Waves

Waves

1. The Nature of Waves

   • A progressive (moving) wave carries energy from one place to another without transferring any material.
     • Transfers energy away from the source so the source loses energy eg. electromagnetic waves causing things to heat up
     • Caused by a source making particles or fields vibrate
   • Waves experience...
     • Refraction
     • Reflection
     • Diffraction
   • Measuring waves
     • DISPLACEMENT is how far a point on the wave has moved from its undisturbed position
     • AMPLITUDE is the maximum displacement from undisturbed position, ie. at a peak or trough
     • WAVELENGTH is the length of one whole wave cycle
     • PERIOD is the time taken for one whole wave cycle
     • FREQUENCY is the number of wave cycles passing a point in one second.
   • Equations

2. Transverse and Longitudinal Waves

   • TRANSVERE - vibration is at right angles to direction of travel
     • EG. Electromagnetic waves, S-waves, ripples on water
     • Can be shown on displacement/time or displacement/distance graphs
     • Can be polarised
   • LONGITUDINAL - vibrations are parallel to direction of travel
     • EG. sound waves, P-waves
     • Hard to plot graphically, as when they are plotted they look like transverse waves
   • Transverse waves with oscillations in many directions can be POLARISED.
     • A polarising filter filters out all but one of the oscillation directions
       • Polarised waves have oscillations in one directions ONLY
       • If a second filter is then used at right angles to the first polarising filter, no light can pass through
     • Used for glare reduction. Light reflected off Tarmac, glass and water is partially polarised so using polarising filters in sunglasses and cameras, only one oscillation of light comes through and glare is reduced.
     • TV aerials are all horizontal so that the receiving aerial is aligned with the transmitting aerial. This polarises them to receive the strongest signals.

3. Refractive Index

   • The more OPTICALLY DENSE a material is, the more light slows down when it enters it.
     • More optically dense, higher refractive index
   • Refractive index is the ratio between the speed of light in a vacuum and the speed of light in the material.
   • Refractive index of air = 1
   • Can use the LAW OF REFRACTION to calculate angles as light passes between substances
     • Angle of Incidence = θ1
     • Angle of Refraction = θ2
     • Law of Refraction: n1sinθ1=n2sinθ2
   • Refraction happens as light changes direction when it changes speed
     • Less optically dense >>> more optically dense TOWARDS THE NORMAL
     • More optically dense >>> less optically dense AWAY FROM THE NORMAL

4. Critical Angle and TIR

   • If the angle of incidence is at the CRITICAL ANGLE, the angle of refraction is 90° (it refracts along the boundary)
   • Refractive index of air is 1, so critical angle formula for material to air boundary becomes: n1=1/sinθc
   • If the angle of incidence is larger than the critical angle, the light undergoes TOTAL INTERNAL REFLECTION
   • An OPTICAL FIBRE is a thin, flexible tube of plastic or glass that can carry signals long distances and round corners using TIR.
     • The CORE has a high refractive index
     • The CLADDING has a low refractive index
     • The fibre is NARROW so that the light hits the boundary between the core and the cladding at an angle of incidence greater than the critical angle.
     • Shape of fibre does not matter
     • Used for phone and cable TV signals
     • BENEFITS: Light has high frequency so the fibre can carry more information, light doesn't heat up the fibre so almost no energy is lost as heat, no electrical interference, cheaper to produce, signals travels a long way very quickly.
     • A digital signal in a fibre can suffer some degradation.
       • Absorption and scattering by the material and light escaping from the fibre can cause the amplitude to be reduced. This can be reversed by a signal booster.
       • A signal that travels straight down the fibre takes less time than one which reflects off the sides. This means the signal is broadened and less clear.

5. Superposition and Interference

6. Stationary Waves

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