Transcript Refraction
Refraction Light http://arrowheadradio.com/images/fiber-optics.jpg Tends to travel in straight lines If you need to bend light or shine it into difficult-to-reach places, optical fibres can be used Fibre Optics http://static.howstuffworks.com/gif/fiber-optic-fiber-edit.jpg Fiber optics (optical fibers) are long, thin strands of very pure glass about the diameter of a human hair. Core - Thin glass center of the fiber where the light travels Cladding - Outer optical material surrounding the core that reflects the light back into the core Buffer coating - Plastic coating that protects the fiber from damage and moisture Refraction http://thescienceroom.pbworks.com/f/brokpen.jpg Light also bends when it passes from one medium into another such as from water to air The pencil appears to bend because light rays change direction as the move between mediums Refraction in a Pool http://www.wildpackets.com/images/compendium/FDRflct.gif You may have stood by a pool and seen an object on the bottom that you want to pick up When you dived in to get the object, it wasn’t where it appeared to be Why? This happens because the light rays change direction at the surface of the water as they pass between the water and the air When we view the refracted light rays we assume they have traveled in a straight line If you trace the light rays back in a straight line you will find that they don’t lead to the object How Light Refracts If light strikes a new medium at and angle it will bend Think of a light beam as the leading edge of a wave, the part that strikes the surface first slows down How Light Refracts http://optics.mbhs.edu/snells-law/image1.JPG As light enters the plastic block it slows down and refracts As light leaves the block and enters the air it speeds up and refracts Light does not refract inside the block, only when changing mediums Speed of Light http://2.bp.blogspot.com/_2ii88_lzmW0/TSSZcbuLLOI/AAAAAAAAAMM/x_iuQyjc5Ak/s1600/speed_of_light_highw ay.jpg In a vacuum, light travels at almost 300 million m/s or 3.0X108 m/s Light travels almost that fast in air The more dense the medium, the slower light can travel through it. The Index of Refraction The amount by which a transparent medium decreases the speed of light is indicated by a number called the index of refraction The larger the refractive index, the more the medium decreases the speed of light The refractive index of the speed of light in a vacuum (and approx. air) is 1.00 Calculating the Index of Refraction Index of refraction = Speed of light in a vacuum Speed of light in medium Or n = c/v The refractive index of a medium, n, is calculated by comparing the speed of light in the medium, v, with the speed of light in a vacuum, c. Practice Read example problems page 438 Do all 6 calculations page 438 Angles of Refraction Angles of refracted light rays are usually measured from the normal, drawn at 90º to the surface where the light ray crosses between two media Low to High Refractive Index When light travels from air, with a low refractive index to water, with a high refractive index, it bends toward the normal Low to High Refractive Index When light travels from water, with a higher refractive index (denser medium) to air, with a lower refractive index (less dense medium), it bends away from the normal Dispersion http://www.educationalelectronicsusa.com/l/images/light-XVa.gif The splitting of a ray into its component colours is known as dispersion of light and the band of colours is known as a spectrum Rainbows The most common type of dispersion is in the formation of a rainbow When sunlight passes through a rain drop, some of the light is refracted, once on entering and again on exiting the drop Both refractions cause separation of the white light Snell’s Law A formula that uses the index of refraction to calculate the new angle that a ray will take as a beam of light strikes the interface between two media Calculations with Snell’s Law n1 sin 1= n2 sin 2 Let the indices of the two media be n1 and n2 Let the angle of incidence be 1 Let the angle of refraction be 2 Practice Read example problems page 441 - 442 Do all 6 calculations page 441 - 442 Total Internal Reflection http://www.hsc.csu.edu.au/senior_science/core/info_systems/9_4_6/Laserpath.gif Sometimes light does not pass from one medium to another, but stays within the medium This happens in the case of fibre optics Total Internal Reflection http://labspace.open.ac.uk/file.php/6211/t305_002i.jpg Light reflects completely off the inside wall of a denser medium (higher index of refraction) rather than passing through the wall into a less dense medium (lower index of refraction) RECALL! When light passes from a denser material into a less dense material, the light refracts away from the normal The Critical Angle As the angle of incidence increases, the angle of refraction increases At the critical angle, the refracted ray of light follows a path exactly along the surface of the water Past The Critical Angle If the angle of incidence is increased past the critical angle, the light ray is reflected not refracted Mirages Both total internal reflection and refraction play a role in forming a mirage Mirages Light rays pass through layers of air with progressively lower indices of refraction, eventually the light is totally internally reflected Mirages http://www.islandnet.com/~see/weather/graphics/photos/infmirg2.gif The End