photometry and interactions
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LIGHT: MEASURABLE PROPERTIES AND INTERACTIONS Understanding Light Phenomena
ContentsCalculating speed of light Photometry Interactions of Light and Matter
CALCULATING SPEED OF LIGHT
Speed of Light Fastest recorded speed of object
in the universe Approximately equal to 3 x 108
m/s or 186,000 mi/s
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
Roemer’s Observation
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
Fizeau’s Setup
c = 2.9979 x 108 m/s
PHOTOMETRY
Photometry The measurement of brightness
of a light source There are three measurable
quantities of light ◦Luminous intensity (I)◦Luminous flux (F)◦Illumination (E)
Luminous Intensity The brightness of a light sourceMeasured 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
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
Illuminance The amount of illuminationThe amount of luminous flux
falling on a unit area of surface Measured in terms of lumens per
square meter or lux E = F/A
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.
Recommended Intensities of Illumination
Place Illuminance (lux)
Library 200-250Basketball court 200-250Classroom or office
300-400
Drafting or sewing room
500-800
Typical Luminous Efficiency
Source Efficiency
Tungsten filament 20 W 40 W 60 W 100 W
10121416
Fluorescent lamp 20 W 40 W
5075
Sodium vapor lamp 55
INTERACTIONS OF LIGHT AND MATTER
Light and Matter InteractionsReflection Absorption Transmission Scattering
Light and Matter InteractionsTransparent Translucent Opaque