Phase-contrast microscopy

Phase-contrast microscopy and the more complicated differential interference contrast microscopy may be used to visualize living cells. To alter the phase of the light waves the microscope is adapted to produce an image in which the degree of brightness of a region of the specimen based on its refractive index.

The phase of the light wave is changed according  to the refractive  index of the cell When light passes through a living cell,: light passing  through  a relatively or dense or thick part of the cell, like the nucleus, is retarded; therefore  its phase  is shifted  relative  to light  that  has  passed  through  an adjacent  thinner region   of  the  cytoplasm.   Both phase-contrast   microscopy   and, in a more complicated way, differential interference contrast microscopy (or Nomarski interference microscopy), exploit the interference effects generated when the 2 sets of light waves recombine, thus creating an image of the cell's structure.  As these types of microscopy  do not need specimens  to be ?xed or stained for  examining   the  structure   and  movement   of  larger  organelles they are  useful  (mitochondria, nucleus  etc.) in living cells.