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

Phys 102 recent lectures Lecture 14 – EM waves Lecture 15 – Polarization Lecture 20 & 21 – Interference & diffraction Lecture 16 – Reflection Lecture 17 – Spherical mirrors & refraction Lecture 18 – Refraction & lenses (today!) Lecture 19 – Lenses & your eye Light as a wave Light as a ray

Snell’s Law: A Quick Review n1n1 n2n2 When light travels from one medium to another the speed changes v=c/n, but the frequency is constant. So the light bends: n 1 sin(  1 )= n 2 sin(  2 ) 11 22 44

Total Internal Reflection normal 22 11 n2n2 n1n1 Recall Snell’s Law: n 1 sin(  1 )= n 2 sin(  2 ) (n 1 > n 2   2 >  1 )  1 = sin -1 (n 2 /n 1 ) then  2 = 90 cc Light incident at a larger angle will only have reflection (  i =  r ) ii rr 06 “critical angle” For water/air: n 1 =1.33, n 2 =1  1 = sin -1 (n 2 /n 1 ) =

Fiber Optics 13 Telecommunications Arthoscopy 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

Can the person standing on the edge of the pool be prevented from seeing the light by total internal reflection ? 1) Yes2) No Preflight % 43% “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.” 8

ACT: Refraction As we pour more water into bucket, what will happen to the number of people who can see the ball? 1) Increase2) Same3) Decrease 11

ACT: Refraction 11 As we pour more water into bucket, what will happen to the number of people who can see the ball? 1) Increase2) Same3) Decrease

Brewster’s angle 15 When angle between reflected beam and refracted beam is exactly 90 degrees, reflected beam is 100% horizontally polarized ! Reflected light is usually unpolarized (mixture of horizontally and vertically polarized). But… n 1 sin  B = n 2 sin (90-  B ) n 1 sin  B = n 2 cos (  B ) horiz. and vert. polarized BB BB 90º-  B 90º horiz. polarized only! n1n1 n2n2

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. 19

Polarizing sunglasses are often considered to be better than tinted glasses because they… Preflight 18.3, 18.4 block more light are safer for your eyes block more glare are cheaper When glare is around  B, it’s mostly horiz. polarized! 21 26% 9% 66% 0% Polarizing sunglasses (when worn by someone standing up) work by absorbing light polarized in which direction? horizontal vertical 53% 47%

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 =

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

LIKE SO!In second rainbow pattern is reversed 25

Flat Lens (Window) n1n1 n2n2 Incident ray is displaced, but its direction is not changed. tt 11 11 If  1 is not large, and if t is small, the displacement, d, will be quite small. d 27

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. Assumptions: monochromatic light incident on a thin lens. rays are all “near” the principal axis. Converging Lens Principal Rays F F Object P.A. Image is: real, inverted and enlarged (in this case). 35 Image

Converging Lens All rays parallel to principal axis pass through focal point F. Double Convex 30 P.A. F n lens > n outside F At F Inside F Outside F P.A. F F 62% 12% 25% 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?

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

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. 40 F F Object P.A.

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. Diverging Lens Principal Rays F F Object P.A. Only 1 case for diverging lens: Image is always virtual, upright, and reduced. 45 Image

Which way should you move object so image is real? 1)Closer to lens 2)Further from lens 3)Diverging lens can’t create real image. ACT: Diverging Lenses Demo 50 F F Object P.A.

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