Diffraction

1. ROBERT ESHUN
S.L.T Department
Accra Polytechnic
Chapter 6
Diffraction

2. 2
 Diffraction
 The spreading or bending of waves as they pass
through an aperture or round the edge of a barrier.
 The change in direction of waves that occurs when
they pass the edge of an obstacle or through a
narrow opening.
 Diffraction of light takes place only if the size of
an obstacle is comparable to the wavelength of
light.

3. 3

4. 4

5. 5
 Examples
 CD or DVDs: closely spaced tracks act as
diffraction grating to form rainbow pattern seen
when looking at a disk.
 Sound waves: Sound can bend around corners.
Sound can be heard even though a listener may not
be in line of sight of the source.
 TV and radio reception (such as mobile phones):
Diffraction around large hills reduces so people
below the hill are unable to receive the signal.
Telcos set up more transmitters for their customers
to receive the service.

6. 6
 In Fraunhoffer diffraction:
 The source and the screen are far away from each
other.
 Incident wave fronts on the diffracting obstacle are
plane.
 Diffracting obstacle give rise to wave fronts which
are also plane.
 Plane diffracting wave fronts are converged by
means of a convex lens to produce diffraction
pattern.

7. 7
 In Fresnel diffraction:
 Source and screen are not far away from each
other.
 Incident wave fronts are spherical.
 Wave fronts leaving the obstacles are also
spherical.
 Convex lens is not needed to converge the
spherical wave fronts.

8. Diffraction by a single slit
8
d
d/2
d/2
θ
1
2
3
4
5

9. 9
d/2
x
ø

10. 10
Therefore the path difference x is:

11. 11
 If this path difference is half of a wavelength,
destructive interference occurs, the waves
annihilate each other and a dark fringe is obtained.

12. 12
 Dividing the slit in 4, a dark fringe is obtained when
 Dividing the slit in 6, a dark fringe is obtained when
 Dividing the slit in 8, a dark fringe is obtained when
 Generally, a dark fringe is obtained when
where

13. Diffraction by a circular aperture
(Circular hole)
13
 Airy’s disc - circular disc surrounded by much
fainter concentric circular rings
 First dark ring is formed an angle θ, such that
 d - diameter of aperture,
 d << l, l being the perpendicular distance from the
screen to the hole

14. 14
 Radius of first dark ring
 If diffracted light is converged with a convex lens
onto screen placed at its focal length, f, radius of
the first dark ring is