WAVES 1
- Created by: CPev3
- Created on: 27-05-20 21:56
Electromagnetic waves
- Transverse waves
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- Electric and magnetic fields oscillating perpendicular to each other
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- Do not require a medium to propagate
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- Travel at a speed of 3 x 108 ms-1 in a vacuum
Electromagnetic spectrum
Radio waves = < 106
Microwaves = 10-1
Infrared = 10-3
Visible light = 7 x 10-7
Ultraviolet = 4 x 10-7
X-rays = 10-8 to 10-13
Gamma rays = 10-10 to 10-16
Electromagnetic waves can be...
- Reflected
- Refracted
- Diffracted
- Plane polarised
Progressive wave
- Oscillation that travels through matter (or in some cases a vacuum)
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- Transfers energy from one place to another, but not matter
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- Particles in the matter vibrate, but do not move along the wave
Progressive wave travelling through a medium
- Particles in the medium move from their equilibrium position to a new position
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- Exert forces on each other
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- A displaced particle experiences a restoring force from its neighbours
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- Pulled back to its equilibrium position
Transverse (S) wave
- Medium displaced perpendicular to direction of energy transfer
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- Oscillations of medium particles perpendicular to direction of wave travel
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- Peaks and troughs
Longitudinal (P) wave
- Medium displaced in same line as direction of energy transfer
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- Oscillations of medium particles parallel to direction of wave travel
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- Compressions and rarefactions
Displacement
- Distance from the equilibrium position in a particular direction
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- A vector, so positive or negative
Amplitude
- Maximum displacement from the equilibrium position
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- Positive or negative
Wavelength
Minimum distance between two points oscillating in phase on adjacent waves
Period
Time taken to complete one oscillation
Frequency
Number of wavelengths passing a given point per unit time
Wave speed
Distance travelled by the wave per unit time
Wave equation
v (wave speed) = f (frequency) x λ (wavelength)
Frequency equation
f (frequency) = 1 / T (period)
Wave profile
- A graph showing the displacement of the particles in the wave against the distance along the wave
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- Can be used to determine the wavelength and amplitude of both types of wave
Phase difference
Difference between the displacements of particles along a wave/ on different waves
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0 λ = 0o = 0π radians = in phase
0.25 λ = 90o = 0.5π radians
0.5 λ = 180o = 1π radians = in antiphase
0.75 λ = 270o = 1.5π radians
1 λ = 360o = 2π radians
In phase/ antiphase
In phase
- Parcticles oscillate in step with each other
- Both reach their maximum +ve displacement at the same time
- Phase difference of zero
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In antiphase
- Parcticles oscillate out of step with each other
- One reaches their maximum +ve displacement at the same time as the other reaches their maximum -ve displacement
- Phase difference of 180o/ 1π radians
Intensity
Radiant power passing through a surface per unit area
Intensity equation
I = P / A
- I = intensity
- P = radiant power
- A = cross-sectional area
Relationship between intensity and distance
Wave travels out from source
Radiant power spreads out
Intensity is reduced
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For a point source of a wave
...energy and power spread uniformly in all directions
......over the surface of a sphere
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I = P / 4πr2
Relationship between intensity and amplitude
↓ intensity = ↓ ampitude as energy more spread out
......1/2 amplitude = 1/2 average speed of oscillating particles
............= 1/4 kinetic energy as Ek = ½mv2
..................= 1/4 intensity as I ∝ amplitude2
Refractive index equation
n = c / v
- n = refractive index of the material
- c = speed of light through a vacuum, 3 x 108 ms-1
- v = speed of light through the material
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↑ n = ↑ refraction towards the normal
Refraction law
n1sinθ1 = n2sinθ2
- θ1 = angle between normal and incident ray
- θ2 = angle between normal and refracted ray
Total internal reflection
- Occurs at the boundary between two different media
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- Light strikes the boundary at a large angle to the normal
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- All the light is reflected back into the original medium
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- No light energy is refracted out of the original medium
Conditions for TIR
Light must be travelling through a ↑ n medium as it strikes the boundary with a ↓ n medium
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Θ < C: refraction and partial reflection
Θ = C: refraction along the boundary between the two different media
Θ > C: total internal reflection
Critical angle equation
sinC = 1 / n
Reflection
- Wave changes direction at the boundary between two different media, remaining in the original medium
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- Constant v, f and λ
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- Law of reflection: angle of incidence = angle of reflection
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- Ray shows the direction of energy transfer/ path taken by the wave
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- Wavefront is a line joining points of the wave that are in phase
Refraction
- Wave changes direction as it changes speed when passing from one medium to another
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- Always partial reflection
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- Constant f
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Refraction: ↑ v
- Wave refracts towards the normal
- ↓ λ
- Constant f
- When longitudinal waves enter a denser medium
Refraction: ↓ v
- Wave refracts away from the normal
- ↑ λ
- Constant f
- When transverse waves enter a denser medium
Diffraction
- Waves passing through a gap/ around an obstacle spread out
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- Constant v, f and λ
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- ↓ gap/ obstacle = ↑ diffraction
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- Size of gap/ obstacle = wavelength of the wave = maximum diffraction
Polarisation
Plane polarised
- Oscillations of a transverse wave confined to a single plane
- Plane of oscillation = oscillations + direction of wave travel
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Partially polarised
- Wave reflects off a surface
- More oscillations in a particular plane
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Unpolarised
- Oscillations in many possible planes
Polarising filters
Unpolarised light passing through a polarising filter becomes plane polarised
Rotating a second filter causes the intensity of light transmitted through to decrease
When the filters are perpendicular to each other, the intensity is zero
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