Lecture_25

# Lecture_25 - 26.3 Total Internal Reflection Consider a...

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26.3 Total Internal Reflection Consider a light ray in air striking the surface of water: Part of the ray is refracted, and part is reflected. The refracted ( θ R ) and reflected ( θ r ) angles will both change if we change the incident angle ( Law of Reflection and Snell’s Law of Refraction ). Now consider what happens when a light ray emerges from a more dense medium into a less dense medium: water to air, for example. Air Water Normal Incident ray Refracted ray Reflected ray θ i θ r θ R

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Part of the ray is reflected, and part is refracted. Air Water Notice that the refracted angle is greater than the incident angle since n air < n water . Now let’s keep increasing the incident angle so that the refracted angle keeps increasing. Notice: There is a point when the refracted angle = 90 o (red ray). This occurs when θ i = θ c , where θ c is known as the critical angle . Thus, when θ i = θ c , θ refracted = 90 0 . Now, if θ i > θ c , then there is no refracted ray (purple ray)! All the incident light is completely reflected back into the medium.
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