Thursday, 21 May 2009

Physics notes– total internal reflection

Total internal reflection By Shell's Law, we know that the value of sin x can't be larger than 1. But if one side of the equation is larger than the refractive index on another side, we'll found that another side of the equation can never equal to another side, and the total internal reflection occur. It's a “reflection” so that it obeys the law of reflection. For example, the light strike from glass (R.I. 1.7) to air, 1.7sin (angle of incidence) =1sin (angle of refraction), if the angle of incidence is big enough, equation can’t hold. The “big enough” limit is called the critical angle, and c=sin^-1 (1/x). It follows that when the R.I. is bigger, critical angle is smaller and total internal reflection is easier to happen. This explained why it mostly happen in medium with higher R.I.. Applications: 1) Views under water – when you're under water, you may not see the image above water if the angle of incidence is big enough, and the range that you can see image above water is in cone shape, with the axis from your eyes, perpendicular to the water surface. (This is because the angle of incidence is limited.) 2) Periscope – If we use glass with enough big R.I.(bigger than 1.5, usually, is enough), we can make two 45-90-45 triangular prisms to reflect light just like plane mirrors. 3) Diamond – it's so bright because it's R.I. is too high (above 2) such that all light (especially after process) will be reflected back. 4) “Cat's eye” in front of the car uses the same rule to give out strong light. Optical fibres Optical fibre is a tube-like device which can transmit light. It's made by glass and some other material, layer by layer. Since total internal reflection always occur inside the tube, and the tube can be bent into a certain angle, it can transmit light in the curved path. There is various application of optical fibre. The most important use is the usage on telecommunication services. Comparing with the traditional wire, it's much cheaper since the traditional wire uses a lot of copper. Even the material cost is more expensive than the production cost of optical fibre considering the same volume. Another important use is endoscope for surgery. ->"optical fibre" as extention reading , here!

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