways to increase the angle of diffraction

Answer 56

Larger wavelength

Smaller gap size

Question 57

Define coherence

Answer 57

The spreading of waves on passing through a gap or near an edge

Question 58

Using the double slit equation state 3 ways to increase the width between the fringes

Answer 58

• Larger wavelength
• Increased distance to screen
• Decrease the slit separation

Question 59

In fibre optics cables why is absorption a problem?

Answer 59

Absorption is where some of the signal’s energy is absorbed by the material of the fibre. This energy loss results in the amplitude of the signal being reduced.

Question 60

In fibre optics cables why is dispersion a problem?

Answer 60

dispersion cause pulse broadening. The received pulse is broader than the initial signal. Broadened pulses can overlap each other leading to information loss.

Question 61

What is modal dispersion?

Answer 61

Is caused by light rays entering the cable at different angles. This causes them to take different paths and therefore take different amounts of time to travel.

Question 62

How can modal dispersion be reduced?

Answer 62

To reduce modal dispersion by using a single mode fibre in which light is only allowed to follow a very narrow path.

Question 63

What is material dispersion

Answer 63

Is caused by different amounts of refraction expienced by different wavelengths of light. This will cause some wavelengths to be slowed down more and take longer to travel down the cable.