Section 3 - Waves and Optics Flashcards
What are the two types of wave?
Transverse & Longitudinal
True or false transverse waves can be polarised?
True
True or false longitudinal waves can be polarised?
False
What is the direction of energy transfer for a transverse wave?
Perpendicular to the direction of travel
What is the direction of energy transfer for a longitudinal wave?
Parallel to the direction of travel
Give two examples of a transverse wave
Examples include:
* Any EM wave
* S-wave
Give two examples of a longitudinal wave
Examples include:
* Sound wave
* P-wave
Features of a transverse wave?
Has troughs and peaks
Features of a longitudinal wave?
Has compressions and rarefactions
What does a transverse wave look like?
Looks like a sinusoidal wave
What does a longitudinal wave look like?
Looks like a spring
Where can we get unpolarised light from?
Sources include:
* The sun
* Flourecent lamps
* Fire
Define plane polarisation
Wave particles oscillate in one plane
How do we polarise a wave?
Using a polarising filter
How much light does parallel polarisation let through?
All of the light passed through the filter
How much light does perpendicular polarisation let through?
None of the light
Define amplitude
Maximum displacement from the equilibrium position
Define frequency
Number of complete wave cycles per second
Define time period
The time taken for one wavelength to pass a specific point
Define displacement
Distance from the equilibrium position
Define wavelength
The distance between two points in phase
What is the wave speed equation?
C = f. lambda
Equation for finding the phase difference between two points?
(x % lambda) * 2pi
When does a wave refelct?
When it hits a hard surface
What is the reflection law?
The angle of reflection is equal to the angle of incidence
When does refraction occur?
When a wave enters a medium with a different optical density
Why does refraction occur?
The beam changes velocity, this causes the light to bend
What is diffraction?
Waves spread out after passing through a gap
What is superposition?
When two waves meet the total displacement is the sum
How are stationary waves formed?
- Progressive wave hits a hard surface; it reflects
- reflected wave has similar amplitude, same frequency and wavelength
- Two wavse superpose
- they interfere constructively at points in phase - node formed
- interfere destructively at points out of phase - anti-node formed
True or false the amplitude of a stationary wave is the same at all points?
False
Define refractive index
The ratio of the speed of light in a vacuum and the speed of light in substance
Does frequency change when entering a new medium?
No it does not
Does wavelength change when entering a new medium?
Yes
Do longer wavelengths get refracted the most or the least?
The least
Do shorter wavelengths get refracted the most or the least?
The most
When does TIR occur?
When the angle of incidence is greater than the critical angle
What condition must be met for TIR to occur?
n1 >n2
How does an optical fibre work?
- Light enters the fibre at varying angles
- The lights undergoes TIR until it exits the fibre
What is the purpose of the cladding?
- To protect the core
- To lower the critical angle
- To prevent cross talk
What is modal dispersion?
- Light takes different paths to the end of the fibre.
- This varies the time taken by different rays
- As a result the light doesn’t arrive all at once
How to fix modal dispersion?
Narrow the core
What is material dispersion?
- Differerent wavelengths of light causes the light to take different paths down the fibre.
- This causes the light to take varying amounts of time to reach the end of the fibre
How to fix material dispersion?
Use monochromatic light
Examples of when optical fibres are used
Data transfer
Medical endoscopes
Define coherent
Constant Phase relationship
Define monochromatic
A single wavelength
Why do maxima form on a screen when passed thorugh a double slit?
- The light source passed through the double slit forms two coherent wave sources
- The two waves superpose
- They interfere constructively at points in phase and form bright fringes
- They interfere destructively at points out of phase and from dark fringes
- At bright fringes the path difference is nλ
- At dark fringes the path diffenrence is n/2 * λ
What is the approximate wavelength for red light?
650nm -700 nm
What is theapproximate wavelenght for violet light
350nm
What seen on the screen when white light is passed through a double slit?
- Bright white central maxima
- Fringes are a full spectra
- Fringes have a wider W
- Blue light is closest to the central maxima
What is single slit diffraction?
- There are an infinite amount of wavelets coming through the slit
- Wavelets at either end of the slits have a path difference by the thime they reach the screen
- Talk about interference in the sam eway for double slit
What does the intensity graph look like for a single slit?
- Peak intensity of the fringe drop off very quickly
- Central maxima is twice the width of the other fringes
What must the two wave sources do in order for diffraction to occur in a double slit?
They must overlap
How can we ensure that the two wave sources overlap?
- Making sure the slits are narrow enough to diffract the light sufficently
- Making sure the slits are close enough so the light overlaps on the screen
Formula for wavelength in a single slit?
λ = W.s/2D
What is a diffraction grating?
A plate with closely placed parallel plates
What does a diffraction grating cause?
Concentrated maxima
What is the central beam often reffered to as?
The zero order beam
How to increase the angle of diffraction?
- Use light of a longer wavelength
- Use a grating with closer slits
What is the diffraction grating equation?
nλ = d>sin(θ)
How do you find the biggest possible value of n and why?
- Set θ to 90 the solve for n
- Because this the greatest possible angle of diffraction
What colour of light has the lowest refractive index?
Red
What colour of light has the highest refractive index?
Blue
What is the equation for light intensity?
Why do different wavelengths of light have different refractive indices?
What kind of wave does not require a medium to propagate through?
Electromagnetic
What kind of wave consists of an oscillating electric field and a perpendicular oscillating magnetic field?
Electromagnetic
graph of displacement versus time for a wave applies to a particular ____?____ on the wave.
Particle
A graph of displacement versus distance for a wave shows the position of the particles at a particular ____?____.
time
Why do different wavelengths of light take different amounts of time to travel through an optical fibre?
Wavelength changes with different optical densities.
Different colours of light refract by a different amount so they will reflect at different angles.
Longer wavelengths will refract the least to their path will be shorter than shorter wave lengths of light.
