PHYSICSPROJECT By Prithviraj Debnath XII Kendriya Vidyalaya , Sukna Board Roll no. TOTAL INTERNAL REFLECTION . KENDRIYA VIDYALAYA .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 PRITHVIRAJ DEBNATH of class XII . SUKNA regarding his project titled “TOTAL INTERNAL REFLECTION”. Teacher’s Signature . Her valuable guidance. support and supervision all through this project are responsible for attaining its present form. .ACKNOWLEDGEME NT It would be my utmost pleasure to express my sincere thanks to my Physics teacher Miss. I would also like to thank my parents as they encouraged me to put forward my project. Rima Biswas in providing a helping hand in this project. . III. II. Total internal reflection in diamond VI.CONTENTS I. Introduction Optical description Critical angle Phase shift upon total internal reflection V. IV. Applications of total internal reflection VII. Examples in everyday life Bibliography . For example.e. . if the angle of incidence is greater (i. it will occur when passing from glass to air.INTRODUCTION Total internal reflection is an optical phenomenon that happens when a ray of light strikes a medium boundary at an angle larger than a particular critical angle with respect to the normal to the surface. If the refractive index is lower on the other side of the boundary and the incident angle is greater than the critical angle. This can only occur where light travels from a medium with a higher [n1=higher refractive index] to one with a lower refractive index [n2=lower refractive index]. The critical angle is the angle of incidence above which the total internal reflectance occurs. no light can pass through and all of the light is reflected. However. the ray is closer to being parallel to the boundary) than the critical angle – the angle of incidence at which light is refracted such that it travels along the boundary – then the light will stop crossing the boundary altogether and instead be totally reflected back internally. and partially reflected. When a light beam crosses a boundary between materials with different kinds of refractive indices. the light beam will be partially refracted at the boundary surface. but not when passing from air to glass. . and some will refract as it passes through. At the glass/air boundary of the flat surface. this will prevent refraction at the air/glass boundary of the curved surface. This is not total internal reflection. This physical property makes optical fibres useful and prismatic binoculars possible. as diamond has an unusually high refractive index. A "ray box" shines a narrow beam of light (a "ray") onto the glass. This is called total internal reflection. • If θ > θC. It is also what gives diamonds their distinctive sparkle.OPTICAL DESCRIPTION Total internal reflection can be demonstrated using a semi-circular block of glass or plastic. the ray will split. The semicircular shape ensures that a ray pointing towards the centre of the flat face will hit the curved surface at a right angle. the entire ray reflects from the boundary. what happens will depend on the angle? Where is θC the critical angle measurement which is caused by the sun or a light source (measured normal to the surface): • If θ < θC. Some of the ray will reflect off the boundary. None passes through. . . we get incidence sin θi= n2 sin θt n1 To find the critical angle. we find the value for when θt =90 ° sin θt =1 and thus of is equal to the critical angle Now. The angle of incidence is measured with respect to the normal at the refractive boundary (see diagram illustrating Snell's law). n1 sinθ i=n2 sin θt Rearranging Snell's Law. The light emanating from the interface is bent towards the glass. When the incident angle is increased sufficiently. we can solve for θi θi . and we get the equation for the critical angle: θc =θ i=sin −1 n2 n1 ( ) . The critical angle is given by Snell's law. . Consider a light ray passing from glass into air. It is at this point no light is transmitted into air.CRITICAL ANGLE The critical angle is the angle of incidence above which total internal reflection occurs.The resulting value θc . the transmitted angle (in air) reaches 90 degrees. 8 ( 1.50 ) .If the incident ray is precisely at the critical angle. If for example. the calculation would give the critical angle for light from acrylic into air.00).50) into air (with an index of refraction of 1. visible light were travelling through acrylic glass (with an index of refraction of 1.00 1. the refracted ray is tangent to the boundary at the point of incidence. which is θc =sin−1 =41. This phase shift is polarization dependent and grows as the incidence angle deviates further from the critical angle toward grazing incidence. . The polarization dependent phase shift is also the reason why TE and TM guided modes have different dispersion relations. The polarization dependent phase shift is long known and was used by Fresnel to design the Fresnel rhomb which allows transforming circular polarization to linear polarization and vice versa for a wide range of wavelengths (colours). Mathematically this means that the Fresnel reflection coefficient becomes a complex rather than a real number. in contrast to the quarter wave plate.PHASE SHIFT UPON TOTAL INTERNAL REFLECTION A lesser-known aspect of total internal reflection is that the reflected light has an angle dependent phase shift between the reflected and incident light. TOTAL INTERNAL REFLECTION IN DIAMOND From glass to air the critical angle is about 42o but it varies from one medium to another. Having a small critical angle.4o for the diamond-air boundary is extremely small. Most rays approach the diamond at angles of incidence greater than the critical angle (as it is so small) so a light ray will typically undergo TIR several times before finally refracting out of the diamond. the critical angle 24. The diagram to the left depicts the total internal reflection within a diamond gemstone with a 'strategic' and a 'non-strategic' cut. light has the tendency to become "trapped" inside of a diamond once it enters. This property of the diamond-air boundary plays an important role in the brilliance of a diamond gemstone. The material that gives the smallest critical angle is diamond. The effect can be enhanced by the cutting of a diamond gemstone with a 'strategically' planned shape. This gives diamond a tendency to sparkle. That is why they sparkle so much! Rays of light can easily be made to 'bounce around inside them' by careful cutting of the stone and the refraction at the surfaces splits the light into a spectrum of colours! Relatively speaking. . . Total internal reflection is the operating principle of automotive rain sensors. which are used in endoscopes and telecommunications. Prismatic binoculars use the principle of total internal reflections to get a very clear image. Another application of total internal reflection is the spatial filtering of light. which control automatic windscreen/windshield wipers. Optical fingerprinting devices use frustrated total internal reflection in order to record an image of a person's fingerprint without the use of ink. Gonioscopy employs total internal reflection to view the anatomical angle formed between the eye's cornea and iris. .APPLICATIONS OF TOTAL INTERNAL REFLECTION Total internal reflection is the operating principle of optical fibres. A Total internal reflection fluorescence microscope uses the evanescent wave produced by TIR to excite fluorophores close to a surface. This is useful for the study of surface properties of biological samples. . the water in the tumbler drains out leaving the glass filled with air. Refraction occurs when light travels from different mediums. While water remains both in the upturned tumbler and in the sink surrounding it.EXAMPLES IN EVERYDAY LIFE Total internal reflection can be observed while swimming. This is due to both and a mixture of both. the plug hole and plug are visible since the angle of refraction between glass and water is not greater than the critical angle. taking a glass tumbler. the tumbler now appears mirrored because light reflects off the air/glass interface. its surface appears mirrorlike. One can demonstrate total internal reflection by filling a sink or bath with water. . when one opens one's eyes just under the water's surface. and placing it upside-down over the plug hole (with the tumbler completely filled with water). If the water is calm. This is different phenomenon from reflection and refraction. and this then acts as the plug. Viewing this from above. Reflection occurs when light goes back in same medium. If the drain is opened and the tumbler is kept in position over the hole. Here both are not happening. SINGHAL. “Why toast lands jelly-side down: Zen and the art of physics demonstrations”. APC Laboratory Manual Physics for class XII by R.Another common example of total internal reflection is a critically cut diamond. NCERT Physics Textbook for class 12 Feynman Lectures on Physics Robert Ehrlich.MITTAL & S. .S. This is what gives it maximum spark BIBLIOGRAPHY Following Books were a source for my project.
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