Physics 1161 Pre Lecture 18 Total Internal Reflection

Physics 1161: Pre. Lecture 18 Total Internal Reflection, Dispersion • Section 26 -8

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 qc = n 2 sin 90 o • n 1 sin qc = n 2 • sin qc = n 2 / n 1 qc = 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 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. noutside ninside Telecommunications Arthroscopy Laser surgery Total Internal Reflection only works if noutside < ninside

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. noutside ncladding ninside Add “cladding” so outside material doesn’t matter! We can be certain that ncladding < ninside

Dispersion The index of refraction n depends on color! In glass: nblue = 1. 53 nblue > nred = 1. 52 prism White light Blue light gets deflected more

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