Waves

• A progressive wave is one that transfers energy without transferring material
• Transverse waves vibrate particles at right angles to the direction of energy transfer
• Longitudinal waves vibrate along the direction of energy transfer
• A polarised wave oscillates in only one direction. Only transverse can be polarised.
• Light waves are a mixture of electric and magnetic fields vibrating. Polarising filters will polarise light waves so they only vibrate in one direction. If two polarising filters are at right angles to each other no light passes through.
• Superposition occurs when waves pass through each other. As the waves cross, their displacements are combined and this causes different interference patterns.
• Destructive interference occurs when two antiphase waves meet and cancel each other out. Constructive interference occurs when two in-phase waves meet each other and double the amplitude of the original wave.
• To achieve interference patterns two sources must be coherent. Coherence is when the two sources have the same wavelength and frequency, as well as a fixed phase difference.
• Path difference is the extra distance which a wave has travelled compared to the reference wave, to get to the same point.
• • Constructive interference will occur at a point where path difference = $n\lambda$ . This is because the two waves are in phase at that point.
  • Destructive interference will occur when path difference = $(n + \frac{1}{2})\lambda$. This is because the two waves are out of phase at that point.
• A stationary wave is formed by the superposition of two progressive waves with same frequency/wavelength, moving in opposite directions. This only occurs when the resonant frequency is achieved.
• Total internal reflection only occurs when:

  • The angle of incidence is greater than the critical angle
  • The incident light ray is 'entering' a less optically dense medium ($n_2 <n_1$)
• Modal/multipath dispersion:

  Light rays can diverge/reflect different amounts in an optic fibre, so will reach their destination at different times. This is called pulse broadening
• Pulse broadening can be reduced by using a thinner core (less divergence) or adding repeaters to re transmit pulses
• Young's double slit experiment: A coherent light source is passed through two slits. The result is a screen showing bright and dark fringes, with a central maxima, with equally spaced maxima either side. Maxima are equally thick.
• Increasing the width of diffraction gaps in youngs double slit experiment will affect the separation of maxima, but not the width of them.
• Single slit experiment:

  Using monochromatic, coherent light, a pattern is formed where the width of the central maxima is double the width of other maxima. Light intensity diminishes either side of the central maxima, equally spaced still.

  Using white light, the maxima are spectrums, with central maximum being white light (still double width), and maxima either side showing red light on the outer edge, and blue light on the inside (diminishing intensity)
• Diffraction gratings show clearly defined maxima (orders), with a finite number of orders being observed.
• Youngs double slit equation is only accurate when the distance from the slits to the screen (D) is much larger than the distance between slits (s)