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Chap 16-1

Chap 16-1. Light Fundamentals. What is Light?. A transverse electromagnetic wave. What is Light?. A small part of the electromagnetic spectrum The part which stimulates the retina of the human eye Visible light has wavelengths 400-700 nm. RED Longer λ Lower f Lower E. BLUE

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Chap 16-1

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  1. Chap 16-1 Light Fundamentals

  2. What is Light? • A transverse electromagnetic wave

  3. What is Light? • A small part of the electromagnetic spectrum • The part which stimulates the retina of the human eye • Visible light has wavelengths 400-700 nm RED Longer λ Lower f Lower E BLUE Shorter λ Higher f Higher E

  4. How does light interact with matter? • Light does not require matter (no medium) for transmission BUT if light does pass through matter • Light travels in a straight line through a • Vacuum or a • Uniform medium

  5. Ray Model • A ray is a straight line that represents the path of a narrow beam of light • ignores the wave nature of light • a useful model in analyzing reflection and refraction (ray diagrams)

  6. Speed of Light • Before 17th century: light thought to be instantaneous • Galileo: thought speed of light was finite but could not measure it • Ole Roemer (~1700) • through measurements of the period of Io, a moon of Jupiter, • Calculated that light took 22 min to cross the diameter of Earth’s orbit • Calculated speed of light at 2.2 x 108 m/s (75% of true value)

  7. Roemer’s Method for calculating the speed of light:

  8. Albert Michelson Michelson (1926) made a more precise measurement for speed of light, c with the Michelson-Morely Experiment:

  9. Michelson • Won the Nobel Prize • Successfully measured the speed of light • c = 3.00 x 108 m/s in a vacuum • this is a “defined value” for light • Now objects lengths are determined by how long it takes for light to travel from one end to the other • Examples: • Definition of the meter • Light year

  10. Sources of Light • A luminous body emits light waves. • An illuminated body reflects light waves produced by an outside source. • Incandescent: light produced by a hot body.

  11. Luminous Flux • The rate at which visible light is emitted by a source • Represented by the letter P • The unit is the lumen, abbreviated lm • A typical 100 W light bulb emits 1750 lm

  12. Illuminance • The rate at which light falls on a surface • Represented by the letter E • The unit is lumens per square meter, lm/m2 = lux, abbreviated lx

  13. Consider a 100-W light bulb in the middle of a sphere. What is the illumination (illuminance) of the sphere?

  14. Note that illumination is proportional to 1/r2 Inverse-Square Relationship: Light from a point source spreads out over an area proportional to the square of the distance from the source:

  15. Luminous Intensity • the luminous flux that falls on 1 m2 of a sphere 1m in radius. • Unit: candela, cd which is the SI unit of light intensity. • Luminous Intensity = P/4 π

  16. A lamp is moved from 30 cm to 90 cm above the pages of a • book. Compare the illumination on the book before and after • the lamp is moved.

  17. 7. What is the illumination on a surface 3.0 m below a 150 Watt incandescent lamp that emits a luminous flux of 225 lm?

  18. A 64 cd point souce of light is 3.0 m above the surface • of a desk. Wht is the illumination on the desk’s surface in lux?

  19. The illumination on a tabletop is 20 lx. The lamp providing • the illumination is 4.0 m above the table. • What is the intensity of the lamp?

  20. A public school law requires a minimum illumination of • 160 lx on the surface of each student’s desk. An architect’s • specifications call for classroom lights to be located 2.0 m • above the desks. What is the minimum luminous flux the • lights must deliver?

  21. End 16-1

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