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Total Internal Reflection, Dispersion Section 26-8 Physics 1161: PreLecture 24
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Internal Reflection When light travels from a slow medium to a fast medium, the angle of refraction is larger than the angle of incidence. When the angle of refraction should be 90 o or greater, there is total internal reflection. The incident angle which predicts a 90 o refraction angle is called the critical angle. Applet
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Critical Angle n 1 sin c = n 2 sin 90 o n 1 sin c = n 2 sin c = n 2 / n 1 c = sin -1 (n 2 / n 1 ) What is the critical angle for a water-air boundary?
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Internal Reflection All rays reflect internally, but the top three rays reflect only a small percentage internally; most energy leaves the prism. The fourth and fifth rays are reflected 100 % internally
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Water Critical Angle
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Fiber Optics Telecommunications Arthroscopy Laser surgery Total Internal Reflection only works if n outside < n inside At each contact w/ the glass air interface, if the light hits at greater than the critical angle, it undergoes total internal reflection and stays in the fiber. n inside n outside
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Fiber Optics At each contact w/ the glass air interface, if the light hits at greater than the critical angle, it undergoes total internal reflection and stays in the fiber. We can be certain that n cladding < n inside n inside Add “cladding” so outside material doesn’t matter! n cladding n outside
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Dispersion prism White light Blue light gets deflected more n blue > n red The index of refraction n depends on color! In glass: n blue = 1.53n red = 1.52
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Dispersion Blue Bends Best
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Different Frequencies Different Indices of Refraction
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Refractive Index Function of Wavelength
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Table of Indices
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Refraction & Reflection in a Raindrop
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Rainbow Formation An observer sees red light coming from droplets of water higher in the sky. Droplets of water lower in the sky send violet light to the eye.
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LIKE SO!In secondary rainbow pattern is reversed
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