Concepts
 Single slit
 Seeing the single slit diffraction pattern
 Resolving power of optical instruments
 The validity of ray optics
What is diffraction ?
Phenomenon of bending of light at sharp edges.
SINGLE SLIT
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.
replace w by a in the diagram.
a = width of the slit
RESOLVING POWER OF AN OPTICAL INSTRUMENT
 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
Telescope