(A) FRESNEL DIFFRACTION
3.1. Diffraction
Diffraction is an ordinary property of each wave (mechanical or electromagnetic). In
our daily experience, we find that if there is an obstacle in the path of
bendat the corners of obstacle. The sound of a person speaking in one sound waves, thev
in the adjacent room. The bending of'sound waves at the room is easily heard
of sound. corners, is called the diffraction
Similarly, if there is an opaque obstacle or an aperature in between the screen
light source, its distinct image is obtained on the screen. This and the
straight lines. But if the size of the obstacle or aperture is of theconcludes that light travels in
order of wavelength of light
(nearly10 m), light deviates from its rectilinear path at the corners of obstacle or aperture
(i.e., it bends at the corners) and some light reaches in the
where there should be complete darkness. Apart from this, geometrical shadow part as well
of light outside the geometrical shadow region, but there isthere is no uniform illumination
a definite manner depending upon the nature and size the distribution of intensity in
of the obstacle (or aperture). This is
called the diffraction of light waves.
The diffraction of light waves can be
Experiment
demonstrated by the following experiments :
(1)A plate of glass is first blackened with smoke and then a
Now the filament of a bulb kept at somne distance is seen through thenarrow slit is drawn on it.
slit, we find the filament
tospread with other filaments in an order of successive
decreasing intensity on either side of it.
(2) If light from a distant hole is seen by keeping a
wesee number of holes, in place of a single hole, with thin piece of cloth near the eye,
either side of the central hole. intensity successively decreasing on
(3) Diffraction at astraight edge In Fig. 3.1 (a), S is a slit and MN is a
The slit S is illuminated with a monochromatic light source. AD is a screen.
sharp edge kept in
, Diffraction 149
between the slit and the screen. The slit S, screen MN and the straight edge AD, all are
perpendicular to the plane of paper. According to the rectilinear propagation of light, the
part MP of the screen must be completely illuminated since light rays can directly fall on
this part, while the umbra part PN must becompletely dark since light rays coming to this
part are obstructed by the face AD of the edge. The result obtained in an actual experiment
is represented in Fig. 3.1 (b). Experimentally we find that
(i) In the part PN of the screen
(where there must be complete MILLUMINATED MILLUMINATED
REGION REGION
darkness) there is no complete FRINGES
darkness. But the intensity of light
gradually decreases from the point
Pto the point Q and then only after
the point Q, complete darkness is
obtained. S P P
REGION
(ii) In the part PM of the scre DARK INTENSITY REGION
DARK
(where there must be a complete
illumination), the alternate bright D
and dark fringes are obtained above
the point P. N
(iii) As we move from the point a) EXPERIMENTAL (b) INTENSITY
P towards the point M, we find that ARRANGEMENT DISTRIBUTION CURVE
the intensity of light at the successive
bright fringe decreases, while the Fig. 3.1. Diffractionat astraightedge
intensity of light at the successive dark fringe increases. Apart from this, the distance
between the two consecutive fringes decreases.
(iv) In the part PM, after some fringes there is aperfect uniform illumination on the screen.
We conclude from this experiment that light does not travel exactly ina straight line,
otherwise the part PN of the screen would have been completely dark. Hence light waves
bend at the corners of the sharp edge, changing their direction of propagation. This bending
of light at the corners is called the difraction.
, Distinction between the Interference and Diffraction
No. Interference Diffraction
Interference takes place due to Diffraction takes place due to the
the superposition of two separate super-position of secondary wavelets
wavefronts obtained from the two obtained from the different points of
coherent sources. a single wavefront.
2. Interference fringes are generally of|The diffraction fringes are not of equal
equal width. width.
3. In interference pattern, all the bright| In diffraction pattern, the intensity at the
fringes have the same intensity successive bright fringe decreases and
F(a, + a,)']while all dark fringes also the intensity at the successive dark
have the same intensity [-(a, -a) fringes.
3.2. Fresnel's and Fraunhofer's Class Diffraction
The phenomenon of diffraction is divided mainly in the
(1) Fresnel's diffiraction, and following two classes:
(2) Fraunhofer's diffraction.
Distinction between the Fresnel and Fraunhofer diffraction
The main differences in Fresnel and
Fraunhofer diffraction are :
(i) In Fresnel'sdiffraction, either thesource of light or
pattern is seen) or both the source and screen are at a finitethe screen (where the diffraction
aperture, while in Fraunhofer's diffraction, both the sourcedistance
of light
from the obstacle or
and the screen are
effectively at an infinite distance from the obstacle oraperture.
(ii) In Fresnel's class, the incident wavefront may be plane, spherical or cylindrical
(Fig. 3.3),while in Fraunhofer'sclass, the incident
wavefront is always plane.
APERTURE
INCIDENT WAVE FRONT
A DIFFRACTED SCREEN
WAVE FRONT
S
B
Fig. 3.3. Fresnel'sdiffraction
3.1. Diffraction
Diffraction is an ordinary property of each wave (mechanical or electromagnetic). In
our daily experience, we find that if there is an obstacle in the path of
bendat the corners of obstacle. The sound of a person speaking in one sound waves, thev
in the adjacent room. The bending of'sound waves at the room is easily heard
of sound. corners, is called the diffraction
Similarly, if there is an opaque obstacle or an aperature in between the screen
light source, its distinct image is obtained on the screen. This and the
straight lines. But if the size of the obstacle or aperture is of theconcludes that light travels in
order of wavelength of light
(nearly10 m), light deviates from its rectilinear path at the corners of obstacle or aperture
(i.e., it bends at the corners) and some light reaches in the
where there should be complete darkness. Apart from this, geometrical shadow part as well
of light outside the geometrical shadow region, but there isthere is no uniform illumination
a definite manner depending upon the nature and size the distribution of intensity in
of the obstacle (or aperture). This is
called the diffraction of light waves.
The diffraction of light waves can be
Experiment
demonstrated by the following experiments :
(1)A plate of glass is first blackened with smoke and then a
Now the filament of a bulb kept at somne distance is seen through thenarrow slit is drawn on it.
slit, we find the filament
tospread with other filaments in an order of successive
decreasing intensity on either side of it.
(2) If light from a distant hole is seen by keeping a
wesee number of holes, in place of a single hole, with thin piece of cloth near the eye,
either side of the central hole. intensity successively decreasing on
(3) Diffraction at astraight edge In Fig. 3.1 (a), S is a slit and MN is a
The slit S is illuminated with a monochromatic light source. AD is a screen.
sharp edge kept in
, Diffraction 149
between the slit and the screen. The slit S, screen MN and the straight edge AD, all are
perpendicular to the plane of paper. According to the rectilinear propagation of light, the
part MP of the screen must be completely illuminated since light rays can directly fall on
this part, while the umbra part PN must becompletely dark since light rays coming to this
part are obstructed by the face AD of the edge. The result obtained in an actual experiment
is represented in Fig. 3.1 (b). Experimentally we find that
(i) In the part PN of the screen
(where there must be complete MILLUMINATED MILLUMINATED
REGION REGION
darkness) there is no complete FRINGES
darkness. But the intensity of light
gradually decreases from the point
Pto the point Q and then only after
the point Q, complete darkness is
obtained. S P P
REGION
(ii) In the part PM of the scre DARK INTENSITY REGION
DARK
(where there must be a complete
illumination), the alternate bright D
and dark fringes are obtained above
the point P. N
(iii) As we move from the point a) EXPERIMENTAL (b) INTENSITY
P towards the point M, we find that ARRANGEMENT DISTRIBUTION CURVE
the intensity of light at the successive
bright fringe decreases, while the Fig. 3.1. Diffractionat astraightedge
intensity of light at the successive dark fringe increases. Apart from this, the distance
between the two consecutive fringes decreases.
(iv) In the part PM, after some fringes there is aperfect uniform illumination on the screen.
We conclude from this experiment that light does not travel exactly ina straight line,
otherwise the part PN of the screen would have been completely dark. Hence light waves
bend at the corners of the sharp edge, changing their direction of propagation. This bending
of light at the corners is called the difraction.
, Distinction between the Interference and Diffraction
No. Interference Diffraction
Interference takes place due to Diffraction takes place due to the
the superposition of two separate super-position of secondary wavelets
wavefronts obtained from the two obtained from the different points of
coherent sources. a single wavefront.
2. Interference fringes are generally of|The diffraction fringes are not of equal
equal width. width.
3. In interference pattern, all the bright| In diffraction pattern, the intensity at the
fringes have the same intensity successive bright fringe decreases and
F(a, + a,)']while all dark fringes also the intensity at the successive dark
have the same intensity [-(a, -a) fringes.
3.2. Fresnel's and Fraunhofer's Class Diffraction
The phenomenon of diffraction is divided mainly in the
(1) Fresnel's diffiraction, and following two classes:
(2) Fraunhofer's diffraction.
Distinction between the Fresnel and Fraunhofer diffraction
The main differences in Fresnel and
Fraunhofer diffraction are :
(i) In Fresnel'sdiffraction, either thesource of light or
pattern is seen) or both the source and screen are at a finitethe screen (where the diffraction
aperture, while in Fraunhofer's diffraction, both the sourcedistance
of light
from the obstacle or
and the screen are
effectively at an infinite distance from the obstacle oraperture.
(ii) In Fresnel's class, the incident wavefront may be plane, spherical or cylindrical
(Fig. 3.3),while in Fraunhofer'sclass, the incident
wavefront is always plane.
APERTURE
INCIDENT WAVE FRONT
A DIFFRACTED SCREEN
WAVE FRONT
S
B
Fig. 3.3. Fresnel'sdiffraction
