Section 3: Waves and Optics

Question 1

How can both absorption’s and dispersion’s signal degradation be reduced?

Answer 1

Optical fibre repeaters.

Question 2

What are maxima?

Answer 2

At any points an equal distance from two sources in phase you will get constructive interference, these points are known as maxima. This also occurs at any point where the path difference is a whole number of wavelengths.

Question 3

What is the second harmonic? Describe its attributes.

Answer 3

A stationary wave vibrating at 2x its lowest possible resonant frequency. It has two “loops” with a node at each end and one in the middle. Two half wavelength fits onto the string, and so the wave length is the length of the string.

Question 4

Define Displacement

Answer 4

How far a point on the wave has moved from its undisturbed position.

Question 5

Define Diffraction

Answer 5

The wave spreads out as it passes through a gap or round an obstacle

Question 6

What is a polarised wave?

Answer 6

A transverse wave that oscillates in one direction only.

Question 7

What is a source of monochromatic light?

Answer 7

A laser.

Question 8

How can a stationary sound wave be produced?

Answer 8

Powder in a tube of air can show standing waves. A loud speaker produces a sound wave which is reflected at the end, created a stationary wave. The powder laid along the bottom of the tube is shaken away from the antinodes, but left undisturbed at the nodes.

Question 9

How does a step index optical fibre work?

Answer 9

Light is shone at one end of the fibre. The fibre is so narrow that the light always hits the boundary between the fibre and cladding at an angle greater than the critical angle. So all the light is TIR from boundary to boundary until it reaches the end.

Question 10

How does increasing slit width affect the central fringe?

Answer 10

This decreases the amount of diffraction. This means the central maximum is narrower, and the intensity is higher.

Question 11

What are nodes and antinodes?

Answer 11

Nodes are points on a standing wave where the amplitude of the waves vibration is 0, Antinodes are points of maximum amplitude.

Question 12

What is absorption? What does it result in?

Answer 12

Some of the signal’s energy is absorbed by the material the fibre is made from. This energy loss results in the amplitude of the signal being reduced.

Question 13

Describe the two-source interference pattern produced by white light.

Answer 13

Central white fringe, followed by patterns of dark fringes and spectra of colours, containing central white fringes, with red on the outside and violet light on the inside. This is because light is made up of different frequencies and wavelengths which all diffract by different amounts.

Question 14

Give examples of polarisation in the real word.

Answer 14

• Glare Reduction.
• The alignment of aerials for transmission and reception of radio signals.

Question 15

How does superposition work?

Answer 15

Superposition happens when two or more waves pass through each other. At the instant the waves cross, the displacements due to each wave combine.

Question 16

What is a progressive wave?

Answer 16

A moving wave carries energy from one place to another without transferring any material.

Question 17

Define path difference.

Answer 17

The amount by which the path travelled by one wave is longer than the path travelled by the other wave.

Question 18

Define Amplitude

Answer 18

The maximum magnitude of displacement, the distance from the undisturbed position to the crest, or trough.

Question 19

What are minima?

Answer 19

At points where the path difference is half a wavelength, one and a half wavelengths, two and a half wavelengths, etc.,the waves arrive out of phase and destructively interfere. These points are known as minima.

Question 20

How does increasing the wavelength affect the central maximum?

Answer 20

This increases the amount of diffraction. This means the central maximum is wider, and the intensity of the central maximum is lower.

Question 21

What are EM waves?

Answer 21

EM waves are transverse, they travel as oscillating magnetic and electric fields - with oscillations perpendicular to the direction of energy transfer.

Question 22

What are Longitudinal Waves?

Answer 22

In longitudinal waves the oscillations are parallel to the direction of energy transfer.

Question 23

Define monochromatic and coherent.

Answer 23

Monochromatic light is light of a single wavelength and frequency, so a single colour.

Two waves are coherent if they have the same wavelength and frequency and a fixed phase difference between them.

Question 24

Define Period

Answer 24

The time taken for one whole wave cycle.

Question 25

Define Refraction

Answer 25

The wave changes direction as it enters a different medium. the change in direction is a result of the wave slowing down or speeding up.

Question 26

Explain the single slit diffraction pattern for monochromatic light.

Answer 26

The pattern has a bright central fringe (central maximum) with alternating dark and bright fringes on either side of it. The fringe pattern is due to interference. - The bright fringes are due to constructive interference, where waves from across the width of the slit arrive at the screen in phase. - The dark fringes are due to total destructive interference, where waves from across the width of the slit arrive at the screen completely out of phase.

Question 27

What does dispersion cause and how does it affect the signal?

Answer 27

Both types of dispersion cause pulse broadening - the received signal is broader, broadened signals can overlap, leading to information loss.

Question 28

What safety precautions should you take when working with lasers?

Answer 28

1) Never shine the laser towards a person

2) Wear laser safety goggles

3) Avoid shining laser at reflective surfaces

4) Turn the laser off when its not needed.

Question 29

Explain the single slit diffraction pattern for white light.

Answer 29

White light is a mixture of colours, each with different wavelengths. All these wavelengths are diffracted by different amounts. This means that instead of getting clear fringes, you get a spectra of colours, with a bright white central fringe. Red light is a longer wavelength then violet, so is diffracted more and seen on the outside of the spectra whereas violet is seen on the inside.

Question 30

What is a step index optical fibre?

Answer 30

A very thin tube of glass or plastic fibre that can carry light signals using TIR. Step index fibres have a optically dense core surrounded by less optically dense cladding to allow TIR. Cladding helps to prevent scratches which could allow light to escape

Question 31

What is material dispersion? And how can it be prevented?

Answer 31

This is caused by the different amounts of refraction experienced by different wavelengths of light. They all travel at different speeds, causing parts of the signal to arrive at different times. Using monochromatic light prevents this.

Question 32

How can a stationary wave be formed on a string?

Answer 32

An oscillator at one end, with the other end fixed. The wave produced is reflected back and forth. If the oscillator produces an exact number of wavelengths in the time it takes for a wave to get to the end and back again, then the original and reflected waves reinforce each other and a visual standing wave is produced.

Question 33

How is a wave caused and how and where does it transfer energy?

Answer 33

A wave is caused by something making particles or fields oscillate at a source. These oscillations pass through the medium as the wave travels, carrying energy with it away from the source.

Question 34

What is the principle of superposition?

Answer 34

When two or more waves cross, the resultant displacement equals the vector sum of the individual displacements.

Question 35

How is a standing wave formed?

Answer 35

Two progressive waves that have the same frequency, amplitude and wavelength travel in opposite directions. This causes superposition, and a standing wave is formed. No energy is transferred along a stationary wave. Nodes are points of no displacement, and they are formed when the two waves destructively interfere, cancelling out each displacement. Antinodes are points of maximum displacement, and they are formed when two waves constructively interfere.

Question 36

What do all EM waves share?

Answer 36

They travel at the same speed in a vacuum. 3.00x10⁸ ms^-1

Question 37

How can a stationary wave be formed from a microwave?

Answer 37

You can set up a stationary wave by reflecting a microwave beam at a metal plate . The superposition of the wave and its reflection produce a stationary wave. Use a probe to measure the signal between, no signal appears at the nodes and maximum signal at the antinodes.

Question 38

What is Total Internal Reflection?

Answer 38

At angles greater than the critical angle, refraction can’t happen. That means all the light is reflected back into the material.

Question 39

What is phase difference and what is it measured in?

Answer 39

The amount that one wave lags behind another, measured in radians, degrees and fractions of a cycle.

Question 40

How does polarisation give evidence for the nature of EM waves?

Answer 40

Scientists discovered that light was polarised by reflection. At the time light was thought of as a longitudinal wave, so polarisation was hard to explain. It was suggested that light was a transverse wave consisting of oscillating electric and magnetic fields perpendicular to the direction of energy transfer. This explained why light could be polarised as only transverse waves can be polarised.

Question 41

What are the applications of diffraction gratings?

Answer 41

1) X-Ray crystallography. The average wavelength of X-rays is of a similar scale to the spacing between atoms in crystalline solids, They therefore can be used to form diffraction patterns, helps us learn more about the structure of small molecules.

2) Analysis of spectra of stars. Certain elements in a stars atmosphere absorb certain wavelengths of light. Therefore when you pass light through a star and diffraction grating, you can see a line absorbtion spectra, with dark lines corresponding to the different wavelengths of light that have been absorbed.

Question 42

What is the critical angle? How do you calculate it?

Answer 42

The angle of incidence where the refracted ray is refracted along the boundary

Question 43

Define Frequency

Answer 43

The number of whole wave cycles per second passing a given point.

Question 44

What is modal dispersion? And how can it be prevented?

Answer 44

It is caused by light rays entering the optical fibre at different angles. This causes them to take different paths down the fibre, causing them to arrive at different times, this can be reduced by using a single-mode fibre in which light is only allowed to follow a very narrow path.

Question 45

Define Wavelength

Answer 45

The length of one whole wave oscillation or wave cycle, the distance between two crests or two troughs.

Question 46

What is the first harmonic? Describe its attributes.

Answer 46

A stationary wave vibrating at its lowest possible resonant frequency. It has one “loop” with a node at each end. One half wavelength fits onto the string, and so the wave length is double the length of the string.

Question 47

What is a stationary wave?

Answer 47

A stationary (standing) wave is the superposition of two progressive waves with the same frequency, wavelength and amplitude, moving in opposite directions. No energy is actually transferred.

Question 48

What is constructive and destructive interference?

Answer 48

When two waves meet, if their displacements are in the same direction, the displacements combine to give a bigger displacement. A crest plus a crest gives a bigger crest. A trough plus a trough gives a bigger trough. This is known as constructive interference.

If a wave with a positive displacement (crest) meets a wave with a negative displacement (trough), they will undergo destructive interference and cancel each other out. If the magnitude of the displacements are equal, they undergo total destructive interference and the resulting wave has no amplitude.

Question 49

When are two waves in phase?

Answer 49

Two points on a wave are in phase if they are both at the same point in the wave cycle. They have equal displacement and velocity.

Question 50

What are Transverse Waves?

Answer 50

In transverse waves the oscillations are at right angles to the direction of energy transfer.

Question 51

How did the knowledge and understanding of EM waves develop over time?

Answer 51

There were disagreements about the nature of light, some argued particles, some argued waves. The particle theory could explain reflection and refraction, but diffraction and interference are both uniquely wave properties. Light has been shown to exhibit all these properties through experiments such as the double slit experiment.

Question 52

Derive the diffraction grating equation.

Answer 52