3. Demo: Snell’s Law n 2 n 1 > n 2 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 ) n 1 > n 2 2 > 1 Light bent away from normal as it goes in medium with lower n 1 2 r incident reflected refracted
4. 1) Total Internal Reflection normal n 2 n 1 > n 2 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 Light incident at a larger angle will only have reflection ( i = r ) For water/air: n 1 =1.33, n 2 =1 1 = sin -1 (n 2 /n 1 ) = 48.8 0 “ critical angle” 1 2 i > c r c
5. Fiber Optics 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
6. Can the person standing on the edge of the pool be prevented from seeing the light by total internal reflection ? 1) Yes 2) No Preflight 18.1
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8. 2) Brewster’s angle 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 90º – B 90º horiz. polarized only! n 1 n 2
9. 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.
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11. 3) Dispersion Prism Blue light gets deflected more n blue > n red The index of refraction n depends on color! In glass: n blue = 1.53 n red = 1.52 White light blue < red red i blue
12. Skier sees blue coming up from the bottom (1) , and red coming down from the top (2) of the rainbow. Rainbow: Preflight 18.5 Wow look at the variation in index of refraction! Which is red ? Which is blue ? Blue light is deflected more!
13. LIKE SO! In second rainbow pattern is reversed
14. 4) Lenses Focal point determined by geometry and Snell’s Law : n 1 sin( 1 ) = n 2 sin( 2 ) Converging lens: – Rays parallel to P.A. converge on focal point Diverging lens: – Rays parallel to P.A. diverge as if emerging from focal point behind lens Larger n 2 /n 1 = more bending, shorter focal length. Smaller n 2 /n 1 = less bending, longer focal length. n 1 = n 2 => No Bending, f = infinity F “ Plano-convex” “ Plano-concave” P.A. F P.A.
15. Converging & Diverging Lenses Converging lens: – Rays parallel to P.A. converge on focal point Diverging lens: – Rays parallel to P.A. diverge as if emerging from focal point behind lens “ Plano-convex” “ Plano-concave” Converging = fat in the middle Diverging = thin in the middle “ Double concave” “ Double convex” = = = = “ Convex-concave” “ Concave-convex”
16. 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. Converging Lens Principal Rays F F Object P.A. Image is: real, inverted and enlarged (in this case). Example Image Key assumptions: • monochromatic light incident on a thin lens. • rays are all “near” the principal axis.
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18. 3 Cases for Converging Lenses This could be used in a camera. Big object on small film Inverted Reduced Real Past 2F This could be used as a projector. Small slide on big screen Inverted Enlarged Real Between F & 2F This is a magnifying glass Upright Enlarged Virtual Inside F Object Image Image Object Image Object
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20. 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 . Example Image
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Editor's Notes
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Demo 281: Snell’s Law
Demo 281: Snell’s Law Only possible if n1>n2
ACT, then demo 1129 (ball in tub of water)
ACT, then demo 664
refraction, reflection, refraction
Start this by :35 Lens in water has larger focal length since n 2 /n 1 is smaller!
Start this by :35 Lens in water has larger focal length since n 2 /n 1 is smaller!