AREPORT 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 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” Teacher’s Signature I would also like to show my gratitude towards Madam Principal “DR. 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. I thank our physics teacher for assistance and guidance as provided by him. although they may not agree with all of the interpretations/conclusions of this project work. . SUNITA VASISHTHA” for giving me such a great valuable. interesting and knowledgeable project. about its occurrence. I acknowledge the support of my teacher Mr. mechanism. . Rajat Pareek who guided me during the investigation. 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. how did diffraction got its name from. its relation with interference and many more. types of Diffraction . 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. Table of Contents S.no Topic Page no. 1 Chapter-1 (Introduction) 2 Chapter-2 (Understanding Diffraction) 3 Chapter-3 (Experimental Analysis) 4 Report Conclusion 5 References . In classical physics. . the bending will be almost unnoticeable. What is Diffraction? “Diffraction is a slight bending of light as it passes around the edge of an object.” Diffraction refers to various phenomena that occur when a wave encounters an obstacle or a slit. and electromagnetic waves such as visible light. If the opening is much larger than the light’s wavelength. including sound waves. These characteristic behaviors are exhibited when a wave encounters an obstacle or a slit that is comparable in size to its wavelength. the diffraction phenomenon is described as the interference of waves according to the Huygens–Fresnel principle. rays and radio waves. 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. The amount of bending depends on the relative size of the wavelength of light to the size of the opening. Matter of Report Chapter-1 Introduction to Diffraction A. water waves. Diffraction occurs with all waves. he deduced that light must propagate as waves. referring to light breaking up into different directions. Explaining his results by interference of the waves emanating from the two Different slits.If we look clearly at the shadow cast by an opaque object. B. which is a general characteristic exhibited by all types of the waves. diffraction also occurs with matter and can be studied according to the principles of quantum mechanics. Since wavelength of light is much smaller than the dimensions of most of the obstacles. This is just due to the phenomenon of the diffraction. who also coined the term diffraction. there are alternate dark and bright regions. close to the region of geometrical shadow. just like in interference. 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. Thomas Young performed an experiment in 1803 demonstrating interference from two closely spaced slits. we do not generally encounter the effects of diffraction of light in the everyday life observations. Isaac Newton studied these effects and attributed them to inflexion of light rays. History of Diffraction The effects of diffraction of light were first carefully observed and characterized by Francesco Maria Grimaldi. Italian scientist Francesco Maria Grimaldi coined the word "diffraction" and was the first to record accurate observations of the phenomenon in 1660. . from the Latin diffringere. 'to break into pieces'. its effects are generally most pronounced for waves whose wavelength is roughly comparable to the dimensions of the diffracting object or slit. When Does Diffraction Occurs Diffraction occurs whenever propagating waves encounter changes. of different parts of a wave that travel to the observer by different paths. closely spaced openings. a complex pattern of varying intensity can result. The formalism of diffraction can also describe the way in which waves of finite extent propagate in free space. where different path lengths result in different phases. . If the obstructing object provides multiple. This is due to the addition. or interference.C. Hence. . The wave displacement at any subsequent point is the sum of these secondary waves. 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. Mechanism In traditional classical physics diffraction arises because of the way in which waves propagate. diffraction patterns usually have a series of maxima and minima. CHAPTER-2 UNDERSTANDING DIFFRACTION A. 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. When waves are added together. this is described by the Huygens–Fresnel principle and the principle of superposition of waves. If the incident light is coherent. 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. A slit which is wider than a wavelength produces interference effects in the space downstream of the slit.B. 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. . Such phase differences are caused by differences in the path lengths over which contributing rays reach the point from the slit. 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. The analysis of this system is simplified if we consider light of a single wavelength. these sources all have the same phase. we may expect to find minima and maxima in the diffracted light. a broad pattern with a central bright region is seen. 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. 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. the intensity becoming weaker away from the centre. 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. the greater the angle of the spread. . On both sides there are alternate bright and dark fringes and regions. If light consisted strictly of ordinary or classical particle. The smaller the slit. 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. We can find the angle at which a first minimum is obtained in the diffracted light by the following reasoning.When the double slit in young’s double slit experiment is replaced by a single narrow slit illuminated by a monochromatic source. when the path difference between them is equal to λ/2. Similarly. 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. Changes in the path lengths of both waves result in a phase shift. The modern double . the expected pattern on screen simply be the sum of the two single slit patterns.b) Double Slit Diffraction If light consisted of classical particles and we illuminated two parallel slits. the pattern changes to one with a series of light and dark bands. He believed it demonstrated that the wave theory of light was correct. A simpler form of the double-slit experiment was performed originally by Thomas Young in 1801. . creating an interference pattern. When this phenomenon was studied.slit experiment is a demonstration that light and matter can display characteristics of both classically defined waves and particles. the experiment in which a wave is split into two separate waves that later combine into a single wave. In reality however. it indicated that light consists of waves as distribution of brightness can be explained by the alternately constructive and destructive interference of wave fronts. and the light passing through the slits is observed on a screen behind the plate.In the experiment. the light is always found to be absorbed at the screen at discrete points.000 atomic mass units. illuminates a plate with two parallel slits. the interference pattern appearing via the varying density of these particle hits on the screen Other entities. are found to exhibit the same behavior when fired towards a double slit. such as electrons. as individual particles (not waves). However. although it becomes more difficult as size increases. The experiment can be done with entities much larger than electrons and photons. . 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. such as a laser beam. The wave nature of light causes the light waves passing through the two slits to interfere. a coherent light source. producing bright and dark bands on the screen. . 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.The double slit experiment for its clarity in expressing the results of quantum mechanics. green. indigo and violet (ROYGBIV). yellow. it increases in another region producing a bright fringe. producing a dark fringe. while Interference is the meeting of two waves during the diffraction process and usually happens when there are two or more slits. albeit an orange- brown color instead of the white color. These components are the colors of the rainbow red. In addition to bending. Full lunar eclipses permit light waves to bend around the edges of the moon to let the side facing earth remain visible.Red light has the longest wavelength. . blue. while violet has the shortest. This is a due to the distance of the moon from earth. allowing the moon to completely cover the sun. light is sometimes broken into its basic components. Hence there is no gain or loss of energy which is consistent with the principle of conservation of Energy. If it reduces in one region. Also in diffraction and interference. light energy is redistributed. orange. 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. d) Diffraction and Interference Diffraction is the bending of waves around an obstacle. This is why red is typically the prominent color in a rainbow and appears to be wider than violet light.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. the hologram on a book is an example. i. Iridescent clouds are a diffraction phenomenon caused by small water droplets or small ice crystals individually scattering light. This principle can be extended to engineer a grating with a structure such that it will produce any diffraction pattern desired. . iv. ii. e) Examples and Applications of Diffraction The effects of diffraction are often seen in everyday life. iii. 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 disc. The setting sun appears to be red because of the diffraction of light from the dust particle in the atmosphere. . Diffraction glasses. Light shows.v. the light bends around the water droplets causing the twinkling effect. for Fireworks. DJ/ Party Lights. As light from stars pass through the earth's atmosphere which is laden with water vapor. 3d movies. Lasers are based on diffraction. The light waves become brighter or dimmer and the colors constantly change due to constructive and destructive interference. vi. Twinkling stars are another example of diffraction of light. The speckle pattern which is observed when laser light falls on an optically rough surface is also a diffraction phenomenon. 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. using light from a compact source. it diffracts giving diffraction patterns which depend upon the type. When light passes through solid objects like diamonds.vii. shows small fringes near its edges. A shadow of a solid object. . 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. CHAPTER-3 EXPERIMENTAL ANALYSIS OF DIFFRACTION 1. Requirements: Two Razor Blade. Black Paper Procedure: a) Hold the two blades so that the edges are parallel and have a narrow slit in between. Filter. One glass electric Bulb. e) Use a filter for red and blue to make fringes clearer. they will show some colours. b) Keep the slit parallel to the filament of the bulb which plays the role of first slit. Single Slit Diffraction Aim: Experiment to study the phenomena of single slit diffraction. This can be done easily with thumb and forefingers as shown in figure. . right in front of eye. and cover them with black paper. Compare the fringes. Don’t use sunlight instead of the bulb as sun also produces infrared rays harmful to our eyes.* . Result: Fringes are wider for red compared to blue. Precaution: Protect your eyes by using spectacles while performing the experiment.Observations: Since the position of all the bands depends on wavelength so they will show some colour. *By repeating the above experiment with aluminium foil we can easily show double slit diffraction. More the wavelength. More they will diffract. com www. References www.wikipedia.youtube.com www.google.com NCERT Part II Class XII .