Total Internal Reflection, Dispersion Section 26-8 Physics 1161: PreLecture 24

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

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?

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

Water Critical Angle

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

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

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

Dispersion Blue Bends Best

Different Frequencies Different Indices of Refraction

Refractive Index Function of Wavelength

Table of Indices

Refraction & Reflection in a Raindrop

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.

LIKE SO!In secondary rainbow pattern is reversed