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Snell’s Law, Total Internal Reflection, Brewster’s Angle, Dispersion, Lenses

Physics 102: Lecture 18. Snell’s Law, Total Internal Reflection, Brewster’s Angle, Dispersion, Lenses. 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:. n 1 sin( q 1 )= n 2 sin( q 2 ).

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Snell’s Law, Total Internal Reflection, Brewster’s Angle, Dispersion, Lenses

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

  2. 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) n1 q1 q2 n2

  3. Total Internal Reflection q2 “critical angle” qr qc qi q1 Recall Snell’s Law: n1 sin(q1)= n2 sin(q2) (n1 > n2 q2 > q1 ) q1 = sin-1(n2/n1) then q2 = 90 Light incident at a larger angle will only have reflection (qi = qr) n2 n1 For water/air: n1=1.33, n2=1 q1 = sin-1(n2/n1) = 48.80 normal

  4. 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

  5. Preflight 18.1 Can the person standing on the edge of the pool be prevented from seeing the light by total internal reflection ? 1) Yes 2) No

  6. 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

  7. Brewster’s angle horiz. polarized only! qB qB horiz. and vert. polarized 90º 90º-qB Reflected light is usually unpolarized (mixture of horizontally and vertically polarized). But… 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)

  8. 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.

  9. Polarizing sunglasses (when worn by someone standing up) work by absorbing light polarized in which direction? • horizontal • vertical Preflight 18.3, 18.4 Polarizing sunglasses are often considered to be better than tinted glasses because they… • block more light • block more glare • are safer for your eyes • are cheaper

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

  11. Rainbow: Preflight 18.5 Wow look at the variation in index of refraction! Which is red? Which is blue? 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!

  12. LIKE SO! In second rainbow pattern is reversed

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

  14. Converging Lens Principal Rays Image Assumptions: • monochromatic light incident on a thin lens. • rays are all “near” the principal axis. Example F 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).

  15. 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? F F F P.A. Converging Lens All rays parallel to principal axis pass through focal point F. Double Convex F P.A. nlens > noutside • At F • Inside F • Outside F

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

  17. F P.A. Object 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.

  18. Diverging Lens Principal Rays Image Example F 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 toward F emerge parallel to principal axis. Only 1 case for diverging lens: Image is always virtual, upright, and reduced.

  19. F P.A. Object F 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.

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