Fiber Optics: Endoscopes to Telephonesįiber optics is one application of total internal reflection that is in wide use. A number of interesting applications of total internal reflection follow. There is no total reflection for rays going in the other direction-for example, from air to water-since the condition that the second medium must have a smaller index of refraction is not satisfied. The same calculation as made here shows that the critical angle for a ray going from water to air is 48.6 o while that from diamond to air is 24.4 o, and that from flint glass to crown glass is 66.3 o. Different combinations of materials have different critical angles, but any combination with n 1 > n 2 can produce total internal reflection. This will make the inside surface of the clear plastic a perfect mirror for such rays without any need for the silvering used on common mirrors. This means that any ray of light inside the plastic that strikes the surface at an angle greater than 42.2 owill be totally reflected. If the incident angle θ 1 is greater than the critical angle, as shown in Figure 1(c), then all of the light is reflected back into medium 1, a condition called total internal reflection. That is, θ c is the incident angle for which θ 2= 90 o. The largest the angle of refraction θ 2 can be is 90 o, as shown in Figure 1(b).The critical angle θ c for a combination of materials is defined to be the incident angle θ 1 that produces an angle of refraction of 90 o. Now imagine what happens as the incident angle is increased. Since n 1 > n 2, the angle of refraction is greater than the angle of incidence-that is, θ 1 > θ 2. If, as shown in the figure, the index of refraction for the second medium is less than for the first, the ray bends away from the perpendicular. Part of the light crosses the boundary and is refracted the rest is reflected. Interestingly, we can produce total reflection using an aspect of refraction.Ĭonsider what happens when a ray of light strikes the surface between two materials, such as is shown in Figure 1(a). But it would be useful to have a mirror that reflects all of the light that falls on it.
Explain the phenomenon of total internal reflection.
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