- 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.
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