F U N N Y W A V E SPHYSICS - HARD

F U N N Y W A V E SPHYSICS - HARD

1. SNELL'S LAW

   • n = c/v
     • n = refractive index, c =speed of light, v = speed of wave in that medium
   • • n1 = refractive index of first medium, n2 = refractive index of second medium, sinϴ1 = angle of incidence, sinϴ2 = angle of refraction

2. Refraction

   • Different colours of visible light have different wavelengths and therefore refract by different amounts. Red is refracted Less than blue.

3. Critical angle

   • Waves at the interface between two media are reflected and transmitted
     • This then means, at every interface, a percentage of the wave is reflected and the corresponding percentage is transmitted
       • As we change the angle of incidence, the ratio of reflected:transmitted changes until no more is transmitted. This is the critical angle
         • If this happens inside a medium, then we call this total internal reflection
   • The critical angle is the angle of incidence at which no more light is transmitted through the interface

4. Optical Fibres

   • The refractive index of the glass core must be higher than the refractive index of the plastic cladding for total internal reflection to occur
     • This type of of optical fibre is called a step index fibre because the refractive index of the cladding is a step up from the air outside.
       • Problem: multipath dispression can occur where basically the light can take multiple paths throughout the core. To fix this the core can be made more narrow or to have the refractive index of the core vary gradually to a peak in the middle
         • Cross talk can occur when two fibre optic cables touch one another, the light would refract from one fibre to the next

5. Wave basics

   • Type
     • Electromagnetic: Do not need a medium. Examples: Micro, Radio, Infra red, visible light, ultra violet, x-rays, gamma rays
     • Mechanical: Travel through a medium. Examples: Sound (air) Water (Water) , Seismic (Earth)
     • Longitudinal: The oscillations are parallel to direction of the wave. Examples: P seismic waves, Sound waves. Longitudinal waves are a series of compressions are rarefactions. Molecules within the medium are vibrating and colliding with one another.
     • Transverse: Oscillations are perpendicular to the direction of propagation. Examples: Waves on a string/rope, S-Seismic wave, Electromagnetic waves. Oscillations are always generated from something but in transverse waves they are generated from oscillations in the electromagnetic field by oscillating charges, like electrons
   • v= fλ
   • Intensity
     • I = P/A
     • Intensity is the rate and density of energy transfer of a wave.
   • Polarisation: When the wave only osciallates in one plane/direction, perpendicular to the direction of energy propagation
   • Echo Location
     • Pulses are emitted and detected by the emitter and detector. Pulses are reflected and trasmitted, However they also scatter because of diffraction. The smaller the wavelength, the less scattering occurs, meaning a higher resolution of the detection
     • Pulse duration: Each reflection that is detected must be attributed with the initial pulse that caused it. This means the reflection must be detected before the next pulse is emitted

6. Cool facts for the kids

   • 180degress == π (radians) == ½ λ
   • Fundamental (First Harmonic) == ½ λ

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