2. Objectives:
Objectives: After completing this
After completing this
module, you should be able to:
module, you should be able to:
•
• Define
Define light
light, discuss its properties, and give
, discuss its properties, and give
the range of wavelengths for visible spectrum.
the range of wavelengths for visible spectrum.
•
• Apply the relationship between
Apply the relationship between frequencies
frequencies
and
and wavelengths
wavelengths for optical waves.
for optical waves.
•
• Define and apply the concepts of
Define and apply the concepts of luminous
luminous
flux
flux,
, luminous intensity
luminous intensity, and
, and illumination
illumination.
.
•
• Solve problems similar to those presented in
Solve problems similar to those presented in
this module.
this module.
3. A Beginning Definition
A Beginning Definition
All objects are emitting
All objects are emitting
and absorbing EM
and absorbing EM radia
radia-
-
tion
tion. Consider a poker
. Consider a poker
placed in a fire.
placed in a fire.
As heating occurs, the
As heating occurs, the
emitted EM waves have
emitted EM waves have
higher energy and
higher energy and
eventually become visible.
eventually become visible.
First red . . . then white.
First red . . . then white.
3
4
2
1
Light may be defined as electromagnetic radiation
that is capable of affecting the sense of sight.
Light
Light may be defined as electromagnetic radiation
may be defined as electromagnetic radiation
that is capable of affecting the sense of sight.
that is capable of affecting the sense of sight.
4. Electromagnetic Waves
Electromagnetic Waves
c
c
E
E
B
B
Electric
Electric E
E
Magnetic
Magnetic B
B
Wave Properties:
Wave Properties:
1.
1. Waves travel at the
Waves travel at the
speed of light
speed of light c
c.
.
2.
2. Perpendicular electric
Perpendicular electric
and magnetic fields.
and magnetic fields.
3.
3. Require no medium
Require no medium
for propagation.
for propagation.
For a complete review of the electromagnetic
properties, you should study module 32C.
For a complete review of the electromagnetic
properties, you should study module 32C.
3 x 10
3 x 108
8 m/s
m/s
5. The Wavelengths of Light
The Wavelengths of Light
The electromagnetic spectrum spreads over a
tremendous range of frequencies or wavelengths.
The wavelength is related to the frequency f:
The electromagnetic spectrum spreads over a
The electromagnetic spectrum spreads over a
tremendous range of frequencies or wavelengths.
tremendous range of frequencies or wavelengths.
The
The wavelength
wavelength
is related to the
is related to the frequency
frequency f
f:
:
c = fc = 3 x 108 m/s
c = fc = 3 x 108 m/s
Those EM waves that are visible (light) have wave
Those EM waves that are visible (light) have wave-
-
lengths that range from 0.00004 to 0.00007 cm.
lengths that range from 0.00004 to 0.00007 cm.
Red,
Red,
0.00007 cm
0.00007 cm
Violet,
Violet,
0.00004 cm
0.00004 cm
6. The EM Spectrum
The EM Spectrum
A wavelength of one
A wavelength of one
nanometer 1 nm is:
nanometer 1 nm is:
1 nm = 1 x 10-9 m
1 nm = 1 x 10-9 m
Red 700 nm
Red 700 nm
Violet 400 nm
Violet 400 nm
c = fc = 3 x 108 m/s
c = fc = 3 x 108 m/s
1024
1023
1022
1021
1020
1019
1018
1017
1016
1015
1014
1013
1012
1011
1010
109
108
107
106
105
104
Frequency wavelength
f (Hz) nm)
10-7
10-6
10-4
10-3
10-1
1
10
102
103
104
105
106
107
108
109
1010
1011
1012
1013
Gamma rays
X-rays
Infrared rays
Short Radio
waves
Broadcast Radio
Long Radio
waves
Ultraviolet
400 nm
400 nm
700 nm
700 nm
Visible Spectrum
Visible Spectrum
7. Example 1.
Example 1. Light from a Helium
Light from a Helium-
-Neon laser
Neon laser
has a wavelength of
has a wavelength of 632 nm
632 nm. What is the
. What is the
frequency of this wave?
frequency of this wave?
8
-9
3 x 10 m/s
632 x 10 m
c
c f f
f = 4.75 x 1014 Hz
f = 4.75 x 1014 Hz Red light
Red light
The Helium Neon Laser
The Helium Neon Laser Wavelength
Wavelength
= 632 nm
= 632 nm
Laser
8. Properties of Light
Properties of Light
• Rectilinear propagation: Light travels in
straight lines.
• Reflection: Light striking a smooth surface
turns back into the original medium.
• Refraction: Light bends when entering a
transparent medium.
Any study of the nature of light must
explain the following observed properties:
Any study of the nature of light must
Any study of the nature of light must
explain the following observed properties:
explain the following observed properties:
9. The Nature of Light
The Nature of Light
Physicists have studied light for centuries, finding
that it sometimes behaves as a particle and
sometimes as a wave. Actually, both are correct!
Physicists have studied light for centuries, finding
Physicists have studied light for centuries, finding
that it sometimes behaves as a particle and
that it sometimes behaves as a particle and
sometimes as a wave. Actually, both are correct!
sometimes as a wave. Actually, both are correct!
Reflection and
Reflection and
rectilinear propagation
rectilinear propagation
(straight line path)
(straight line path)
Dispersion of white
Dispersion of white
light into colors.
light into colors.
10. Photons and Light Rays
Photons and Light Rays
Light may be thought of as little bundles of waves
Light may be thought of as little bundles of waves
emitted in discrete packets called
emitted in discrete packets called photons
photons.
.
photons
photons
The wave treatment uses
The wave treatment uses rays
rays to show
to show
the direction of advancing wave fronts.
the direction of advancing wave fronts.
Light
Light
ray
ray
Light rays are
convenient for
describing how
light behaves.
Light rays are
Light rays are
convenient for
convenient for
describing how
describing how
light behaves.
light behaves.
11. Light Rays and Shadows
Light Rays and Shadows
A geometric analysis may be made of shadows
A geometric analysis may be made of shadows
by tracing light rays from a point light source:
by tracing light rays from a point light source:
shadow
shadow
screen
screen
Point
Point
source
source
The dimensions of the shadow can be found
by using geometry and known distances.
The dimensions of the shadow can be found
by using geometry and known distances.
12. Example 2:
Example 2: The diameter of the ball is
The diameter of the ball is 4 cm
4 cm
and it is located
and it is located 20 cm
20 cm from the point light
from the point light
source. If the screen is
source. If the screen is 80 cm
80 cm from the
from the
source, what is the diameter of the shadow?
source, what is the diameter of the shadow?
4 cm
20 cm
80 cm
h
The ratio of
shadow to
the source
is same as
that of ball
to source.
Therefore:
The ratio of
The ratio of
shadow to
shadow to
the source
the source
is same as
is same as
that of ball
that of ball
to source.
to source.
Therefore:
Therefore:
(4 cm)(80 cm)
20 cm
h h = 16 cm
4 cm
80 cm 20 cm
h
13. Shadows of Extended Objects
Shadows of Extended Objects
Extended
Extended
source
source
penumbra
penumbra
umbra
umbra
The
The umbra
umbra is the region where no light reaches
is the region where no light reaches
the screen.
the screen.
•
• The
The penumbra
penumbra is the outer area where
is the outer area where
only part of the light reaches the screen.
only part of the light reaches the screen.
•
• The
The umbra
umbra is the region where no light
is the region where no light
reaches the screen.
reaches the screen.
14. The Sensitivity Curve
The Sensitivity Curve
Sensitivity curve
Sensitivity curve
Wavelength
Wavelength
Sensitivity
Sensitivity
Human eyes are not
Human eyes are not
equally sensitive to
equally sensitive to
all colors.
all colors.
Eyes are most
Eyes are most sensi
sensi-
-
tive
tive in the mid
in the mid-
-range
range
near
near
= 555 nm
= 555 nm.
.
555 nm
555 nm
400 nm
400 nm 700 nm
700 nm
40 W
40 W 40 W
40 W
Yellow light appears brighter
to the eye than does red light.
Yellow
Yellow light appears brighter
light appears brighter
to the eye than does
to the eye than does red
red light.
light.
15. Luminous Flux
Luminous Flux
Luminous flux
Luminous flux is the portion of total radiant power
is the portion of total radiant power
that is capable of affecting the sense of sight.
that is capable of affecting the sense of sight.
Typically only about 10%
Typically only about 10%
of the power (flux) emitted
of the power (flux) emitted
from a light bulb falls in
from a light bulb falls in
the visible region.
the visible region.
The unit for luminous flux is the
The unit for luminous flux is the lumen
lumen which
which
will be given a quantitative definition later.
will be given a quantitative definition later.
16. A Solid Angle: Steradians
A Solid Angle: Steradians
Working with luminous flux requires the use of a
solid angle measure called the steradian (sr).
Working with luminous flux requires the use of a
solid angle measure called the steradian (sr).
A
R
The
Steradian 2
A
R
A solid angle of one
steradian (1 sr) is
subtended at the
center of a sphere
by an area A equal
to the square of its
radius ( R2 ).
A solid angle of one
A solid angle of one
steradian
steradian (
(1
1 sr
sr) is
) is
subtended at the
subtended at the
center of a sphere
center of a sphere
by an area
by an area A
A equal
equal
to the square of its
to the square of its
radius (
radius ( R
R2
2 ).
).
17. Example 3.
Example 3. What solid angle is subtended at
What solid angle is subtended at
the center of a sphere by an area of
the center of a sphere by an area of 1.6 m
1.6 m2
2?
?
The radius of the sphere is
The radius of the sphere is 5 m
5 m.
.
A
1.6 m2
R
5 m
The
Steradian 2
A
R
2
2
1.60 m
(5.00 m)
2
A
R
= 0.00640 sr
= 0.00640 sr
18. The Lumen as a Unit of Flux
The Lumen as a Unit of Flux
One
One lumen
lumen (lm)
(lm) is the
is the luminous flux
luminous flux emitted from
emitted from
a
a 1/60 cm
1/60 cm2
2 opening in a standard source and
opening in a standard source and
included in a solid angle of
included in a solid angle of one steradian
one steradian (1
(1 sr
sr).
).
In practice, sources of light are usually rated
In practice, sources of light are usually rated
by comparison to a commercially prepared
by comparison to a commercially prepared
standard light source.
standard light source.
A typical
A typical 100
100-
-W
W incandescent
incandescent
light bulb emits a total radiant
light bulb emits a total radiant
power of about
power of about 1750 lm
1750 lm. This is
. This is
for light emitted in all directions.
for light emitted in all directions.
19. The Lumen in Power Units
The Lumen in Power Units
Recalling that luminous flux is really radiant
Recalling that luminous flux is really radiant
power allows us to define the lumen as follows:
power allows us to define the lumen as follows:
One lumen is equal to 1/680 W of yellow-
green light of wavelength 555 nm.
One lumen is equal to 1/680 W of yellow
One lumen is equal to 1/680 W of yellow-
-
green light of wavelength 555 nm.
green light of wavelength 555 nm.
Wavelength
Wavelength
Sensitivity curve
Sensitivity curve
A disadvantage of this
A disadvantage of this
approach is the need to
approach is the need to
refer to sensitivity curves
refer to sensitivity curves
to determine the flux for
to determine the flux for
different colors of light.
different colors of light.
20. Luminous Intensity
Luminous Intensity
The
The luminous intensity
luminous intensity I
I for a light source
for a light source
is the luminous flux per unit solid angle.
is the luminous flux per unit solid angle.
F
I
Luminous intensity:
F
I
Unit is the candela (cd)
A source having an intensity of one candela
emits a flux of one lumen per steradian.
A source having an intensity of one candela
emits a flux of one lumen per steradian.
21. Total flux for Isotropic Source
Total flux for Isotropic Source
R
R
3 m
3 m
An isotropic source emits in
An isotropic source emits in
all
all directions; i.e., over a
directions; i.e., over a
solid angle of
solid angle of 4
4
steradians
steradians.
.
Total flux: F = 4I
Total flux: F = 4I
The flux confined to area A is:
The flux confined to area A is:
F = I A
F = I A
= 4
= 4
sr
sr
Thus, for such
Thus, for such
a source, the
a source, the
intensity is:
intensity is: 4
F F
I
22. Example 4.
Example 4. A
A 30
30 cd
cd spotlight is located
spotlight is located 3 m
3 m
above a table. The beam is focused on a surface
above a table. The beam is focused on a surface
area of
area of 0.4 m
0.4 m2
2. Find the intensity of the beam.
. Find the intensity of the beam.
Total flux: F = 4I
Total flux: F = 4I
F
FT
T = 4
= 4
(30
(30 cd
cd) = 377 lm
) = 377 lm
R
R
3 m
3 m
The luminous intensity of
The luminous intensity of
the beam depends on
the beam depends on
2
2 2
0.4 m
; 0.0444 sr
(3 m)
A
R
754 lm
0.0444 sr
F
I
Beam Intensity:
Beam Intensity:
I = 8490 cd
I = 8490 cd
23. Illumination of a Surface
Illumination of a Surface
The illumination E of a surface A is defined as
the luminous flux per unit area (F/A) in lumens
per square meter which is renamed a lux (lx).
The
The illumination
illumination E
E of a surface
of a surface A
A is defined as
is defined as
the luminous flux per unit area (
the luminous flux per unit area (F/A
F/A) in
) in lumens
lumens
per square meter
per square meter which is renamed a
which is renamed a lux
lux (lx)
(lx).
.
Unit: lux (lx)
F
E
A
An illumination of one lux
occurs when a flux of
one lumen falls on an
area of one square
meter.
An illumination of one lux
occurs when a flux of
one lumen falls on an
area of one square
meter.
R
R
Area
Area A
A
Illumination,
Illumination, E
E
24. Illumination Based on Intensity
Illumination Based on Intensity
R
Area A
; ;
F F
E I F I
A
2
but so that
I A
E
A R
2
Illumination,
I
E
R
This equation applies for
perpendicular surfaces.
The illumination E of a surface is directly
proportional to the intensity I and inversely
proportional to the square of the distance R.
The
The illumination
illumination E
E of a surface is directly
of a surface is directly
proportional to the
proportional to the intensity
intensity I
I and inversely
and inversely
proportional to the
proportional to the square
square of the
of the distance R
distance R.
.
25. Example 5.
Example 5. A
A 400
400-
-cd
cd light is located
light is located 2.4 m
2.4 m
from a tabletop of area
from a tabletop of area 1.2 m
1.2 m2
2. What is the
. What is the
illumination and what flux
illumination and what flux F
F falls on the table?
falls on the table?
R
2 2
400 cd
(2.40 m)
I
E
R
E = 69.4 lx
Illumination:
Illumination:
Now, recalling that
Now, recalling that E = F/A
E = F/A, we find
, we find F
F from:
from:
F = EA =
F = EA = (69.4 lx)(1.20 m
(69.4 lx)(1.20 m2
2)
) F = 93.3 lm
26. The Inverse Square Relationship
The Inverse Square Relationship
2
I
E
R
E/9
E/4
E
3 m
2 m
1 m
1 m2
4 m2
9 m2
If the intensity is 36 lx at 1 m, it will
be 9 lx at 2 m and only 4 lx at 3 m.
27. Summary
Summary
Light may be defined as electromagnetic radiation
that is capable of affecting the sense of sight.
Light
Light may be defined as electromagnetic radiation
may be defined as electromagnetic radiation
that is capable of affecting the sense of sight.
that is capable of affecting the sense of sight.
• Rectilinear propagation
• Reflection
• Refraction c = fc = 3 x 108 m/s
c = fc = 3 x 108 m/s
General Properties of Light:
Red,
Red,
700 nm
700 nm
Violet,
Violet,
400 nm
400 nm
28. Summary (Continued)
Summary (Continued)
Luminous flux
Luminous flux is the portion of total radiant power
is the portion of total radiant power
that is capable of affecting the sense of sight.
that is capable of affecting the sense of sight.
Extended
Extended
source
source
penumbra
penumbra
umbra
umbra
The formation of shadows:
The formation of shadows:
29. Summary (Continued)
Summary (Continued)
Unit: lux (lx)
F
E
A
Total flux: F = 4I
Total flux: F = 4I
Luminous intensity:
F
I
Unit is the candela (cd)
A
R
The
Steradian 2
A
R
