A REPORT ON INVESTIGATORY PROJECT
“PHENOMENA OF DIFFRACTION OF LIGHT”
(Session 2017-18)
Submitted To:
Submitted By:
MR. RAJAT PAREEK
PRAKHAR GUPTA XII-A 32
CENTRAL ACADEMY SENIOR SECONDARY SCHOOL NEAR WATER TANK, AMBABARI, JAIPUR, RAJASTHAN (INDIA)
CERTIFICATE
This is hereby to certify that, the original and genuine investigation work has ha s been carried out to investigate about the subject matter and the related data collection and investigation has been completed solely, sincerely, and satisfactorily by Prakhar Gupta of class XII A, Central Academy regarding his Investigatory Project Report entitled “Phenomena of Diffraction of Light” Light ”
Teacher’s Signature
ACKNOWLEDGEMENT
This research was supported by our respected Physics Teacher MR. RAJAT PAREEK. I thank my friends and my teachers who provided their help and expertise that greatly assisted
the
research,
although
they
may
not
agree
with
all
of
the
interpretations/conclusions of this project work.
I thank our physics teacher for assistance and guidance as provided by him.
I
would
also
like
to
show
my
gratitude
towards
Madam
Principal
“DR. SUNITA VASISHTHA” for giving me such a great valuable, interesting and knowledgeable project.
PREFACE
It is a matter of great pleasure for me to present my investigatory report on topic entitled “Diffraction of Light”. During my investigation I came to know about the various phenomena of diffraction of light like about the discovery of diffraction, how did diffraction got its name from, about its occurrence, mechanism, types of Diffraction , its relation with interference and many more. My investigation included understanding based on real life examples which helped me to understand the above listed topics easily, as well as some experiments which made me more clear about the topic. I acknowledge the support of my teacher Mr. Rajat Pareek who guided me during the investigation.
Table of Contents
S.no
Topic
1
Chapter-1 (Introduction)
2
Chapter-2 (Understanding Diffraction)
3
Chapter-3 (Experimental Analysis)
4
Report Conclusion
5
References
Page no.
Matter of Report Chapter-1 Introduction to Diffraction
A. What is Diffraction?
“Diffraction is a slight bending of light as it passes around the edge of an object. The amount of bending depends on the relative size of the wavelength of light to the size of the opening. If the opening is much larger than the light’s wavelength, the bending will be almost unnoticeable.”
Diffraction refers to various phenomena that occur when a wave encounters an obstacle or a slit. It is defined as the bending of light around the corners of an obstacle or aperture into the region of geometrical shadow of the obstacle.
In classical physics, the diffraction phenomenon is described as the interference of waves according to the Huygens – Fresnel principle. These characteristic behaviors are exhibited when a wave encounters an obstacle or a slit that is comparable in size to its wavelength. Diffraction
occurs
with
all
waves,
including sound
waves, water
and electromagnetic waves such as visible light, ra ys and radio waves.
waves,
If we look clearly at the shadow cast by an opaque object, close to the region of geometrical shadow, there are alternate dark and bright regions, just like in interference. This is just due to the phenomenon of the diffraction, which is a general characteristic exhibited by all types of the waves.
Since wavelength of light is much smaller than the dimensions of most of the obstacles, we do not generally encounter the effects of diffraction of light in the everyday life observations. However the finite resolution of our eye or of the optical fiber instruments such as telescopes or microscopes is limited due to the phenomenon of diffraction
Since physical objects have wave-like properties, diffraction also occurs with matter and can
be
studied
according
to
the
principles
of quantum
mechanics.
Italian
scientist Francesco Maria Grimaldi coined the word "diffraction" and was the first to record accurate observations of the phenomenon in 1660.
B. History of Diffraction
The effects of diffraction of light were first carefully observed and characterized by Francesco Maria Grimaldi, who also coined the term diffraction, from the Latin diffringere, 'to break into pieces', referring to light breaking up into different directions. Isaac Newton studied these effects and attributed them to inflexion of light rays. Thomas Young performed an experiment in 1803 demonstrating interference from two closely spaced slits. Explaining his results by interference of the waves emanating from the two Different slits, he deduced that light must propagate as waves.
C. When Does Diffraction Occurs
Diffraction occurs whenever propagating waves encounter changes, its effects are generally most pronounced for waves whose wavelength is roughly comparable to the dimensions of the diffracting object or slit. If the obstructing object provides multiple, closely spaced openings, a complex pattern of varying intensity can result. This is due to the addition, or interference, of different parts of a wave that travel to the observer by different paths, where different path lengths result in different phases. The formalism of diffraction can also describe the way in which waves of finite extent propagate in free space.
CHAPTER-2 UNDERSTANDING DIFFRACTION
A. Mechanism
In traditional classical physics diffraction arises because of the way in which waves propagate;
this
the Huygens – Fresnel
is
described
by
principle and
the principle of superposition of waves. The propagation of a wave can be visualized by considering every particle of the transmitted medium on a wave front as a point source for a secondary spherical wave. The wave displacement at any subsequent point is the sum of these secondary waves. When waves are added together, their sum is determined by the relative phases as well as the amplitudes of the individual waves so that the summed amplitude of the waves can have any value between zero and the sum of the individual amplitudes. Hence, diffraction patterns usually have a series of maxima and minima.
B. Types of Diffraction a) Single-slit diffraction
A long slit of infinitesimal width which is illuminated by light diffracts the light into a series of circular waves and the wave front which emerges from the slit is a cylindrical wave of uniform intensity. A slit which is wider than a wavelength produces interference effects in the space downstream of the slit. These can be explained by assuming that the slit behaves as though it has a large number of point sources spaced evenly across the width of the slit. The analysis of this system is simplified if we consider light of a single wavelength. If the incident light is coherent, these sources all have the same phase. Light incident at a given point in the space downstream of the slit is made up of contributions from each of these point sources and if the relative phases of these contributions vary by 2π or more, we may expect to find minima and maxima in the diffracted light. Such phase differences are caused by differences in the path lengths over which contributing rays reach the point from the slit.
When the double slit in young’s double slit experiment is replaced by a single narrow slit illuminated by a monochromatic source, a broad pattern with a central bright region is seen. On both sides there are alternate bright and dark fringes and regions, the intensity becoming weaker away from the centre.
We can find the angle at which a first minimum is obtained in the diffracted light by the following reasoning. The light from a source located at the top edge of the slit interferes destructively with a source located at the middle of the slit, when the path difference between them is equal to λ/2.
Similarly, the source just below the top of the slit will interfere destructively with the source located just below the middle of the slit at the same angle. Along the entire height of the slit, the condition for destructive interference for the entire slit is the same as the condition for destructive interference between two narrow slits a distance apart that is half the width of the slit. If light consisted strictly of ordinary or classical particle, and these particles were fired in a straight line through a slit and allowed to strike a screen on the other side we would expect to see a pattern corresponding to the size and shape of the slit. However when the single slit experiment is actually performed the pattern on the screen is a diffraction pattern in which the light is spread out. The smaller the slit, the greater the angle of the spread.
b) Double Slit Diffraction
If light consisted of classical particles and we illuminated two parallel slits, the expected pattern on screen simply be the sum of the two single slit patterns. In reality however, the pattern changes to one with a series of light and dark bands. When this phenomenon was studied, it indicated that light consists of waves as distribution of brightness can be explained by the alternately constructive and destructive interference of wave fronts. The modern double - slit experiment is a demonstration that light and matter can display characteristics of both classically defined waves and particles. A simpler form of the double-slit experiment was performed originally by Thomas Young in 1801. He believed it demonstrated that the wave
theory
of
light was
correct,
the
experiment in which a wave is split into two separate waves that later combine into a single wave. Changes in the path lengths of both waves result in a phase shift, creating an interference pattern.
In the experiment, a coherent light source, such as a laser beam, illuminates a plate with two parallel slits, and the light passing through the slits is observed on a screen behind the plate. The wave nature of light causes the light waves passing through the two slits to interfere, producing bright and dark bands on the screen. However, the light is always found to be absorbed at the screen at discrete points, as individual particles (not waves), the interference pattern appearing via the varying density of these particle hits on the screen Other entities, such as electrons, are found to exhibit the same behavior when fired towards a double slit. The experiment can be done with entities much larger than electrons and photons, although it becomes more difficult as size increases. The largest entities
for
which
the
double-slit
experiment has been performed were molecules that each comprised 810 atoms, whose total mass was over 10,000 atomic mass units.
The double slit experiment for its clarity in expressing the results of quantum mechanics. Because
it
demonstrates
the
fundamental
limitation of the ability of the observer to predict
experimental
results,
Richard
Feynman called it "a phenomenon which is impossible to explain.
c) Diffraction Events
The amount of bending which occurs is based on the wavelength of the light or the objects size in relation to light's wavelength. In addition to bending, light is sometimes broken into its basic components. These components are the colors of the rainbow red, orange, yellow, green, blue, indigo and violet (ROYGBIV).Red light has the longest wavelength, while violet has the shortest. This is why red is typically the prominent color in a rainbow and appears to be wider than violet light. Full lunar eclipses permit light waves to bend around the edges of the moon to let the side facing earth remain visible, albeit an orange brown color instead of the white color. This is a due to the distance of the moon from earth, allowing the moon to completely cover the sun. d) Diffraction and Interference
Diffraction is the bending of waves around an obs tacle, while Interference is the meeting of two waves during the diffraction process and u sually happens when there are two or more slits. Interference of the light waves with each other causes the diffracted light to become brighter or dimmer during the diffraction process because of what we call destructive and constructive interference. Also in diffraction and interference, light energy is redistributed. If it reduces in one region, producing a dark fringe, it increases in another region producing a bright fringe. Hence there is no gain or loss of energy which is consistent with the principle of conservation of Energy.
e) Examples and Applications of Diffraction
The effects of diffraction are often seen in everyday life. i.
The closely spaced tracks on a CD or
DVD act as a diffraction grating to form the familiar rainbow pattern seen when looking at a di sc.
ii.
This principle can be extended to engineer a grating with a structure such
that
it
diffraction
will
produce
pattern
any
desired;
the hologram on a book is an example.
iii.
Iridescent clouds are a diffraction phenomenon caused by small water droplets or small ice crystals individually scattering light.
iv.
The setting sun appears to be red because of the diffraction of light from the dust particle in the atmosphere.
v.
DJ/ Party Lights, Diffraction glasses, for Fireworks, Light shows, 3d movies, Lasers are based on diffraction.
vi.
Twinkling stars are another example of diffraction of light. As light from stars pass through the earth's atmosphere which is laden with water vapor, the light bends around the water droplets causing the twinkling effect. The light waves become brighter or dimmer and the colors constantly change due to constructive and destructive interference.
vii.
When light passes through solid objects diffracts
like
diamonds,
giving
it
diffraction
patterns which depend upon the type, nature and shape of the material.
Diffraction in the atmosphere by small particles can cause a bright ring to be visible around a bright light source like the sun or the moon. A shadow of a solid object, using light from a compact source, shows small fringes near its edges. The speckle pattern which is observed when laser light falls on an optically rough surface is also a diffraction phenomenon.
CHAPTER-3 EXPERIMENTAL ANALYSIS OF DIFFRACTION 1. Single Slit Diffraction Aim: Experiment to study the phenomena of single slit diffraction. Requirements: Two Razor Blade, One glass electric Bulb, Filter, Black Paper
Procedure:
a) Hold the two blades so that the edges are parallel and have a narrow slit in between. This can be done easily with thumb and forefingers as shown in figure, and cover them with black paper. b) Keep the slit parallel to the filament of the bulb which plays the role of first slit, right in front of eye. c) Adjust the width of the slit and the parallelism of the edges the pattern the pattern of light and dark bands is visible. d) As the position of the bands (except the central one) depends on the wavelength, they will show some colours. e) Use a filter for red and blue to make fringes clearer, Compare the fringes.
Observations: Since the position of all
the bands depends on wavelength so they will show some colour. More the wavelength, More they will diffract. Result: Fringes are wider for red
compared to blue. Precaution: Protect your eyes by using spectacles while performing the experiment.
Don’t use sunlight instead of the bulb as sun also produces infrared rays harmful to our eyes.
*By repeating the above experiment with aluminium foil we can easily show double slit diffraction.*
References
www.google.com www.wikipedia.com www.youtube.com NCERT Part II Class XII
