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Refraction & Lenses

Chapter 18. Refraction & Lenses. Recognize Anyone?. In your journal, define:. Constructive interference When two waves interact to give a higher amplitude. Destructive interference When two waves interact to cancel each other out. Reflection When a wave “bounces” off a surface.

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Refraction & Lenses

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  1. Chapter 18 Refraction & Lenses

  2. Recognize Anyone?

  3. In your journal, define: Constructive interference When two waves interact to give a higher amplitude. Destructive interference When two waves interact to cancel each other out. Reflection When a wave “bounces” off a surface. Refraction A wave bends when changing media

  4. Refraction • Occurs when light hits a boundary at a non-zero angle. • Light bends or changes direction if media have differing optical densities. • Light bends towards the normal when going form less dense (air) into more dense (water). θi Normal θr Faster medium Slower medium

  5. The wagon on the pavement scenario • What happens when the wagon goes from pavement (fast) to sand (slow)? • What happens when it goes from slow to fast?

  6. Refraction - Continued • Light is refracted away from the normal when entering a medium that is less dense. θi normal θr water air

  7. Snell’s Law • Light bends such that sinθi/sinθr = constant • The constant = n = index of refraction • It is characteristic of the medium (see page 353 for values) nisinθi = nrsinθr The index of refraction for air = 1.00 (how nice is that???)

  8. Index of Refraction • n for a substance, s, = ns =c/vs • Where c is the speed of light in a vacuum • Vs is velocity of light in substance s. • Why will the index of refraction always be greater than 1 ? • Because light travels faster in a vacuum than anywhere else. • Does it make sense that n for air = 1 (or close to it)?

  9. Indices of Refraction • How does the index of refraction relate to the speed of light in a substance? • Why?

  10. Practical Application of Refraction • The next time you go spear fishing for Nemo, remember refraction!

  11. Total Internal Reflection • Remember that light traveling from more dense to less dense medium moves away from the normal. • When θr = 90°, there is no refracted ray.

  12. Total Internal Reflection (con’t) • When θr = 90° there is no refracted ray. This is total internal reflection • Critical angle is the incident angle where no refraction occurs. nisinθi = nrsinθr 90° Θi = 48.8° This is the critical angle 1.33 water air

  13. Fiber Optics • Use solid glass rod “fiber”. • Coating (cladding) with lower n value cause total internal reflection. • Total internal reflection allows light to be carried through rod. • Allows for flexible cable that can carry much more data than copper wire (i.e. arthroscopic surgery, phone lines, etc.)

  14. A Bit About Your Brain… • When light rays hit your eyes, you assume they have come to you in a straight line. • “bending light” does very strange things with your perception.

  15. Mirages • Light travels faster in hot air than cool air. • Hot road heats air directly above it. • Air further from road remains cool. • Different velocities of light in warm vs. cool air give different values for n. • Different n values means refraction occurs. Sky is reflected off road so it appears there is a puddle there.

  16. Somewhere over the… • A few bits of info: • Different frequencies of light refract differently • Dispersion is the separation of light into the spectrum by refraction.

  17. Rainbows • Think of raindrops as teeny tiny prisms in the air. Each one splits white light into the spectrum. • Location of the observer determines which color is seen coming from each drop. • Different drops at different angles give the complete spectrum.

  18. Why is the sky blue? (Actually, it’s not quite) • Remember resonance? • Vibrating atoms can resonate. • Smaller particles emit higher frequencies of light. • Sunlight hits air molecules and causes the to send out light in all directions (it’s scattered). • Of the visible frequencies, violet is scattered most (then blue, then green, etc.) • Our eyes are not as sensitive to violet so we see blue.

  19. What happens as the sun sets? • Sun’s rays go through more atmosphere and there are larger particles (due to smog, etc.) These scatter the red/orange/yellow end of the spectrum more so we see more of those colors.

  20. What about clouds? • Clouds contain water droplets and ice crystals. • These can scatter all colors of light. • When you see all colors of light, you perceive it as what color? • White  • And now for a Spencer Thomson break…

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