2. Nature of Light
Optical Waves
Waves
• Longitudinal Waves or P waves (Eg. Sound/Spring waves)
• Transverse Waves or S waves (Eg. String wave)
• Longitudinal Waves + Transverse Waves (Eg. Sesmic/Earthquake wave)
A transverse wave is one in which the displacement is perpendicular to
the direction of travel. In longitudinal waves, the displacement is
parallel to the propagation direction. The "closer together" region is
called a compression, and the "farther apart" region is called a
rarefaction.
Frequency : number of oscillation
f 
1
T
Earthquake Waves
The faster earthquake waves is called
the primary wave (P wave). It is a
longitudinal wave, as it spreads out, it
alternately pushes (compresses) and
pulls (dilates) the rock. The P waves
are able to travel through solid rock.
The slower earthquake wave through
the rock is called the secondary wave
(S wave). It shears the rock sideways
at right angles to the direction of
travel.
The other type of earthquake wave is
called the surface wave. Its motion is
restricted to near the ground surface.
The surface waves in earthquakes can
be divided into two types. The first is
called a Love wave. It has no vertical
displacement; it moves the ground
from side to side in a horizontal plane
but at right angles to the direction of
propagation. The horizontal shaking
of Love waves is particularly
damaging to the foundations of
structures. The second type of surface
wave is known as a Rayleigh wave. It
moves both vertically and
horizontally in a vertical plane
pointed in the direction in which the
waves are traveling. Love waves
generally travel faster than Rayleigh
waves.
A person is located from
the epicenter of the
earthquake. When
earthquake occurs, he/he
moves forwards and
backwards, firstly. And
then she/he vibrates up and
down.
Constructive/Destructive Interferences
Phase Velocity and Group Velocity
Example of Observing Phase VelocityBarber’s Pole
Electromagnetic Spectrum
• Radiometry
• ( :1mm~1nm)
• infrared light, visible light,
ultraviolet
• light in optoelectronics
• Photometry
• ( : 400~700nm)
• visible light in human
vision
Representations of Electromagnetic Waves
• In EE theory, the electric and magnetic field
components of the z-propagating light are often


 jkz
expressed by
H  yˆH e
E  xˆE0 e
 jkz
0

E( z, t )  xˆE0 cos(t  kz   )
• In Physics, the electric and magnetic field
components of the z-propagating light are often


expressed by
jkz
jkz
E  xˆE e
ˆ
0
H  yH 0 e

E( z, t )  xˆE0 cos(kz  t   )
TE-polarized: the electric field ⊥ the plane spanned by the incident and the normal
lines
TM-polarized: the magnetic field ⊥ the plane spanned by the incident and the
normal lines
General Traveling Wave Expression
in Homogenous Medium
Maxwell’s EM Wave Equations in
Simple Medium
Oblique Incidence for TE-polarized Light
Oblique Incidence for TM-polarized Light
Normal incidence at interface
Regardless of polarizations,
Photon
Photon energy: E  hf
(J)
• h : planck’s constant
• f : frequency of
radiation (Hz)
• Joule (J) is the unit of
energy, and Watt (W)
is the unit of power.
• Power is the energy
generated/dissipated
per time interval.
Note: 1 W=1 J/sec
Radiometric and Photometric Units
Flux—to describe a flow phenomenon
Radiometric flux
Photometric flux
Symbol
w
p
Unit
watt (w)
lumen (lm)
Relationship :
1 W 555nm  683lm
Radiometric and Photometric Intensity
Radiometric intensity: radiometric flux density per steradian (w/sr)
IR 
R
(W/sr)

Photometric intensity: luminous flux density per steradian
IP 
P
(lm/sr or cd)

Eg. Photon population
P hotonenergy at   555 nm
8
3

10
19
E  hf  6.6261034 

3
.
582

10
J
9
555 10
1
1 lm 
 1.464103 W
683
Number of phot onper second for one lumen
1.464103
15  photons 
n

4
.
087

10


19
s


3.58210
Efficacy  Conversion from Radiometric Flux to
Photometric Flux
Definition of efficacy

Efficacy: k  P
R
where k = efficacy (lm/w)
 P = photometric flux (lm)
 R = radiometric flux (w)
0
[Eg-1] 40w incandescent lamp delivers 460 lumens . (Assume that 90%
0 of the power is
radiated and 10 % is conducted away as heat)
 R  40 90 0 0  36 W (radiometric flux)
 P  460lum ens (photometr
ic flux)
efficacy: k 
 
460
 12.8 lm
w
36
[Eg-2] 40w fluorescent lamp delivers 3200 lm .
efficacy: k 
 
3200
 88.9 lm
w
40 90 0 0
Note : fluorescent lamp > incandescent lamp in efficacy .
Efficacy of Thermal Radiators
Block-body radiation – perfect radiator (absorber)
Stefan-Boltzman law :
total radiation
I  T 4
: Stefan-Boltzman constant
( 5.67 108 watt
m2 o K 4
)
T : degree Kelvin( oK )
Wien’s displacement law:
m  T  const. ( 3000)
m: maximum radiated wavelength
(m)
Radiant profiles
I  I O  cos
2
1
Point source
Lambertian source
Exponential intensity source
I()=I0 (=constant)
I()=I0 cos 
I()=I0 cosn 
I(): intensity in direction of angle
(Lambert’s cosine law)
n :radiation pattern exponent
I0 : intensity in direction of symmetry
axes
Radiant Incidence and Illuminance
Radiant incidance: Radiant flux distribution on a surface
ER 
R
A
(W/m2 )
R : radiant flux (W)
A : area of flux distribution (m2 )
Illuminance :
Ep 
p
A
(lm/m2 , lux or lx))
P : luminousflux (lm)
Eg. Illumination from a point source
s
I cos
A
d2
For   0 (source  surface)
Illuminat on
i : E
I
E 2
d

Eg. Illumination from a wide-area Lambertian source

Intensity in the direction of the receiver:
Flux into the receiver:
I   I o cos 
r 
S AR cos  cos 
(S  I o )
2
d
S : source flux
AR: receiver area
: receiver direction from the perpendicular to the source
: receiver-angle from the perpendicular
d: source-receiver distance (m)