Waves 1 - Progressive waves and Refraction Flashcards
What is a progressive wave?
Oscillations that have a resultant transfer of energy in one direction
How are mechanical and electromagnetic waves different?
Mechanical waves require a medium to oscillate through
Electromagnetic waves don’t require matter (oscillate through electric and magnetic fields)
What makes a wave transverse?
Oscillations are perpendicular to the transfer of energy
What makes a wave longitudinal?
Oscillations are parallel to the transfer of energy
What 2 properties do all electromagnetic waves possess?
- Always transverse
- Propagate with velocity of 3×108ms-1 through vacuum
Name 3 longitudinal waves
- Sound
- P-waves (Earthquakes)
- Water waves (beneath surface)
Name 3 transverse waves
- E-M waves (Light, X-rays, UV etc)
- Waves on string
- S-waves (Earthquakes)
- Water waves (surface)
List in order all waves on the E-M spectrum
How are displacement and amplitude of a wave different?
Displacement → Current distance of a point from the equilibrium position
Amplitude → Maximum distance a point reaches from equilibrium position
Why do all points on a progressive wave have the same amplitude?
All points have the same maximum displacement from equilibrium position
What is the time period of a wave?
Time taken for each particle to complete one full oscillation
(Return to same position)
How is frequency of a wave defined?
The number of complete oscillations per second
What is the wavelength of a wave?
Distance between two adjacent corresponding points on a wave
(Same displacement, no phase difference)
What is the phase difference between A and B on this progressive wave?
360° ∼ 0°
2π∼ 0π
What is the phase difference between A and B on this progressive wave?
180°
π ∼ Antiphase
What is the phase difference between points A and B on this progressive wave?
540° ∼ 180°
3π ∼ π ∼ antiphase
How is phase difference calculated in degrees?
How is phase difference calculated in radians?
How do you convert from degrees → radians?
What is the phase difference between A and B on this progressive wave?
420° ∼ 60°
14π/6 ∼ π/3
How are frequency and wavelength related?
What are the 2 key features of longitudinal waves?
Compressions and rarefactions
Why can’t sound waves be polarised?
Only transverse waves can be polarised
(Sound is longitudinal)
What is the final intensity?
- Light vertically polarised through first grating
- Vertically p[olarised light can’t pass through horizontal grating
- Final intensity = 0
What is the final intensity?
- Light vertically polarised through first grating (intensity halves)
- Vertically polarised light passes through second grating
- Final intensity = ½
How do sunglasses reduce glare?
- When sunlight reflects off surfaces it is polarised
- Sunglasses have filter to block polarised light
- Only unpolarised light passes through
What is the refractive index of a material?
Ratio of speed of light in a vacuum : speed light passes through material
(The greater n > the more light slows down)
How does θ2 compare to θ1?
θ2 > θ1
(Light speeds up and bends away from normal)
How does θ2 compare to θ1?
θ2 < θ1
(Light slows down and bends towards normal)
Is the light refracting here?
Yes
It hasn’t bent towards or away from normal
But it has slowed down
How does refraction affect the frequency of a wave?
Frequency does not change
(But wavespeed and wavelength do)
What is dispersion?
Different wavelength refract by different amounts
So light passing through a prism separates into wavelengths
What is wrong here?
In Snell’s law θ1 is the angle between normal and incident ray
What are the 2 conditions for total internal reflection?
- θ1 > θc
- n2 < n1
How is the critical angle calculated?
How do you calculate the angle of incidence in the fibre?
Using basic geometry (angles in triangle add to 180°)
Why are optical fibres better than copper cables?
- Information transmission faster
- More information can be transmitted (harder to intercept signals)
- Less energy loss (copper heats up)
In optical fibres what does cladding do?
- Protects the core from scratches and spills
- Stops data loss to adjacent fibres
- Increases critical angle (reducing modal dispersion)
What is modal dispersion and how is it combatted?
Different modes (angles) take different amount of time to propagate through an optical fibre
Leads to pulse broadening
Combatted by making core narrow and using cladding with low n (increasing θc)
What is spectral (material) dispersion and how is it combatted?
Different wavelengths (colours) of light refracted by different amounts
Leads to pulse broadening
Combatted using monochromatic light
What is oscillating in the case of a mechanical wave?
Particles are oscillating about their equilibrium position
What is oscillating in the case of an electromagnetic wave?
Electric and magnetic fields
No particles are involved in oscillations
This is why electromagnetic waves can travel through a vacuum
What do waves not transfer?
They DO NOT transfer matter
Are all longitudinal waves mechanical?
Yes
EM waves transfer energy perpendicular to oscillations and so cannot be longitudinal
Therefore, any longtudinal wave can only be mechanical
Are all mechanical waves longitudinal?
No
Mechanical waves can be longitudinal (sound waves) but can also be transverse (string)
What occurs when the gradient of a displacement-distance graph for a longitudinal wave is:
- Positive
- Negative
- When the gradient is positive, rarefactions occur
- When the gradient is negative, compressions occur
What is partial polarisation and when does it occur?
Where more light is transmitted in one plane than in other planes
This occurs during reflection, such as reflection off water, glass or metal
How is light intensity emitted related to the angle of the second filter?
Intensity follows a cosine graph where initial intensity is max
If initial intensity is 0 it will follow a sine graph
How does refractive index affect the speed of light?
Inversely
Higher refractive index → Slower speed of light
Lower refractive index → Higher speed of light
Describe the process of reaching the critical angle
- A ray of light passing from higher to a lower refractive index will refract away from the normal when the angle of incidence < critical angle and there will be a partially reflected ray
- As the angle of incidence increases, the angle of refraction increases
- The intensity of the reflected ray increases as the intensity of the refracted ray decreases
- When the angle of incidence = critical angle, the angle of refraction is 90 degrees and travels along the boundary. There is still a reflected ray
- If the angle of incidence increases further, (angle of incidence > critical angle), total internal reflection occurs. There is no refracted ray.
What is attenuation and how is it overcome?
The decrease in the intensity of a wave over long distances
Solutions:
- Using very high purity glass
- This keeps the intensity as high as possible along the fibre
Which end of the visible light spectrum has the highest refractive index?
Violet light
(shortest wavelength)
(slows down the most in refraction)
Which end of the visible light spectrum has the highest refractive index?
Violet light
(shortest wavelength)
(slows down the most in refraction)
