Physics 102: Lecture 18 Snell’s Law, Total Internal Reflection, Brewster’s Angle, Dispersion, Lenses 1

Snell’s Law: A Quick Review When light travels from one medium to another the speed changes v=c/n, but the frequency is constant. So the light bends: n1 sin(q1)= n2 sin(q2) ACT: n1 q1 q1 < q2 sinq1 < sinq2 n1 > n2 Demo 281: Snell’s Law 1) n1 > n2 2) n1 = n2 3) n1 < n2 q2 n2 Compare n1 to n2. 44

Total Internal Reflection Recall Snell’s Law: n1 sin(q1)= n2 sin(q2) (n1 > n2  q2 > q1 ) q1 = sin-1(n2/n1) then q2 = 90 “critical angle” q2 Light incident at a larger angle will only have reflection (qi = qr) n2 n1 qr Demo 281: Snell’s Law Only possible if n1>n2 qi qc q1 normal 06

Snell’s Law Practice Example Usually, there is both reflection and refraction! A ray of light traveling through the air (n=1) is incident on water (n=1.33). Part of the beam is reflected at an angle qr = 60o. The other part of the beam is refracted. What is q2? 1 r n1 n2 normal 48

Snell’s Law Practice Example Usually, there is both reflection and refraction! A ray of light traveling through the air (n=1) is incident on water (n=1.33). Part of the beam is reflected at an angle qr = 60o. The other part of the beam is refracted. What is q2? q1 =qr =60o 1 r sin(60o) = 1.33 sin(q2) n1 q2 = 40.6o Question/ACT: is total internal reflection possible? n2 normal 48

Preflight 18.1 Can the person standing on the edge of the pool be prevented from seeing the light by total internal reflection ? “There are millions of light ’rays’ coming from the light. Some of the rays will be totally reflected back into the water, but most of them will not.” 1) Yes 2) No 51% 49% 10 8

Green Flash See http://mintaka.sdsu.edu/GF/ for all you could ever want to know about green flashes For students looking for an honors project, this would be an interesting topic see me if you are interested Tell Hawaii story

ACT: Refraction As we pour more water into bucket, what will happen to the number of people who can see the ball? 1) Increase 2) Same 3) Decrease ACT, then demo 1129 (ball in tub of water) 11

ACT: Refraction As we pour more water into bucket, what will happen to the number of people who can see the ball? 1) Increase 2) Same 3) Decrease 11

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 Arthoscopy Laser surgery Total Internal Reflection only works if noutside < ninside 13

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 Fiber optics are used for probing and micro-surgery. Cladding lets optics be used to probe tissues with variable indices of refraction. Demo 289 ninside Add “cladding” so outside material doesn’t matter! We can be certain that ncladding < ninside 13

horiz. and vert. polarized Brewster’s angle Reflected light is usually unpolarized (mixture of horizontally and vertically polarized). But… horiz. and vert. polarized qB 90º-qB 90º horiz. polarized only! n1 n2 When angle between reflected beam and refracted beam is exactly 90 degrees, reflected beam is 100% horizontally polarized ! n1 sin qB = n2 sin (90-qB) n1 sin qB = n2 cos (qB) 15

ACT: Brewster’s Angle When a polarizer is placed between the light source and the surface with transmission axis aligned as shown, the intensity of the reflected light: (1) Increases (2) Unchanged (3) Decreases T.A. ACT, then demo 664 19

Preflight 18.3, 18.4 Polarizing sunglasses are often considered to be better than tinted glasses because they… 27% 62% 9% 1% block more light block more glare are safer for your eyes are cheaper When glare is around qB, it’s mostly horiz. polarized! Polarizing sunglasses (when worn by someone standing up) work by absorbing light polarized in which direction? horizontal vertical 60% 40% 21

Dispersion The index of refraction n depends on color! In glass: nblue = 1.53 nred = 1.52 nblue > nred prism White light Blue light gets deflected more 23

Wow look at the variation in index of refraction! Rainbow: Preflight 18.5 Wow look at the variation in index of refraction! Which is red? Which is blue? refraction, reflection, refraction Skier sees blue coming up from the bottom (1), and red coming down from the top (2) of the rainbow. Blue light is deflected more! 25

In second rainbow pattern is reversed LIKE SO! In second rainbow pattern is reversed 25

Flat Lens (Window) Incident ray is displaced, but its direction is not changed. n2 n1 q1 q1 Start this by :35 d If q1 is not large, and if t is small, the displacement, d, will be quite small. t 27

Converging Lens Principal Rays Example F Image P.A. Object F 1) Rays parallel to principal axis pass through focal point. 2) Rays through center of lens are not refracted. 3) Rays through F emerge parallel to principal axis. Image is: real, inverted and enlarged (in this case). Assumptions: • monochromatic light incident on a thin lens. • rays are all “near” the principal axis. 35

Converging Lens Preflight 18.6 All rays parallel to principal axis pass through focal point F. Double Convex Preflight 18.6 A beacon in a lighthouse produces a parallel beam of light. The beacon consists of a bulb and a converging lens. Where should the bulb be placed? P.A. F F nlens > noutside P.A. F 53% 20% 27% At F Inside F Outside F 30

3 Cases for Converging Lenses Object Image Past 2F Inverted Reduced Real This could be used in a camera. Big object on small film Between F & 2F Image Object Inverted Enlarged Real This could be used as a projector. Small slide on big screen Demo 71 Image Object Inside F Upright Enlarged Virtual This is a magnifying glass 40

ACT: Converging Lens Which way should you move object so image is real and diminished? (1) Closer to lens (2) Further from lens (3) Converging lens can’t create real diminished image. F Object P.A. Demo 71 40

Diverging Lens Principal Rays Example F P.A. Image Object F 1) Rays parallel to principal axis pass through focal point. 2) Rays through center of lens are not refracted. 3) Rays toward F emerge parallel to principal axis. Only 1 case dir dirverging lens: Image is always virtual, upright, and reduced. 45

ACT: Diverging Lenses Which way should you move object so image is real? Closer to lens Further from lens Diverging lens can’t create real image. F Object P.A. Demo 50

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