Section 3: Waves

Question 1

When are two points on a wave in phase?

Answer 1

When they are at the same point in the wave cycle - they have a phase difference that is a multiple of 360 degrees. They also have the same displacement and velocity.

They do not need to have the same amplitude, just frequency and wavelength.

Question 2

When are two points on a wave completely out of phase?

Answer 2

When they are an odd integer of half cycles apart, where one half cycle is 180 degrees. E.g. 1, 3, 5 etc multiplied by 180

Question 3

Describe the conditions for the formation of a standing wave

Answer 3

Two progressive waves, travelling in opposite directions in the same plane, with the same frequency, wavelength and amplitude

Question 4

What is the first harmonic?

Answer 4

The lowest frequency at which a stationary wave forms. This wave will have two nodes and one antinode. The distance between the nodes (for this wave and any other harmonic) is equal to half a wavelength.

Question 5

How do you find the frequency of harmonics beyond the first harmonic?

Answer 5

Multiply the frequency of the first harmonic by the number of the harmonic you are trying to find.

Question 6

What is path difference?

Answer 6

The difference in the distance travelled by two waves.

Question 7

What is a coherent light source?

Answer 7

A light source that has the same frequency and wavelength and a fixed phase difference.

Question 8

What is a monochromatic light source?

Answer 8

A light source that emits a single frequency of light

Question 9

Which form of EM radiation has the longest wavelength?

Answer 9

Radio

Question 10

Which form of EM radiation has the shortest wavelength?

Answer 10

Gamma

Question 11

Which colour of visible light has the longest wavelength?

Answer 11

Red

Question 12

Which colour of visible light has the shortest wavelength?

Answer 12

Violet

Question 13

Describe Young’s double slit experiment

Answer 13

Shine a coherent light source through two parallel slits approximately the same size as the wavelength of the light, which causes the light to diffract through each slit.

Each slit acts as a coherent point source, which causes a pattern of alternating light and dark fringes to form on a screen.

The light fringes form where the two waves meet in phase and interfere constructively, and the dark fringes form where the two waves meet completely out of phase and interfere destructively.

Question 14

Describe how the diffraction pattern formed through Young’s double slit experiment differs if white light is used

Answer 14

The maxima are wider and provide a less intense diffraction pattern, with a central white fringe and subsequent fringes being spectra, with violet nearest to the centre.

Question 15

What scientific theory did Young’s double slit experiment help prove?

Answer 15

The wave nature of light, because diffraction and interference are wave properties, meaning that light must act as a wave (at least some of the time).

Question 16

What is diffraction?

Answer 16

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

Question 17

When does the greatest diffraction occur?

Answer 17

When the size of the gap is equal to the wavelength of the light passing through it.

Question 18

What is the only property of a wave that changes when it is diffracted?

Answer 18

Amplitude, as some energy is dissipated in the process.

Question 19

Describe and explain the interference pattern formed by a diffracted monochromatic light source

Answer 19

A pattern of alternating light and dark fringes, with a bright central fringe double the width of the rest. The light fringes are caused by constructive interference where the waves meet in phase, and the dark fringes are caused by destructive interference where the waves meet completely out of phase.

Question 20

Describe the interference pattern formed by white light diffracted through a single slit

Answer 20

A central white maximum with alternating bright fringes which are spectra, with violet closest to the centre.

Question 21

How does increasing slit width affect a diffraction pattern?

Answer 21

It decreases the amount of diffraction so the central maximum
becomes narrower and its intensity increases.

Question 22

How does increasing the wavelength of light affect a diffraction pattern?

Answer 22

It increases the amount diffraction as the slit is closer in
size to the light’s wavelength, therefore the central maximum becomes wider and its intensity decreases

Question 23

What is the advantage of a diffraction grating over a double slit?

Answer 23

The interference pattern is much brighter, as there are many more sources reinforcing the pattern. This means the measurements are much more accurate as they are easier to take.

Question 24

Name some practical applications for diffraction gratings

Answer 24

Used in spectrometers to: analyse light from stars, analyse compositions of stars, analyse absorption/emission spectra in stars, chemical analysis.

Used in x-ray crystallography.

Question 25

What is Snell’s law?

Answer 25

(n1)sin(θ1) = (n2)sin(θ2)

Question 26

What does a higher refractive index indicate?

Answer 26

A more optically dense material.

Question 27

How do a wave’s properties change when it is refracted?

Answer 27

The frequency stays the same, while the speed and wavelength change.

Question 28

Define the critical angle

Answer 28

The angle of incidence at which the wave is refracted along the boundary between the two materials.

Question 29

What are the physical conditions for a critical angle and total internal reflection to be possible?

Answer 29

The medium the wave is in currently must be more optically dense than the medium it is trying to enter.

Question 30

When does total internal reflection occur?

Answer 30

When the angle of incidence is greater than the critical angle.

Question 31

Describe the structure of an optical fibre

Answer 31

An optically dense core surrounded by a less optically dense cladding.

Question 32

Describe the purpose of an optical fibre’s cladding

Answer 32

As it has a lower optical density than the core, total internal reflection can occur to help prevent signal degradation through light escaping the core. It also helps to protect the delicate core from damage.

Question 33

Describe how signal degradation occurs through absorption

Answer 33

Part of the signal’s energy is absorbed by the fibre, which reduces the signal’s amplitude and can lead to a loss of information.

Question 34

What is dispersion?

Answer 34

Where the signal passing through an optical fibre experiences pulse broadening, which is where the received signal is wider than the original transmitted signal. This can cause signals to overlap and cause loss of information.

Question 35

Describe modal dispersion

Answer 35

Dispersion caused by light rays entering the fibre at different angles, therefore they take different paths along the fibre. This leads to the rays taking a different amount of time to travel along the fibre, causing pulse broadening.

Question 36

Describe material dispersion

Answer 36

Dispersion caused by using light consisting of different wavelengths, meaning light rays will travel at different speeds along the fibre, which leads to pulse broadening.

Question 37

How can material dispersion be prevented?

Answer 37

Using monochromatic light.

Question 38

How can absorption and dispersion be reduced?

Answer 38

Using an optical fibre repeater to regenerate the signal during its travel to its destination.