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
- 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
