Refraction and diffraction


In 3AB, only qualitatively. Quantitatively is out of the syllabus.

  • Refraction is the bending of a wave when it enters a medium with a different wave speed
  • When light moves from a fast medium to a slow medium, it bends towards the normal
  • When light moves from a slow medium to a fast medium, it bends away from the normal


From fast to slow, to the normal we go


Total internal reflection
  • Occurs when
  • The wave is going from a slower medium to a faster one
  • the angle of incidence exceeds the critical angle
  • The wave gets reflected rather than refracted
  • If angle of incidence is equal to the critical angle, the wave refracts at 90o to the normal


  • The bending of a wave around small objects
  • The spreading out of a wave beyond small openings


  • Significant diffraction occurs when the wavelength is the same order of magnitude as the width of the opening or obstacle
  • The greater the wavelength, the greater the diffraction
  • The smaller the obstacle or opening, the greater the diffraction







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