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G.K.BHARAD INSTITUTE OF ENGINEERING Total internal reflection Sub : PHYSICS Sub code: 2110011 Prepared by: AKABARI VIRAL M. 130590107002 TOTAL INTERNAL REFLECTION The picture shows the dim image of a boy formed image by a glass. The glass surface is not smooth enough. Only very small amount of light is reflected by the glass. The environment outside is not dark enough. Light ray should undergo refraction instead of reflection. INTRODUCTION The inside surface of water or a glass block can act like a mirror. 1 INTERNAL REFLECTION A light ray hits the inside face of a semicircular block as follows. air glass What will happen? 1 INTERNAL REFLECTION For a small angle of incidence • The incident ray splits into 2 rays. • angle of refraction < 90o air glass incident ray reflected ray 1 INTERNAL REFLECTION • as angle of incidence , angle of refraction • eventually, angle of refraction = 90 • angle of incidence = critical angle C air glass incident ray refracted ray C C reflected ray 1 INTERNAL REFLECTION • angle of incidence continues to , no refracted ray air glass incident ray reflected ray 1 INTERNAL REFLECTION • Therefore, if angle of incidence > C, the light ray is totally reflected inside. air glass incident ray reflected ray • This is called total internal reflection. CRITICAL ANGLE AND REFRACTIVE INDEX air glass refracted ray C C incident ray o sin 90 n= sin C 1 n= or sin C reflected ray C= sin1 (1) n CRITICAL ANGLE AND REFRACTIVE INDEX • critical angles of different materials Refractive Critical Medium index angle Glass 1.50–1.70 30–42 Water 1.33 49 Perspex 1.5 42 Diamond 2.42 24 EXAMPLES OF TOTAL INTERNAL REFLECTION a Prisms in optical instruments • plane mirror = glass + silvered surface glass sheet silvered surface forms the image • multiple reflection inside the glass • multiple images formed O I1 object • nuisance in optical instruments I2 I3 • If light rays strike the inside face at an angle > 42, glass prism behaves like a perfect mirror. 45 45 45 45 45 45 • Prisms are used as mirrors in periscopes and binoculars. • In single-lens reflex camera mirror film A five-sided ‘pentaprism’ reflects light from the mirror into the eye. 3 EXAMPLES OF TOTAL INTERNAL REFLECTION b Mirage In a desert. On a dry road. 3 EXAMPLES OF TOTAL INTERNAL REFLECTION b Mirage • On a hot day, you may see a pool of water at a distance in a desert. Hurray! Sorry! What you see is just a MIRAGE. • It happens due to total internal reflections. Image of the sky layers of air near the ground – hot – less dense than upper air – lower refractive indices Image of the sky • light refracted more and more towards the horizontal • When light meets a layer of air near the ground at an angle greater than C, Image of the sky Total internal reflections occurs. 3 EXAMPLES OF TOTAL INTERNAL REFLECTION c Light guides • Light can pass through a curved perspex rod! • The rays bounce inside the rod. • None of them is lost to the air from the sides. Applications 1 optical fibre cable • made up of a bundle of glass fibres bundle of fibres glass fibre 2 as a ‘light guide’ for the doctor to see inside the human body THANKING YOU