Waves

Waves

1. Nature of Waves

   • Parts to know
     • Displacement- distance
     • Amplitude- Max magnitude
     • Wavelength- length of a cycle
     • Period- time for a cycle
     • Frequency- cycles per second
     • Phase- the position of a point in a cycle
   • Frequency is the inverse of the period
     • Frequency= 1/period
     • 1Hz= 1s(-1)
   • Wave equation links speed, frequency and wave length
     • Wave speed(v) = Distance (d) / Time(T)
     • Speed(v) = frequency(f) x wavelength(λ)
       • V= f λ

2. Types of Waves

   • Transverse
     • All EM waves
     • Can be plotted on a graph
       • Displacement-Distance
       • Displacement-Time
       • Both usually look simular
   • Longitudinal
     • Most common is sound
     • They consist of compressions and rarefactions
     • Rarely plotted on a graph
   • BOTH can be reflected or refracted

3. Polarisation of Waves

   • A polarised wave can only oscillate in one direction
     • EG horizontally OR vertically
   • You can only polarise transverse waves
   • Polarising filters only transmit vibrations in one direction
   • Reflected light is partially polarised

4. Superposition and Coherence

   • Occurs when 2/more waves pass each other
     • The displacements combine from the waves
     • Superposition means one thing on top of another
   • Interference can be constructive or deconstructive
     • when waves superpose, the effect is interference
     • A crest plus a crest gives a larger crest...a trough plus a trough gives a larger one. (This is constructive)
     • A crest plus an equal trough give nothing (This is deconstructive)
     • Two points are in phase if they are at equal points on a wave
     • To get interference sources must be coherent
       • They need the same wavelength/frequency
   • Interference depends on path differance
     • 'how far one wave has travelled compared to another'
     • CONSTRUCTIVE- Path Difference= n λ (where n is an integer)
     • DECONSTRUCTIVE- Path Difference= (n+0.5) λ

5. Stationary Waves (Standing)

   • The superposition of 2 progressive waves, same wave length but opposite directions
   • No Energy is transmitted
   • The wave is reflected back and forth
   • They form oscillating loops separated by nodes

6. Refractive Index

   • This occurs when a wave passes into a different medium
   • Light can be slowed or sped up
     • FASTER- Light bends away from the normal
     • SLOWER- Light bends towards normal
   • Refractive index is how much light is slowed
     • n= c/v
       • N- refractive index of material
       • C= ratio of light in a vacuum to air
         • C= 3 x 10(8) ms(-1)
       • V= Speed of light in that material

7. Lenses

   • Converging
     • Bulge outwards
     • Focal length is always positive
   • Diverging
     • Cave inwards
     • Focus is behind the lens
     • Focal length is negative
   • Images can be real or virtual
     • A real image is in front of the lens
     • Virtual is behind the lens
     • 1/f = 1/u + 2/v
       • f- focal length
         • u- distance between object and lens axis
         • v- distance between image and lens axis
   • Power
     • How much is bends light
     • P= 1/f
     • For very close lenses total power is just the sum of powers
     • Magnification= image height/object height

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