1. Single slit
  2. Seeing the single slit diffraction pattern
  3. Resolving power of optical instruments
  4. The validity of ray optics

What is diffraction ?

Phenomenon of bending of light at sharp edges.


Image result for single slit diffraction

Intensity distribution of  and photograph of fringes due to diffraction at single slit

Path difference for diffraction by a single slit

LQ = diffracted wave front

  • Path difference between the two edges of the slit =   NP – LP = NQ
  • NQ = a sin theta roughly equal to a.theta, because the angle is very small, sin theta = theta
  • M2P – M1P = y. theta
  • At central point, C on the screen, the angle theta = 0
  • At central point, there is no path difference and the intensity is high.
  • Secondary maxima at

  • Minima  at

  • When n = 1,

  • Let us divide the slit into two equal halves, LM and MN
  • Full gap of the slit, LN = a
  • LM or MN = a/2
  • The path difference between M1 and M2  at P = M2P – M1P

  • Hence, there is a phase difference of 180 degree.
  • Contributions from the two halves of the slit cancel each other.
  • Intensity falls to zero for


  • with the increase of n, the maxima become weaker.


Image result for single slit diffraction experiment

replace w by a in the diagram.

a = width of the slit



  • Ability to produce distinctly separate images of two objects very close together.
  • entering the objective lens

  • lambda = wavelength of light used to illuminate the object