Waves and Optics Flashcards
What is a longitudinal wave?
The vibrations or displacement are parallel to the direction of energy transfer.
What is a transverse wave?
A wave where the displacement or vibrations are perpendicular to the direction of energy transfer.
What is a progressive wave?
A progressive wave transfers energy without transferring material and is made up of particles oscillating.
What is the amplitude of the wave?
A wave’s maximum displacement from the equilibrium position (m)
What is frequency?
The number of complete oscillations passing through a point per second. (Hz)
What is the wavelength of a wave?
The length of one whole oscillation (m)
What is wave speed and how is it calculated?
v = fλ
What is a transverse wave?
A type of wave where energy moves perpendicular to the oscillations
What is the time period?
The time taken for one whole oscillation
What is constructive interference?
When 2 or more waves have displacement in the same direction
What is destructive interference?
When one wave has positive displacement and the other has negative displacement (if they are equal total destructive interference occurs)
What is a node (waves)?
Region of no displacement
What is an antinode?
Region of maximum displacement
What is the fundamental frequency?
The lowest frequency of a stationary wave that can be supported by a system
How do you determine the resultant displacement after interference?
The vector sum of displacements?
What is a stationary wave?
A wave where energy is not transferred from one point to another, but oscillations still happen.
How far must two points be to support a stationary wave?
An integer number of half wave lengths
What are the two ways a station wave can be formed?
- 2 coherent progressive waves travelling in opposite directions.
- Reflecting a wave of a constant frequency and amplitude between two points.
What are coherent waves?
Waves of a fixed phase relationship and same frequency.
What is refraction?
Change in direction of waves as it travels through different media caused by the medium’s optical density.
What is the refractive index of air?
1
What is the refractive index of a material?
A measure of how fast light travels through it compared to the speed of light in a vacuum. (c/v)
What is the relationship between frequency and refraction?
The higher the frequency the larger the change in wavelength and speed.
What is the critical angle of incidence?
One where the angle of refraction is 90 degrees
What are the condition necessary for a critical angle?
- There must be an increase in angle with the normal
- Must be an increase in speed
- n1 > n2
What happens if we exceed the critical angle?
Total internal reflection
What is the role of the cladding in a fibre optic cable?
- Protect the core from damage and scratching
- Prevent signal degradation
- Keeps signal secure
- Keeps core separate from other fibres preventing information crossover
How can we deal with pulse absorption?
- Use repeaters
- Increase the intensity of the pulse
- Use a beam that diffracts less
Describe Spectral/Material Dispersion in reference to fibre optics.
- If white light is used each component frequency has a different speed in a medium
- Violet has the greatest change in speed and red the least
- Violet travels slower than red
- A broadened signal is received by the sensor
Describe Pulse Broadening.
- Pulse is wider as the pulse arrives at different times
- Pulse is shorter as there is a lower intensity as less energy is arriving per second
What is the solution to spectral/material dispersion?
Use a monochromatic source of light
Describe Modal Dispersion
- Each part of the wavefront arrives at different angles of incidence
- Reflects at a different angle so each takes a different path
- Path with more internal reflections take longer
- Pulse broadening is observed
How is Modal Dispersion reduced?
- Using a narrow fibre to reduce the number of alternate paths the light can take
- Increasing core-cladding critical angle to lose rays with a high angle of incidence in the cladding
Why do diamonds sparkle?
- High refractive index of 2.4 so separates colours more
- Gives a small critical angle of 24 degrees
- Total internal reflections occur many times before rays are transmitted
- Colours spread out even more and so the diamond sparkles with different colours
What are some of the required properties of the core?
- Material should have a low absorption
- Refractive index should not vary much across the wavelengths of light that are to be used
What are some of the properties of the cladding?
- Lower refractive index than the core
- Has a high enough refractive index so that a sufficient number of high incidence rays are lost in the cladding
What are some of the required properties of the cladding?
- Lower refractive index than the core
- High enough refractive index so the high incidence rays are not totally internally reflected and are lost in the cladding
Why do we want to reduce pulse broadening?
Prevent individual pulses in a series from overlapping so that we get a clearer signal
What does it mean for two waves to be in phase ?
- phase difference is a multiple of 360°
- same displacement and velocity
What does it mean for two waves to be out of phase ?
- odd integer of half cycles apart
- 180° or π radians
What is the speed and type of wave of a EM wave?
- transverse, 3x10^8
What is a polarised wave?
- oscillates in only one plane
- only transverse waves can be polarised
What is superposition?
- the displacements of two waves are combined as they pass each other
- resultant displacement is vector sum of each displacement
What is the first harmonic and at what length does it occur?
- First harmonic is the lowest frequency at which a stationary wave with two nodes and one antinode.
- The distance between adjacent nodes is half the wavelength
What is diffraction?
- Spreading out of waves as they pass through a gap
- Greatest diffraction occurs when gap is same size as the wavelength
What happens when monochromatic light is shone through a single slit?
This light diffracts as it goes through the slit and forms fringes on the screen. There is a central bright fringe which is double the width of the other fringes.
What happens if white light is shone through a single slit?
- Central bright white maximum
- Alternating bright fringes surrounding which are a spectrum from violet to red
- this happens as different wavelengths of light are all diffracted by different amounts
What causes fringes to occur?
Bright fringes: waves meet in phase and constructively interfere
Dark fringes: waves meet in anti-phase and destructively interfere
What occurs when monochromatic light passes through double slits?
- light must be coherent (same frequency and wavelength)
- produces a pattern of dark and light fringes
- constructively interferes when the path difference between both waves from both sources is a whole number of wavelengths causing a bright fringe to form
- dark fringes from destructive interference at n +1/2 λ path difference
How do you calculate fringe spacing?
w = λD/s
w = fringe spacing
λ = wavelength of light
D = distance between screen and slits
s = slit separation
Why may light be passed through a single slit before the double slit?
ensures light sources are coherent
What happens when white light is shone through a double slit?
central bright white maxima, surrounded by fringes which are spectra with violet closest and red furthest
- fringes are wider and less intense
What is the distance between adjacent nodes of a stationary wave?
λ/2
How can you find frequency of a stationary wave made by a vibrating string for the first harmonic?
L = length of vibrating string (m)
T = tension in string (N)
μ = mass per unit length (kg/m)
What is a diffraction grating?
Slits with equal spacing
What happens when monochromatic light is shone through a diffraction grating?
interference pattern forms on the screen, but sharper and bright as many more rays of light to interfere
What is the zero order line?
Central maximum formed from a diffraction grating
What is the formula for a diffraction grating that can be used to find the maximum visible order?
d sinθ = nλ
d = slit separation
θ =angle between normal and maximum
n = order
λ = wavelength of order
- maximum order found by θ = 90 degrees so n = d/λ and truncate for answer
How to derive diffraction grating formula?
- path difference for rays from adjacent slits for first order is λ
- use right angle triangle with d and and θ and n = 1 to prove for n - 1
- extend this for all n where n is an integer as must be integer wavelengths to constructively interfere
Uses of diffraction grating
line absorption spectra
- show which elements are present in stars or other substances
What is the use of cladding in optical fibres?
- protect the core from scratching
- reduces pulse broadening
- prevents loss of information
- increases critical angle
State the conditions necessary for a stationary wave to be produced.
1) same frequency and wavelength
2) travelling in opposite directions
3) on the same medium
4) superpose to form a stationary wave
What is the purpose of the outer sheath of optical fibres?
1) prevent crosstalk
2) protect the fibre from damage
How to work out the number of visible orders?
1) work out maximum order
2) 2 x max order + 1
