Waves

Waves

1. Nature of Waves

   • A progressive wave(moving wave) carries energy from one place to another without transferring any material
   • A wave is caused by something making particles or fields oscillate at a source
   • Waves can be reflected, refracted or diffracted
     • Reflection: the wave is bounced back when it hits a boundary
     • Refraction: the wave changes direction as it enters a different medium
     • Diffraction: the wave spreads out as it passes through a gap or round an obstacle
   • f=1/T
   • c=d/t
     • c=fλ

2. Transverse and Longitudinal

   • In transverse waves the oscillations are perpendicular to the direction of travel
     • eg. Electromagnetic waves, water waves, s-waves
   • In longitudinal waves the oscillations are parallel to the direction of travel
     • eg. Sound waves, p-waves
     • They consist of compressions and rarefractions

3. Polarisation

   • Transverse waves are plane polarised if the vibrations stay in one plane only
     • If the vibrations change from one plane to another, the waves are unpolarised
     • Polarisation doesn't occur in longitudinal waves
   • If we want the wave to be vertical, the polarised film will be vertically, similarly, if we want the wave to be horizontal, the polarised film will be horizontally
   • Polarisation is used for TV and radio signals as well as glare reduction
     • TV signals are polarised by the orientation of the rods on the transmitting aerial. To recieve a strong signal, you have to line up the rods on the receiving aerial with the rods on the transmitting aerial

4. Refractive Index

   • The refractive index of a material is the ratio between the speed of light in a vacuum and the speed of light in that material
     • n=c/cs
   • Light goes fastest in a vacuum and slows down in other materials because it interacts with the particles in them
     • The more optically dense a material is, the more light slows down when it enters it. The optical density of a material is measured by its refractive index
   • The law of refraction states that the angle of incidence in material 1 equals the angle of refraction in material 2
     • n1sinƟ1=n2sinƟ2

5. Critical angle and TIR

   • Total internal reflection (TIR): When all light is completely reflected back into a medium at a boundary with another medium, instead of being refracted. it only happens at angles of incidence greater than the critical angle
   • Critical angle is the angle of incidence at which the angle of refraction is 90°
     • sinƟc=n2/n1
       • n1sinƟ1=n2sinƟ2
         • n1=1/sinƟc
   • Optical fibres are very thin flexible tubes of glass or plastic fibre that carries light signals over long distnaces and round corners using TIR
     • The core of an optical fibre has a high refractive index (it is optically dense) but is surrounded by cladding with a lower refractive index (less optically dense)

6. Superposition and Interference

   • The superposition of two or more waves can result in interference. Interference can either be constructive or destructive
     • Constructive and Destructive Interference
       • Destructive interference is when two waves interfere to make a wave with a reduced displacement
         • If two waves with equal and opposite displacement meet (i.e a crest and a trough with equal magnitudes), they cancel each other out completely-This is called total destructive interference
       • Constructive interference is when two waves interfere to make a wave with a larger displacement
   • The principle of superposition says that when two or more waves cross, the resultant displacement equals the vector sum of the individual displacements
     • "Superposition" means "one thing on top of another thing"