Online+lesson+--+TIR

=Online learning --- Total Internal Reflection=

= What is total internal reflection? =

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= Live demonstration of Total Internal Reflection =

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= Applet = Click here to play with the applet



= Summary =



The maximum possible angle of refraction is 90-degrees. You will recognize that if the angle of refraction were greater than 90 degrees, then the refracted ray would lie on the incident side of the medium - that's just not possible. So in the case of the laser beam in the water, there is some specific value for the angle of incidence (we'll call it the //critical angle, c//) which yields an angle of refraction of 90-degrees. This particular value for the angle of incidence can be calculated using Snell's Law:

Given that (n, water = 1.33, n, air = 1.000, = 90 degrees,  = critical angle), using Snell's Law:

**1.33 sin = 1.00 sin 1.33 sin c = 1.00 sin 90 c = 49 degrees**

Any angle of incidence which is greater than 49 degrees would not result in refraction. Instead, when the angles of incidence is greater than 49 degrees (the critical angle), all of the energy (the **total** energy) carried by the incident wave to the boundary stays within the water ( **internal** to the original medium) and undergoes **reflection** off the boundary.
 * When this happens, total internal reflection occurs. **

= Conditions for Total Internal Reflection =

Total internal reflection (TIR) is the phenomenon which involves the reflection of all the incident light off the boundary. TIR only takes place when both of the following two conditions are met:
 * the light is in the more dense medium and approaching the less dense medium.
 * the angle of incidence is greater than the critical angle (c) of the denser medium.

= Classroom Demonstrations = = __Light Pipe__ =

A common Physics demonstration of T.I.R involves the use of a large jug filled with water and a laser beam. The jug has a pea-sized hole drilled in its side such that when the cork is removed from the top of the jug, water begins to stream out the jug's side. The beam of laser light is then directed into the jug from the opposite side of the hole, through the water and into the falling stream. The laser light exits the jug through the hole but is still in the water. As the stream of water begins to fall as a projectile along a parabolic path to the ground, the laser light becomes trapped within the water due to total internal reflection. Being in the more dense medium (water) and heading towards a boundary with a less dense medium (air), and being at angles of incidence greater than the critical angle, the light never leaves the stream of water. In fact, the stream of water acts as a light pipe to pipe the laser beam along its trajectory.

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= __Disappearing Coin__ =

Watch the video. Analyse how this is possible and discuss with your partner beside you if he thinks the same way too.

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= Quiz Time = =media type="custom" key="5742801" width="152" height="170"= = =

= = =Applications=

**__1. Optical Fiber__**
Learn more about optical fibers here. Click the tab "Exercise" on top of the applet to take a short quiz!


 * __ 2. Sparkle of Diamonds __**

Of all the possible combinations of materials which could interface to form a boundary, the combination of diamond and air provides one of the largest difference in the index of refraction values. This means that there will be a very small nr/ni ratio and subsequently a small critical angle. This peculiarity about the diamond-air boundary plays an important role in the brilliance of a diamond gemstone.

Having a small critical angle, light has the tendency to become "trapped" inside of a diamond once it enters. A light ray will typically undergo TIR several times before finally refracting out of the diamond. Because the diamond-air boundary has such a small critical angle (due to diamond's large index of refraction), most rays approach the diamond at angles of incidence greater than the critical angle. This gives diamond a tendency to sparkle. The effect can be enhanced by the cutting of a diamond gemstone with a strategically planned shape. The diagram below depicts the total internal reflection within a diamond gemstone with a strategic and a non-strategic cut.



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=__ 3. Rainbow Formation __=

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 * __

4. Mirage __**

Mirage is an optical illusion, which occurs usually in deserts on hot summer days. On such a day, temperature of air near the earth is maximum and hence is rarer or lighter. The upper layers of air, which are relatively cool, are denser. A ray of light from the top of a tree travels from denser to rarer and bend away from the normal. At a particular layer, if the angle of incidence is greater than 'c', total internal reflection occurs. To far away observer, this ray i.e., AE appears to be coming from I i.e., mirror image of O. Thus inverted image of tree creates an optical illusion of reflection from a pond of water.



**END ==> Please attempt your worksheet now. I hope you have enjoyed learning this topic.**