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