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Electromagnetic Waves Physics 202 Professor Vogel Lecture 13 Reflection and Refraction Angles measured from normal !! Angle of incidence (q1) Angle of reflection (q1’) Angle of refraction (q2) Law of Reflection q1’ = q1 Law of Refraction n2 sin q2 = n1 sin q1 General Cases n2 > n1 Light is bent towards the normal q2 < q1 e.g. air to glass n2 < n1 Light is bent away from the normal q2 > q1 e.g. glass to air Index of Refraction Every material has an index of refraction that determines its optical properties Indices of refraction determine bending, by law of refraction Index of refraction also determines the speed of light within the material v=c/n generally, v<c Consequences of Laws Law of Reflection The angle of reflection is equal to the angle of incidence (q1’ = q1) Light changes direction when it reflects but our vision expects light to travel on a straight line we see images where light appears to come from Plane Mirror q1’ = q1 we see images where light appears to come from Consider how light from your eye reflects from a mirror to get to your eye, so you can see it repeat for light from your feet Images in Plane Mirror Upright Same distance behind mirror as object is in front Generally “front” is side light comes from. Same size as object Left and right are reversed. Cardinal directions aren’t reversed. Images in Plane Mirror Based on depth perception. Where do both the purple and the green rays appear to come from? Images in 2-D corner Reflector Can see an image in each mirror Light can also reflect from both mirrors, then back to you. You see image of image 1 Also see image of image 2 Both should be same place Images in 2-D corner Reflector Image of image reflected twice left and right exchanged twice If you are righthanded, so is that image! Images in Curved Mirrors Concave mirror think “cave” like bowl of spoon image is upright and larger – if object is close image is inverted and smaller – if object is far Convex mirror like back of spoon image is upright and smaller – any distance Consequences of Laws Law of Refraction n2 sin q2 = n1 sin q1 Light changes direction when it refracts but our vision expects light to travel on a straight line we see images where light appears to come from Demo Chromatic Dispersion The index of refraction depends on the wavelength of light In general, n is larger for shorter wavelengths Blue light bent more than red Incident white light is spread out into its constituent colors Chromatic dispersion with raindrops causes rainbows Chromatic Dispersion