Photometry and interactions

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Photometry and interactions

  1. 1. LIGHT: MEASURABLE PROPERTIES AND INTERACTIONS Understanding Light Phenomena
  2. 2. Contents Calculating speed of light  Photometry  Interactions of Light and Matter 
  3. 3. CALCULATING SPEED OF LIGHT
  4. 4. Speed of Light Fastest recorded speed of object in the universe  Approximately equal to 3 x 108 m/s or 186,000 mi/s 
  5. 5. Ole Roemer Danish astronomer who made the first successful estimation of the speed of light (1675)  Involved astronomical observations of one of satellites of Jupiter  Speed of light was estimated to be 2.3 x 108 m/s 
  6. 6. Roemer’s Observation
  7. 7. Fizeau’s Method The first successful method for measuring the speed of light by means of purely terrestrial techniques (1849)  Performed by French physicist Armand H. L. Fizeau  Arrived at a value of 3.1 x 108 m/s 
  8. 8. Fizeau’s Setup
  9. 9. c = 2.9979 x 8 10 m/s
  10. 10. PHOTOMETRY
  11. 11. Photometry The measurement of brightness of a light source  There are three measurable quantities of light  ◦ Luminous intensity (I) ◦ Luminous flux (F) ◦ Illumination (E)
  12. 12. Luminous Intensity The brightness of a light source  Measured in terms of candela  Candela is defined as the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540 x 1012 Hz and that has a radiant intensity in that direction of 1/683 watt per steradian 
  13. 13. Luminous flux A measure of the radiant power of light emitted from a source without regard for the direction in which it is emitted  Measured in terms of lumens (lm)  F = 4ΠI 
  14. 14. Illuminance The amount of illumination  The amount of luminous flux falling on a unit area of surface  Measured in terms of lumens per square meter or lux  E = F/A 
  15. 15. Sample Problem  A light source of 500 cd is placed 4 m above the floor of a big hall. ◦ Find the total luminous flux emitted by the source. ◦ Find the intensity of illumination on a point directly below the source.
  16. 16. Recommended Intensities of Illumination Place Illuminance (lux) Library 200-250 Basketball court 200-250 Classroom or 300-400 office Drafting or sewing 500-800 room
  17. 17. Typical Luminous Efficiency Source Tungsten filament 20 W 40 W 60 W 100 W Fluorescent lamp 20 W 40 W Sodium vapor lamp Efficiency 10 12 14 16 50 75 55
  18. 18. INTERACTIONS OF LIGHT AND MATTER
  19. 19. Light and Matter Interactions Reflection  Absorption  Transmission  Scattering 
  20. 20. Light and Matter Interactions Transparent  Translucent  Opaque 

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