AQA Physics A Level definitions - Section 2 Waves and optics

• Amplitude


  The maximum displacement of an oscillating particle from its equilibrium position
• Antinode


  Fixed point on a stationary wave where amplitude is maximum
• Coherent

  When two waves have a constant phase difference and same frequency
• Critical angle

  The angle of incidence of a light ray must exceed this angle for total internal reflection to occur
• Cycle


  Interval for an oscillating particle from a certain displacement and velocity to the next time the particle has identical displacement and velocity
• Diffraction

  The spreading out of waves that pass through gap or near an edge
• Diffraction grating

  A plate with many close-ruled parallel slits on it
• Dispersion


  Splitting a beam of white light into colours by a prism
• Displacement


  Distance in a given direction from the equilibrium position
• Electromagnetic wave


  A wavepacket or photon consisting of transverse electric and magnetic waves in phase and at right angle to each other
• Endoscope


  An optical fibre device used to see inside cavities
• First harmonic


  The pattern of stationary waves on a string when it oscillates at its lowest possible frequency
• Frequency


  The number of cycles of a wave that pass a point per second
• Interference


  The formation of points of cancellation and reinforcement where two coherent waves pass through each other
• Laser


  A device that produces a coherent, parallel beam of monochromatic light
• Modal dispersion


  The lengthening of a light pulse as it travels along an optical fibre due to continual total internal reflection of waves meaning some rays travel longer distance than others in fibre
• Node

  A fixed point on a stationary wave where the amplitude is zero
• Optical fibre


  A thin, flexible transparent fibre used to carry light pulses from one end to another
• Path difference

  The difference in the distances from two coherent sources to an interference fringe
• Period


  The time for one complete cycle of a wave to pass a point
• Phase difference


  The fraction of a cycle between the oscillations of two oscillating particles, measured in radians or degrees
• Photon

  A packet or 'quantum' of electromagnetic waves
• Plane-polarised waves

  Transverse waves that oscillate in one plane only
• Progressive waves


  Waves which travel through a substance, or through space if electromagnetic
• Refraction

  The change of direction of a wave when it crosses a boundary where its speed changes
• Refractive index

  The speed of light in free space/speed of light in substance
• Spectrometer


  An instrument used to measure light wavelengths very accurately
• Stationary waves

  The wave pattern with nodes and antinodes formed when 2 or more progressive waves of same frequency and amplitude pass through each other
• Superposition


  The effect of 2 waves adding together when they meet
• Total internal reflection


  A light ray will totally internally reflect at the boundary of two substances when the boundary has a new substance with a lower refractive index, and the angle of incidence is greater than the critical angle
• Transverse waves


  A wave where direction of oscillation perpendicular to the direction of propagation of the wave
• Types of light spectra

  continuous spectrum:continuous range of colours line emission spectrum:characteristic coloured lines due to discrete frequencies and corresponding wavelengthsline absorption spectrum:dark vertical lines against continuous range of colours
• Wavefronts


  Lines of constant phase (e.g. wavecrests)
• Wavelength


  The least distance between two adjacent vibrating particles with the same displacement and velocity at the same moment in time
• Young's fringes


  The parallel bright and dark fringes observed when light from a narrow slit passes through two closely spaced slits
• X-ray wavelength

  1 nanometre