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Light

Light. Chapter 16 & 17. What is light?. Usually we think of only visible light but visible light is just a part of the electromagnetic spectrum. Light sources: sun, flames, sparks, fireflies, glow sticks. Lamps. LEDs, lasers, TV etc., If there is NO light in a room you cannot see anything.

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Light

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  1. Light Chapter 16 & 17

  2. What is light? • Usually we think of only visible light but visible light is just a part of the electromagnetic spectrum. • Light sources: sun, flames, sparks, fireflies, glow sticks. Lamps. LEDs, lasers, TV etc., If there is NO light in a room you cannot see anything.

  3. Let’s define light • Light is the a range of frequencies in the electromagnetic spectrum that stimulate the retina of the eye. • Light waves have a λ of 4.00 x 10-7 m to 7.00 x 10-7 m ( 400-700nm) • Light travels in a vacuum, does not need medium. Travels in a straight line.

  4. Ray model of light • Light travels in a straight-line path. • That is why you see a shadow. The light in the straight path is blocked and the light does not go around.

  5. Speed of Light • Galileo was the first to guess that light has a certain speed and he tried to measure it. • Albert Michelson measured time of light to pass between 2 mountains. 1st American to receive Nobel prize in science. • Speed of light 2.99899 x 108 m/s • We just say 3.00 x 108 m/s speed of light in a vacuum. Represented by c

  6. Equation for the speed of light • c= λf • EX; What is the frequency of yellow light if it has a wavelength of 556 nm? • 3.00 x 108 m/s= 5.56 x 10-7m(f) • f=5.40 x 1014 Hz

  7. Electromagnetic spectrum

  8. Visible LIGHT White light is split into colors (rainbow). ROYGBIV Longest wavelength is red,lowest f

  9. Color White light is not a single color; it is made up of a mixture of the seven colors of the rainbow. 24 year old Issac Newton did first experiments on splitting light (Spectrum) We can demonstrate this by splitting white light with a prism: ROY G BIV This is how rainbows are formed: sunlight is “split up” by raindrops.

  10. Adding colors White light can be split up to make separate colors. These colors can be added together again. The colors of light are red, blue and green: Adding blue and red makes magenta (purple) Adding blue and green makes cyan (light blue) Adding red and green makes yellow Adding all three makes white again

  11. Color Perception

  12. Using colored light If we look at a colored object in colored light we see something different. For example, consider a soccer uniform : Shirt looks red White light Shorts look blue

  13. In different colors of light this kit would look different: Red light Shirt looks red Shorts look black Shirt looks black Blue light Shorts look blue

  14. Because light travels as wavelengths • If you change what wavelengths are reflected- you change the color. • Try this with polarized filters.

  15. Ready for some confusing terms? • Luminous Flux (P) = the rate at which visible light is emitted from a source. • Or total amount of light from source • Units for luminous flux = lumen (lm) This is over a large (usually circular area)

  16. Illuminance(E)- how much light emitted by a luminous object. Ex. How much light is on the book. (Rate light falls on a surface) • Units for Illuminance = lux (lx) = lm/m2

  17. What is the illumination of a spherical area’s surface if a 100 watt bulb is in the middle of the sphere? • 100 watt bulbs lets off 1750 lm of flux • Area of the surface is 4πr2 • At a distance of 1 m from the bulb, comparer amt of light emitted to area • 1750 lm/4πr2 m2 = 140 lx

  18. Luminousintensity • Measured in candela (cd) or candle power • What is it? It is the luminous flux (rate light comes from a source) that falls on 1 m2 of a sphere 1 m in radius. • So…. Lum flux = 1750 lm = 139 cd 4πd24πd2 • candela is SI unit for light intensity

  19. How can you increase light on a surface? • 1) Use brighter bulbs (increases lum flux –lm) • 2) Move surface closer (decreasing distance) • 3) Use twice as much candle power

  20. Summary of terms

  21. How to use this • An equation that represents this • Illuminance E = P P= lum flux (lm) • 4πd2 d = distance from surface

  22. Example Problem #1 • What is the illumination on your desktop if it is lighted by a 1750 lm lamp that is 2.50 m above your desk? 2.50 m

  23. Solution • Luminous flux, P = 1750 lm • d = 2.50 m • Illuminance, E = ? • E= P E= 1750 lm • 4πd24π(2.50)2 • 22.3 lm/m2 = 22.3 lx

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