Waves and Optics Flashcards
What is an optical fibre?
An optical fibre is a flexible, transparent fibre made of glass or plastic that is used to transmit light signals over long distances.
What is the principle behind the operation of optical fibres?
The principle behind the operation of optical fibres is total internal reflection, where light is reflected back into the fibre when it encounters a boundary between materials with different refractive indices.
What are the advantages of using optical fibres for communication?
Optical fibres offer high bandwidth, low loss, and immunity to electromagnetic interference, making them ideal for high-speed data transmission over long distances.
What is the refractive index of an optical fibre?
The refractive index of an optical fibre is a measure of how much the speed of light is reduced when it passes through the fibre, and is typically between 1.44 and 1.52.
What is dispersion in optical fibres?
Dispersion in optical fibres is the spreading of the light pulse over time due to variations in the speed of light in different wavelengths, and can limit the maximum bandwidth of the fibre.
What is Total Internal Reflection?
The complete reflection of a light ray reaching an interface with a less dense medium where the angle of incidence is greater than the critical angle.
Define Coherent
Sources that have the same wavelength, frequency and phase difference
Use wave theory to explain how the fringe pattern is formed
- Slits act as coherent sources
- Waves diffract at slits
- Waves superpose
- Bright patches: constructive
- Dark patches: destructive
Define Monochromatic
Single colour so single wavelength
Explain how Young’s double slit arrangement produces interference fringes, refer to width of first slit and coherence of second slits
- 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
If Young’s double slit was carried out with red light and then with white light, how would the two differ?
- Central fringe would be white
- Dark fringes would be closer together
- Side fringes are spectra
State two ways a diffraction pattern on single slit would change if the slit became narrower
- Increased separation
- Lower intensity
State and explain what happens to angle θ in λ = d sin θ when wavelength decreases?
- Angle θ gets smaller
- As path difference gets smaller
How can the appearance of a first order beam from light from a star be used to deduce information about the outer layers of the star?
Absorption spectrum reveals the composition of the star’s atmosphere
What is the name for the part of an optical fibre that is around the core?
Cladding
State and explain an advantage of a smaller diameter core
- Reduced multipath
- Increasing resolution
State one application of optical fibres and its benefit to society
- Communications
- Improved transmission of data
Explain why optical fibres used for communication need to have cladding
- Keeps signal secure
- Keeps most light rays in
State and explain the features of Young’s Double Slit(4)
- Light source: Multiple wavelengths of visible light
- Filter: Selection of single wavelength, production of monochromatic source
OR
-Laser: monochromatic light source - Single slit: Diffraction allows single source to strike 2 slits
- Double slit: Distance between single and either double slit is the same so phase difference is the same making the entering source coherent
How can a guitarist raise the fundamental frequency of vibration in their string?
Tightening the string
Which colour light has a higher refractive index, red light or blue light?
Blue light
Blue light has a higher refractive index than red light, explain how this difference results in a change in a pulse of white light by the time it leaves the fibre
- Blue travels slower than red due to greater refractive index
- Red reaches the end of the fibre before blue leading to material dispersion
Suggest an experiment to demonstrate the wave nature of sound
Diffraction through a door
What is a progressive wave?
A moving wave which carries energy from one place to another without transferring any material
What is a transverse wave?
Oscillation of particles is perpendicular to the direction of energy propagation
Give 3 examples of a transverse wave
All Electromagnetic waves
Ripples in water
Waves on strings
What is a progressive wave?
A moving wave which carries energy from one place to another without transferring any material
What is a transverse wave?
Oscillation of particles is perpendicular to the direction of energy propagation
Give 3 examples of a transverse wave
- All Electromagnetic waves
- Water ripples
- Waves on strings
What is a longitudinal wave?
Oscillation of particles is parallel to the direction of energy propagation
Give 2 examples of a longitudinal wave
- Sound waves
- Some types of earthquake shock waves
What is a polarised wave?
A wave that oscillates in one direction only
One polarising filter is placed in front of another. Describe what would as one filter is rotated 360 degress
- Incoming light decreases, minimum when no light gets through at 90 degrees
- Light passing through then increases and reaches max at 180 degrees
- Incoming light then decreases again, reaching minimum at 270 degrees
- Incoming light then increases and reaches maximum at 360 degrees
What are applications of polarised waves
- Glare reduction for items such as sunglasses
- Improving TV and radio signals as TV signals are polarised by the orientation of the rods so by lining the rods of the receiving aerial with the rods of the transmitting aerial, signal strength is increased
When are two points of a wave in phase?
When the points are both at the same point in the wave cycle
What is the phase difference of two points in a wave?
The phase difference of two points in a wave is the difference in their positions in a wave’s cycle. This is measured in degrees, radians or fractions of a cycle
When are two waves in phase?
When their phase difference is 0 degrees or a multiple of 360 degrees
When are two wave exactly out of phase?
When their phase difference is odd multiples of 180 degrees
What is a stationary/standing wave?
- Two identical waves travelling in the opposite direction meet and superpose, usually happening when one wave is a reflection of the other
- When an exact number of half wavelengths fits on the string
- No energy is transmitted
Define resonant frequency
- The frequency at which an exact number of waves are produced in the time it takes for a wave to get to the end of its distance and back to the start
- The original and reflected waves reinforce each other
Define diffraction
Wavefronts spread out after passing through a gap or around an obstacle
Describe why the middle maxima is the brightest
Intensity is power per unit area. For monochromatic light, all photons have the same energy so increasing intensity will increase the number of photons per seconds. More photons hit the central maxima per second than other bright fringes.
What factors affect the width of the central maxima?
- Slit width
- Wavelength of diffracted light
How does slit width affect the width of the central maxima?
Increasing slit width decreases amount of diffraction, making the central maxima narrower and the intensity of the maxima is higher
How does the wavelength of the diffracted light affect the width of the central maxima?
Increasing the wavelength increases the amount of diffraction which makes the central maxima wider and intensity of the maxima is lower
What is two source interference and how can it’s clarity be improved?
When waves from two sources interfere to produce a pattern. To get a clear interference pattern, the waves from the two sources should be monochromatic and coherent
Define Path difference
The amount by which the path travelled by one wave is longer than the path travelled by another wave
When does constructive interference occur?
When the path difference = nλ (Where n = 0,1,2…)
When does destructive interference occur?
When the path difference = (n + 1/2)λ
Define absolute refractive index
The ratio of the speed of light in a vacuum and the speed of light in a material
Define reflective refractive index
The ratio of the speed of light in material 1 to the speed of light in material 2
What is the angle of incidence?
The angle incoming light makes to the normal
What is the angle of refraction
The angle the refracted ray makes with the normal
Define Frequency
The number of wave cycles per second in Hertz(Hz)
Define Time period
The time taken for one complete wave cycle in Seconds(s)
Define node
A point of zero displacement along a stationary wave caused by destructive interference
Define antinode
A point of maximum displacement along a stationary wave caused by constructive interference
Define intensity
The wave power transmitted per unit area at right angles to the wave velocity
Define refraction
The change of direction when a wave travels from one medium to another and changes speed
What is the critical angle?
The angle at which there is a transition between refraction and total internal reflection
Define wavefront
A line or surface in the path of a wave’s motion where all the displacements at any point have the same phase
Define phase
A measurement of the position of a certain point on a wave cycle
Define wavelength
The length of one whole oscillation, in metres(m)`
Define amplitude
The maximum displacement of the wave from the equilibrium position
Define phase difference
The amount by which one wave lags behind another
What type of waves can be polarised?
Transverse waves
What type of waves cannot be polarised?
Longitudinal waves
What is constructive interference?
When two or more waves have displacements in the same direction
What is destructive interference?
When two waves interfere to make a wave with a reduced amplitude
What is total destructive interference?
When two waves meet perfectly out of phase, the resultant amplitude is zero
Two waves have a phase difference of 2π, what is the path difference of these waves?
π
What equation links phase difference and path difference?
Δx/λ = ΔΦ/2π
- Δx = path difference
- λ = wavelength
- ΔΦ = phase difference
- 2π = phase in one period
State the equation linking d and N, including what each letter means
-d = 1/N
- d = slit spacing
- N = number of slits per metre
State the fringe width equation, including what each letter means and their units
- w = λD/s
- w = fringe spacing between two adjacent maxima, or two adjacent minima(m)
- λ = wavelength(m)
- D = distance between slits and screen(m)
- s = distance between slits(m)
State the 2 types of dispersion
- Modal dispersion
- Material dispersion
What is modal dispersion?
- Dispersion caused by the light rays entering the optical fibre at different angles
- This causes them to take different paths along the fibre, with a straight path down the middle being the fastest
What is material dispersion?
- Dispersion caused by the different amount of refraction experienced by different wavelengths of light
- Different wavelengths slow down by different amounts in a material, leading to some parts of the signal to take longer to travel along the fibre than others
What is the role of cladding in an optical fibre?
To make the critical angle as small as possible
State the 2 ways a signal can be degraded in an optical fibre
- Absorption
- Dispersion
What is absorption in an optical fibre?
- Signal degradation caused by some of the signal’s energy being absorbed by the material the fibre is made from
- This results in signal amplitude being reduced
How can material dispersion be fixed?
By using monochromatic light
What type of dispersion can be completely fixed?
Material dispersion
What type of dispersion can be reduced but not completely fixed?
Modal dispersion
How can modal dispersion be reduced?
By using a single-mode fibre, in which light is only allowed to follow a very narrow path
What is the pre-condition the use of the Young’s double slit equation?
Slit-screen distance is a lot greater than slit separation
What happens to a ray of light which is incident on an optical fibre which is bending?
- Angle of incidence on core-cladding boundary decreases
- Angle of incidence becomes less than critical angle
- Some light will be refracted into cladding
Define Resonance
When the driving frequency matches the natural frequency, there is a rapid increase in amplitude
When entering a material from air, What is a way in which a light ray can have no deviation when entering the material?
By having the ray of light enter the material along the normal
When entering a material from another material, What are 2 ways in which a light ray can have no deviation when entering the second material?
- Have the materials be in contact with each other with no gaps
- Have the refractive index of the first material be the same as the refractive index of the second material
Describe the pattern on a screen when light is diffracted through a single slit
- Middle maxima is twice as wide as other maxima
- Central maximum with lower intensity maxima on either side
