1. LIGHT:
MEASURABLE PROPERTIES
AND INTERACTIONS
Understanding Light Phenomena

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. Roemer’s Observation

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. Fizeau’s Setup

9. c = 2.9979 x
8
10
m/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 = 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.

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
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. INTERACTIONS OF
LIGHT AND MATTER

19. Light and Matter Interactions
Reflection
 Absorption
 Transmission
 Scattering


20. Light and Matter Interactions
Transparent
 Translucent
 Opaque
