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
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
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 = 4ΠI
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.
