Waves

• A progressive (moving) wave transfers energy from one place to another without transferring any material.
• Phase is a measurement of the position of a certain point along the wave cycle
• Phase difference is the amount one wave lags behind another.
• 1. A cathode ray oscilloscope measures voltage. It displays waves from a signal generator as a function of voltage over time.
  2. The displayed wave is called a trace.
  3. Screen is split into squares called divisions.
  4. Vertical axis is in volts. The volts per division is controlled by gain dial.
  5. Horizontal axis is in seconds. Seconds per division is controlled by timebase dial
  6. Can alter gain and timebase dial to make it more easy to read measurements.
• Transverse waves oscillate perpendicular to the direction of propagation.
• Intensity is the rate of flow of energy per unit area at right angles to the direction of travel of the wave. Its measured in Wm^-2
• Intensity is proportional to amplitude squared.
• Polarising filters don't work on microwaves - their wavelength is to long. Instead metal grilles are used.
• 
  

  Polarising microwaves

  1. Place a metal grille between the microwave transmitter and reciever. (microwave transmitters only produce vertically polarised microwaves so you only need one metal grille).
  2. The intensity of microwaves passing through the grille is at maximum when the direction of the vibration of the microwaves and the wires on the grille are at right angles to eachother.
  3. As you rotate grille, intensity of polarised microwaves able to pass through grille decrease so reading on voltmeter decreases.
• Intensity drops to zero when the wires are aligned with the direction of the microwaves because the grille is absorbing their energy.
• 1. The vibrating electric field of the microwave excites electrons in the metal grille.
  2. The energy of the incoming microwaves is absorbed and re-emitted in all directions.
  3. Only a few of those re-emitted waves are vibrating in the direction of the microwave receiver.
  4. The microwave receiver only recieves microwaves in one plane, so even if re-emitted wave travels towards receiver it might not be picked up
  5. When the wires and vibrations of waves are aligned, more electrons are excited than when the grille and vibration of waves are at right angles to each other - causing the drop in intensity.
• Diffraction is the way that waves spread out as they come through a narrow gap or go round obstacles.
• Using a ripple tank to investigate diffraction

  1. Ripple tanks are shallow tanks of water that you can generate a wave in.
  2. This is done by an oscillating paddle, which continually dips into the water and creates regular waves with straight parallel wave fronts.
  3. Objects are then placed into the ripple tank to create a barrier with a gap in the middle of it.
  4. This gap can be varied to see the effects this has on how the waves spread through the tank.
  5. The similar the gap is to wavelength the more diffraction that occurs.
• When a wave meets an obstacle you get diffraction around the edges. Behind the obstacle is called a shadow. The wider the obstacle is compared to its wavelength, the less diffraction you get and so the longer the shadow.
• Reflection means the wave is bounced back when it hits a boundary. The angle of incidence always equals the angle of reflection.
• Reflection in a ripple tank

  1. Set up the ripple tank so the oscillating pas is creating straight waves with straight parallel wave fronts. Place a barrier in tank at an angle to wave fronts.
  2. Angle incoming waves make with normal to barrier is called angle of incidence.
  3. Should see waves reflecting off barrier and travelling in a different direction to when they arrived.
  4. Angle between direction of reflected waves and the normal to the barrier is called angle of refelction
  5. Can change angle of incidence to see angle of reflection changes by same amount. They are always equal.
• apertureDiffraction of light waves

  1. If wavelength of light wave is roughly similar to size of aperture, you get a diffraction pattern of light and dark fringes.
  2. Pattern has bright central fringe with alternating dark and bright fringes either side of it.
  3. The narrower the slit, the wider the diffraction pattern.
• Refraction

  Refraction is the way a wave changes direction as it enters a different medium. The change in direction is a result of the wave slowing down or speeding up.

  Speed changes because the wavelngth of the wave is changing and the frequency stays constant (v=fλ)
• If ray bends towards normal - it is slowing down. The ray is going from a less optically dense material to a more optically dense material.
• If ray bends away from normal - the wave is speeding up. It is going from an optically denser material to a less optically dense material.
• Using a ray box and glass block to investigate refraction

  1. Place a glass block on a piece of paper and draw around it.
  2. Use the ray box to shine a beam of light into the glass block. Turn off any other lights so you can see path of light beam through block clearly.
  3. Trace path of incoming and outgoing beams of light either side of block.
  4. Remove block and join up two paths with straight line.
  5. Measure the angle of incidence and refraction. As light enters glass block it bends towards normal (slowing down). Beam speeds back up once exiting the block (bending away from normal).


  Diagram of practical

  
• The absolute refractive index of a material, n, is the ratio between the speed of light in a vacuum, c, and the speed of light in that material, v. (n=c/v)
• Snells Law

  nsinθ=constant
• A refractometer can accurately measue the refractive index of a material. The machine shines a beam of light at the sample. You then view the refracted beam through a microscope and measure its angle of refraction.
• When the angle of incidence causes the angle of refraction to reach 90 degrees, we call this the critical angle. This can only happen when light goes from an optically dense material to a less optically dense material.
• At angles of incidence greater than C (critical angle), refraction is impossible. That means all the light is reflected back into the material. This effect is called total internal reflection. (sinC=1/n only works for material-air assuming material is more dense).
• Fibre optics

  • Cladding protects core and promotes TIR
  • Modal dispersion when signals arrive at differnet times.
  • Absorption is when energy is lost. Can happen when wire is bunched up as incidence angle is to small to cause TIR.
• Investigating critical angle and TIR with glass block

  1. Shine light ray into curved face of a semi-circular glass block so that it always enters at right angles to edge - this means ray won't refract as it enters block, only when it leaves straight edge.
  2. Vary angle of incidence until light beam refracts so much that it exits the block along straight edge. This angle of incidence is called critical angle for glass-air boundary.
  3. If you increase the angle of incidence so that it is larger than critical angle the ray is reflected from the straight edge of the block.
• The principal of superposition states that when two or more waves cross, the resultant displacement equals the vector sum of the individual displacements.
• Two points on a wave are in phase if they are both at the same point in the wave cycle. Points in phase have the same displacement and velocity.
• In order to get clear interference patterns the two or more sources must be coherent.
• Two sources are coherent if they have the same wavelength and frequency and a fixed phase difference between them.
• Whether you get constructive or destructive interference at a point depends on how much further one wave has travelled than the other wave to get to that point (path difference).
• Constructive interference occurs when: path difference=nλ
• Destructive interference occurs when:

  path difference = (n+1/2)λ
• Observing interference with sound waves

  1. Connect two speakers to the same oscillator (so they're coherent) and place them in line with each other.
  2. Walk slowly across the room in front of them.
  3. Will hear varying volumes of sound> At points where sound is loudest, the path difference is a whole wavelength.
  4. The sound will be quietest at points where path difference is an odd number of half wavelengths.
  5. May still hear some sound at quietest points due to sound being reflected of walls and around room.
• Young's double slit experiment

  1. Laser light is coherent and monochromatic (only one wavelength present)
  2. The slits have to be about the same size as the wavelength of the laser light so that it is diffracted - then the light from the slits act as two coherent point sources.
  3. Get a pattern of light ad dark fringes, depending on whether constructive or destructive interference is taking place.
  4. Measure fringe spacing and use λ=aX/D to find wavelength. When measuring fringe spacing X take multiple readings and divide by number of fringes to reduce percentage error
• Youngs Double-slit Formula

  λ=aX/D
• Why was Young's Experiment evidence for wave nature of light?

  1. Newton's theory suggested that light was made up of tiny particles called 'corpuscles'
  2. Huygens put forward a theory using waves.
  3. The corpuscular theory could explain reflection and refraction but diffraction and interference are both uniquely wave properties. If it could be shown light had interference patterns it would end the debate.
  4. Young's double slit experiment provided the necessary evidence. It showed light could both diffract and interfere.
• Interference patterns get sharper when you diffract through more slits.