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November 15.doc

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McGill University
PHYS 101
Kenneth Ragan

Phys 101 Alanna Houston November 15, 2007 Light Propagation: - Light usually travels in straight lines (sharpness of shadows, beams such as headlights, laser pointers etc.) - Therefore, we usually use the ray model of light o Describes reflection, refraction, and behaviour of light in lenses and mirrors o This is called geometrical optics Reflection: - Angle of incidence = angle of reflection - Angles measures from the normal to the surface - In a plane (flat) mirror: - o light rays emitted from object or reflected from object, strike the mirror, and are reflected into the observer’s eyes. She extrapolates these rays backwards and perceives an image “behind the mirror” o image called a virtual image and appears at the image distance di behind the mirror o Here the image distance di is equal to the object distance do - Example: Minimum height of mirror necessary for 1.60m person to see himself? Assume eyes are 10 cm below the top of the head. o o use 5 cm down because the angles are the same Phys 101 Alanna Houston o thetai = thetar o 1.60-0.10 = 1.50 cm o therefore, from the feet to the mirror would be 75 cm o x=160 cm – 75 cm – 5 cm therefore x = 80 cm Refraction: - the ration of speed of light in a vacuum (written as c) to the speed of light in a medium is called the index of refraction: n = c/v lightC = 3.00x10^8 m/s (speed of light in a vacuum) Therefore, the index of refraction in a vacuum is 1.0 - v light for all materials: n for all materials is larger than 1.0 - occurs when light is incident on a boundary where the index of refraction changes. - Typical indices of refraction: - - index of refraction is often a slight function of the wavelength (colour) of the light - For example, for some prism glass, n(blue) = 1.525 and n(red) = 1.514 - The large index of refraction and the large variation of n with l (dispersion) are reasons that diamonds sparkle…the large index of refraction makes total internal reflection more probably and the number of bounces inside the diamond larger - At each bounce, the difference in the colours is enhanced so the diamond ‘flashes’ with brilliance and colour - In general, there will be part of the ray that reflects and part of the ray that refracts: -
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