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# A Clear View

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### A Clear View

1. 1. A Clear View Glass and Light May 29, 2009
2. 2. To Review Homework... <ul><li>Mark work with the answers provided... </li></ul>
3. 3. A Clear View... <ul><li>Light travels at 300,000,000 m/s in a vacuum </li></ul><ul><li>It travels nearly as fast in air </li></ul><ul><li>It slows down when it enters a denser transparent material </li></ul>
4. 4. Refraction
5. 5. Refraction Refraction is when waves ____ __ or slow down due to travelling in a different _________. A medium is something that waves will travel through. When a ruler is placed in water it looks like this: In this case the light rays are slowed down by the water and are _____, causing the pen to look odd. The two mediums in this example are ______ and _______. Words – speed up, water, air, bent, medium
6. 6. Wave diagrams of Refraction Waves change direction when changing speed Waves are slowed down when entering glass
7. 7. Refractive Index <ul><li>The refractive index is linked to: </li></ul><ul><li>the direction that light travels between 2 transparent substances </li></ul><ul><li>The speed the light travels in each substance </li></ul>
8. 8. Consider a glass block <ul><li>Snell’s Law: </li></ul><ul><li>Refractive Index of Material = Sin (Angle of Incidence)* </li></ul><ul><li>Sin (Angle of Refraction) </li></ul><ul><li>Also equal to: Speed of light in air </li></ul><ul><li>Speed of light in Material </li></ul><ul><li>*Angle of Incidence measured in air or vacuum </li></ul>
9. 9. Refraction through a glass block: Angle of Refraction <ul><li>Using Snell’s Law, find: </li></ul><ul><li>The refractive Index of the glass block </li></ul><ul><li>The speed of light in the block </li></ul>Angle of Incidence
10. 10. Using the Critical Angle… 1) Ray gets refracted 4) Ray gets internally reflected 3) Ray still gets refracted (just!) 2) Ray still gets refracted THE CRITICAL ANGLE
11. 11. The Critical Angle <ul><li>At the Critical Angle: </li></ul><ul><ul><li>Sin (i) = 90 degrees </li></ul></ul><ul><ul><li>Snell’s Law - Sin I / Sin r = n </li></ul></ul><ul><ul><li>As Sin 90 = 1 </li></ul></ul><ul><ul><li>And </li></ul></ul><ul><ul><li>Sin r = Sin (critical angle) </li></ul></ul><ul><ul><li>Sin (Critical Angle) = 1 / n or </li></ul></ul><ul><ul><li>n = 1 / Sin CA </li></ul></ul>
12. 12. Homework <ul><li>Use page 95 to summarise the following: </li></ul><ul><ul><li>Why fibre optic are used instead of copper wires for long distance communications </li></ul></ul><ul><ul><li>Draw a diagram to show how the signal intensity can change with distance - give numerical examples </li></ul></ul>