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The Behavior of Light

The Behavior of Light. Chapter 16-1 and 16-2. Light vs. Sound. Similarities: Light and sound are 2 methods by which humans can receive information Both are waves. Light vs. Sound. Differences: Light is visible….sound is not. Light is part of the EM spectrum

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The Behavior of Light

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  1. The Behavior of Light Chapter 16-1 and 16-2

  2. Light vs. Sound • Similarities: • Light and sound are 2 methods by which humans can receive information • Both are waves

  3. Light vs. Sound • Differences: • Light is visible….sound is not. • Light is part of the EM spectrum • Sound is NOT part of the EM spectrum • Light is a TRANSVERSE wave. • Sound is LONGITUDINAL. • Light travels faster than sound • Speed of light c = 300,000,000 m/s • Speed of sound v = 343 m/s

  4. Ray Model of Light • The ray model of light says: • Light travels in a straight path • It acts as a both a wave and a particle

  5. Sunlight vs. Moonlight Differences: • Sunlight is much brighter than moonlight. • Sun is a luminous source---an object that emits (gives off) light • The moon is an illuminated source —an object that becomes visible from light reflecting off of it.

  6. Try this… • List at least two examples of each type of source: • Luminous: sun, lightbulbs, fire, fireworks, glowsticks, tv screen • Illuminated: people, paper, books, cars, etc.

  7. Illuminated Sources • Illuminated sources are visible to us because: • light is reflecting off of it or • transmitting (passing) through the object to our eyes.

  8. Categories of Media • 3 major types of media (materials): • Opaque: do not transmit light, but reflect it • Transparent: transmit light • Translucent: transmit light, but not not permit objects to be seen clearly through them.

  9. Try this… • List two examples of each type of media: (Hint: try to think of every day items) • Opaque: blankets, walls, etc. • Transparent: clear glass, plastic wrap • Translucent: frosted glass, stained glass, lampshades

  10. Quantity of Light • Luminous flux---- the rate at which light energy is emitted (given off) from a luminous source. • Measured in Lumens (lm) • Symbol = P

  11. Quantity of Light • Illuminance– the illumination of a surface or the rate at which light strikes the surface • Measured in lux (lm/m2) • Symbol = E

  12. Illuminance Equation E = P 4π r2

  13. Example (p. 436) • What is the illuminance of your desktop if it is lighted by a 1750lm lamp that is 2.5 meters above your desk? • First---what are we solving for? • Illuminance = E

  14. Example Continued…. • Make your substitutions: • E = 1750 4π (2.5)2

  15. So…the answer is…. E = 22.3 lx

  16. Try one… • A light in a classroom is located 3m above the a desk. If it emits 1300 lumens of light, what is the illuminance of the desk? Remember, you are solving for E!

  17. Answer…. E = P 4π r2 E = 1300 = 11.5 lx 4π (3)2

  18. What if you had to solve for P? • If you have to solve for P… • You will have to plug in for E and do some algebra….

  19. Example… • A lightbulb provides an illuminance of 150lx to a table that is 1.5 meters below. How much luminous flux does the bulb emit? • Try this one on your own!

  20. So we substitute… • E = P 4π r2 150 = P 4π (1.5)2

  21. And do a little algebra.. • P = 150 (4π (1.5)2) • P = 150 (28.3)

  22. To find that….. • P = 4241 lumens

  23. What is a candela? • A candela is a measurement of luminous intensity. • Luminous intensity = luminous flux 4π

  24. In some problems… • You may have to convert luminous intensity (candelas) to luminous flux (lumens). • You can use the formula on the previous slide….

  25. How many lumens? • If a bulb is said to have a luminous intensity of 80 candelas (cd), how much luminous flux (lumens) does it emit? • Try this one….

  26. Formula first, then substitute… • Luminous intensity = luminous flux 4π • 80 = luminous flux 4π

  27. Keep going… • Luminous flux = 80 (4π) • Luminous flux = 1005.3 lumens

  28. Exit Pass… • You will need a sheet of paper for this one….

  29. Exit Pass Question 1 • Explain the difference between luminous flux and illuminance. • Use complete sentences…do NOT copy the definitions directly out of the book.

  30. Exit Pass Question 2 • A ) The symbol for illuminance is _______. • B) The unit for illuminance is __________.

  31. Exit Pass Question 3 • A ) The symbol for luminous flux is _______. • B) The unit for luminous flux is __________.

  32. Exit Pass Question 4 • A spot light provides an illuminance of 200lx to a table that is 0.75 meters below. How much luminous flux does the bulb emit?

  33. Exit Pass Question 5 • A 40W bulb is said to have a luminous intensity of 45 candelas. • How much luminous flux does it produce?

  34. Homework…. • P. 436 • Practice problems # 1-6

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