4.1 Reflection and Refraction

4.1 Reflection and Refraction - Geometrical optics 4.1...

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Unformatted text preview: Geometrical optics 4.1 Reflection and Refraction • Geometrical Optics • Reflection • Refraction In geometrical optics light waves are considered to move in straight lines. This is a good description as long as the waves do not pass through small openings (compared to l) Christian Huygens Light waves perpendicular to wave fronts Wave front Rays are perpendicular to wave fronts Rays are not physical entities but are a convenient representation of a light wave. (surfaces with constant phase ­ e.g. maxima) Reflection • Two general types of reflection – Specular reflection – Diffuse reflection Specular reflection Flat surface Light reflected in one direction Diffuse reflection Rough surface Light reflected in all directions • Most of geometric optics deals with specular reflection. • However, most of the time ambient lighting is due to diffuse reflection. 1 Transmission and Reflection at an interface Reflection Transmission What are some examples of these processes in this picture. Specular Reflection Incident wave Absorption x Diffuse reflection (scattering) Transmission Absorption medium 1 medium 2 Law of Reflection The angle of reflection equals the angle of incidence Full length mirror A 6 ft tall man wants to install a mirror tall enough to see his whole body. How tall a mirror is needed? q1 = q ' 1 Angle of incidence Angle of reflection Reflecting surface Multiple reflections • For multiple reflections use the law of reflection for each reflecting surface. Mirror 1 2­Dimensional Corner reflector Mirror 2 Show that 2 perpendicular mirrors reflect a light beam in a plane perpendicular to both mirrors back along the opposite direction 2 Refraction • Refraction is the bending of light when it passes across an interface between two materials. • The bending is due to the differences in the speed of light in different media. Refraction and Reflection The light beam (3) is refracted at the interface. Speed of light in a medium medium vacuum Transmission across an interface The speed of the wave changes. The frequency remains the same. The wavelength changes c Index of refraction v slower n= c v Snell’s Law of Refraction n1 sin q1 = n2 sin q2 Going from air to glass n > n 2 1 q2 < q1 (sinq increases with q ) 3 Physical picture for Snell’s Law Going from glass to air n < n 2 1 q2 > q1 One end of the wave front slows down. The wave front changes direction. Example Find the angle of refraction for o an angle of incidence of 30 in going from air to glass (n glass =1.52) Example Show that light going through a flat slab is not deviated in angle. question When light goes from air into glass the angle of refraction is ______ the angle of incidence. A) greater than B) less than C) equal to Problem The cylindrical tank in a public aquarium is 10 m deep, 11 m in diameter, and full to the brim with water. If a flashlight shines on the tank from above what is the minimum angle its beam can make with the horizontal if it is to illuminate part of the tank bottom? 4 Refractive index matching • A transparent object can be made invisible if the index of refraction of the surrounding media is made the same as that of the object. 5 ...
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