Photometry and interactions

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2. Contents Calculating speed of light Photometry Interactions of Light and Matter 3. CALCULATING SPEED OF LIGHT 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. 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. Roemers Observation 7. Fizeaus 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. Fizeaus Setup 9. c = 2.9979 x8 10m/s 10. PHOTOMETRY 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. 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. 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 = 4I 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. 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. 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. Typical Luminous Efficiency SourceTungsten filament 20 W 40 W 60 W 100 W Fluorescent lamp 20 W 40 W Sodium vapor lampEfficiency 10 12 14 16 50 7555 18. INTERACTIONS OF LIGHT AND MATTER 19. Light and Matter Interactions Reflection Absorption Transmission Scattering 20. Light and Matter Interactions Transparent Translucent Opaque