Bài giảng cơ sở kỹ thuật ánh sáng Nguyễn Thanh Phương

3/21/17

CƠ SỞ KỸ THUẬT
ÁNH SÁNG
TS. Nguyễn Thanh Phương
Bộ môn Quang học và Quang điện tử

Chương  1
Những  khái  niệm  cơ  bản  về  ánh  sáng
I.1. Lịch sử phát triển của KTAS.

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I.1. Lịch sử phát triển của KTAS.

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I.1. Lịch sử phát triển của KTAS.

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I.1. Lịch sử phát triển của KTAS.

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Chương  1
Những  khái  niệm  cơ  bản  về  ánh  sáng
I.1. Lịch sử phát triển của KTAS.
I.2. Khái niệm về ánh sáng.

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I.2.  Khái niệm về ánh sáng.

Ánh  sáng  là  g ì?

Sóng?

Hạt?

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A  long  time  ago  ……
n

Aristotle  (384  -­ 322  B.C.),  an  ancient  Greek  thinker,  thought  

that  we  saw  the  world  by  sending  “something”  out  of  our  eye
and  that  reflected  from  the  object.

n

In  Plato’s  time  (427  – 347  B.C.),  the  reflection  of  light  from  
smooth  surfaces  was  known.  He  was  also  a  Greek.  

n

The  ancient  Greeks  (about  200  A.D.)  also  first  observed  the  
refraction  of  light  which  occurs  at  the  boundary  of  two  
transparent  media  of  different  refractive  indices.

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Thế  kỷ  17,  hai  nhà  khoa  học  đưa  ra  hai  quan  điểm  
khác  nhau  về  ánh  sáng  tự  nhiên……
Ánh  sáng  
là  hạt

Không!  
Ánh  sáng  
là  sóng

Isaac  Newton
1643  -­ 1727

Christian  Huygens
1629  -­ 1695
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In  the  17th century,  some  p roperties  o f  light  were  
well  known  already.  F or  example:
n

Light  has  different  c olours.

n

Light  can  travel  through   a  vacuum.

n

Light  can  be  reflected  and   refracted,  these  
processes  are  described  by  the  Laws  of  Reflection
and  Laws  of  Refraction.  

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Laws  of  Reflection
n

According  to  the  L aws  o f  Reflection,  
angle  of  incidence  =  angle  of  reflection  (θi =  θr  )
Incident  light  ray

Reflected  light  ray

Normal
θi

θr

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Laws  of  Refraction
n

Willebrord  Snell discovered  in  1621  that  when  a  wave  
travels  from  a  medium  of  refractive  index,  n1  ,  to  one  of  
different  refractive  index,  n2  ,
n1sin(θ1)  =  n2sin(θ2)
This  relationship  is  c alled Snell’s  Law
Incident  light  ray

Normal
θ1

n1

Interface

n2
Light   bends   towards  the  normal  when   it  
travels  from  an  optically  l ess  dense   medium  
to  an  optically  more  dense  medium.

θ2
Refracted  light  ray
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Newton  proposed   his  “particle  theory  of  light”  
(or  “corpuscular  theory  of  light”)  to  explain  
the  characteristics  of  light.
(source:  “Opticks”,  published  by  Isaac  Newton  in  1704)

I  think  light  is  a  stream  of  tiny  
particles,  called  Corpuscles  …

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Particle  Theory
Why  does  light   have  different   colours?
n

The  particles  of  different  c olours have  
different  properties,   such  as  mass,  s ize    
and  speed.

Why  can  light   travel  through   a  vacuum?
n

Light,  being  particles,  c an  naturally  pass  through  
vacuum.  (At  Newton’s  time,  no  known  wave  could  
travel  through   a  vacuum.)
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Why  does  light  travel  in  straight  lines?
n

A  ball  thrown   into  space  follows  a  c urved  path  
because  of  gravity.

n

Yet  if  the  ball  is  thrown  with  greater   and  greater  
speed,  its  path  c urves  less  and  less.

n

Thus,  billions  of  tiny  light  particles  of  extremely  low  
mass  t ravelling  at  enormous  speeds  will  have  paths  
which  are  essentially  s traight  lines.  
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How  does  the  particle  theory  explain  
the  Laws  of  Reflection?
n

The  rebounding   of  a  steel  ball  from  
a  smooth  plate  is  s imilar  to  the  
reflection  of  light  from  the
surface  of  a  mirror.  

Steel  Ball

Many   light  
particles  i n  a  light  r ay

Light

Rebound

Reflection

Mirror

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How   does  Newton's  particle  theory  explain  the  Laws  of  
Refraction?  
n

A  cannon  ball  hits  the  surface  of  water,  it  is  acted  upon  by  a  
“refracting”  force  which  is  perpendicular  to  the  water  surface.  
It  therefore  slows  down  and  bends  away  from  the  normal.  
Light  does  the  opposite.  Newton  explained  this  observation  
by  assuming  that  light  travels  faster  in  water,  so  it  bends  
towards the  normal.
(What  was  the  problem  in  this  explanation?)

n

The  problem:  
Does  light  really  travel
faster  in  water?
In  fact  n obody  could  measure  
the  speed  o f  light  a t  the  time  o f  
Newton  a nd  Huygens

Air
Cannon  ball

Water

Light

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Why  does  a  prism  separates  a  beam   of  
white  light  into  the  colours  of  the  rainbow?
Why  does  red  light  refract  least  and  violet  
light  refract  most?

Newton’s   assumptions:  
1. The  light   particles   of  different   colours  have   mass.
Red  light   particles  have   more  mass  than   violet   particles.
2.  All   light   particles   experience   the  same  refracting   force  
when  crossing  an  interface.
Thus,  red  light   particles   with  more   inertia   will   be  refracted
less  by  the  same  force  than   violet   light   particles   by  the  same  force  .    
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Let’s  see  how  Huygens  used  his  “wave  
theory”  to  explain  the  characteristics  of  
light  …
(source: Treatise  on  light,  published  by  Huygens  in  1690)

I  think  light  is  emitted  as   a  series  

of  waves   in  a  medium  he  called  
“aether”

(“aether”  commonly  a lso  called  “ether”)
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How  do  waves  propagate?

P

A  wave  s tarts  at  P  and  a  “wavefront”  W
moves  outwards  in  all  directions.
After  a  time,  t,  it  has  a  radius  r,  s o  that  
r  =  ct  if  c  is  the  s peed  of  the  wave.  
Each  point  on  the  wavefront  s tarts  
a  s econdary  wavelet.  These  s econdary  
wavelets  interfere  to  form  a  new  wavefront  
W’ at  time  t ’.
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How  can  wave  theory  explain  the  Laws  of  Reflection?
When  wavefront  W1 (AC)  reaches  point  A,  a  secondary  wave  from  A  
starts  to  spread  out.  When  the  incoming  wavefront  reaches  B,  the  
secondary  wave  from  A  has  reached  D,  giving  a  new  wavefront  W2  
(BD).

Angle  of  incidence  =  Angle  of  reflection can  be  proved  by  
geometry.  Refer  to  the  appendix  of  the  worksheet  or  y our  textbook  for  
the  proof.

C

D

W1

W2

A

B

Click  here  for  animation
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How  can  wave  theory  explain  the  Laws  of  Refraction?
Wavefront  W1 reaches  the  boundary  between  media  1  &  2,  point  A  of  
wavefront  W1 starts  to  spread  out.  When  the  incoming  wavefront  
reaches  B,  the  secondary  wave  from  A  has  travelled  a  shorter  
distance  to  reach  D.  It  starts  a  new  wavefront  W2.  As  a  result  the  
wave  path  bends  towards  the  normal.  

θ1
C
Air

W1

A

Optically  
denser  
medium  

D
θ2

B
W2

n1sinθ1 =  n2sinθ2
can  be  proved  by  geometry.
Refer  to   the  a ppendix  of  the  
worksheet  or  y our  textbook  
for  the  proof.

Click  here  for  animation
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If  light  behaves  as  waves,  diffraction  and  interference  
should  be  seen.  These  are   two  important  features  of  

waves.  This  was  known  in  the  17th  century.
(You  can  see  this  easily  with  water  waves  in  a  “ripple  tank”)
n

The  wave  theory  of  light  predicts  interference  and  
diffraction. However,  Huygens  could  not  provide  any  
strong  evidence  to  s how  that  diffraction  and  
interference  of  light  occurred.

Diffraction  and  interference  
of  water  waves
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If  you  were  one  of  the  scientists  in  the  17th century,  
would  you  believe  the  “particle  theory  of  light”  or  the  
“wave  theory  of  light”?  Why?

Hint:  Which  theory  has  a  greater   ability  to  explain the  
characteristics  of  light?

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Newton  was  the  “winner”…..  (at  that  time!)
n

Newton’s  particle  theory  of  light  dominated   optics  
during   the  18th century.

n

Most  s cientists  believed  Newton’s  particle  theory  of  
light  because  it  had  greater   explanatory  power.

n

Let’s  c onsider  the  reasons……

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(1)Waves  do  not  travel  only  in  straight  lines,
so  light  cannot  be  “waves”.

n

Sounds  can  easily  be  heard  around  an  obstacle  
but  light  cannot  be  seen  around  an  obstacle.
Light,  unlike  sound,  does  not demonstrates  the  
property  of  diffraction  and  it  is  unlikely  to  be  a  
type  of  wave.

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(2)  Light,  unlike  sound  waves,  can  travel  
through  a  vacuum.  Particles  can  travel  
through  a  vacuum.  
n

In  the  17th century,  it  was  believed   that  waves  c ould  
not  travel  through   a  vacuum. It  was  difficult  for  
people   at  that  time  to  believe  that  waves  c ould  travel  
through   the  “ether”,  which  was  the  imaginary  
“medium”   that  light  travels  through,   proposed   by  
Huygens.

X

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(3)  Particle  theory  of  light  can  explain  
why  there  are  different  colours  of  light.
n

Huygens  could  not  explain  why  light  has  different  
colours  at  all. He   did  not  k now  that  different  c olours  
of  light  have  different  “wavelengths”.

n

Though   Newton’s  explanation  was  not  c orrect  
(particles  of  different  c olours  of  light  have  different  

mass  and  size),  his  particles  t heory  could  explain  
this  phenomenon   logically  in  the  17th century.

?

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(4)  Reputation  of  Newton
n

People  tend  to  accept  “authority”  when  there  is  not  
enough  evidence  to  make  judgement.  Newton’s  
particle  theory  could  only  explain  refraction  by  
incorrectly  assuming  that  light  travels  faster  in  a  
denser  medium.  No  one  could  prove  he  was  
wrong  at  that  time.

n

The  uncertainty  about  the  speed  of  light  in  water  remained  
unresolved  for  over  one  hundred  years  after  Newton's  
death.

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However,  the  wave  theory  of  light  was  
re-­examined  100  years  after  Newton’s  particle  
theory  of  light    had  been  accepted……

Light   is  not  
particles!

Thomas  Young
1773  -­ 1829
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n

Thomas  Young  
successfully  
demonstrated  the  
interference  of  light  
(which  Huygens  failed  to  
show),  by  his  famous  
double-­slit  experiments.

n

Since  then  the  wave  
theory  of  light  has  been  
firmly  established.

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The  wave  theory  of  light  was  widely  
accepted  until  1905……
Wave  theory  of  
light?   “No  
way!”

Albert  Einstein
1879  -­ 1955

n

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The  photoelectric  effect is  observed  when  light  strikes  a  
metal,  and  emits  electrons.
Einstein  used  the  idea  of  photons  (light  consists  of  tiny  
particles)  to  explain  results  which  demonstrate  the  
photoelectric  effect.
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n

In  the  s etup  investigating  the  photoelectric  effect  (as  
shown  below),   the  intensity  of  the  light,  its  frequency,  
the  voltage  and   the  size  of  the  current  generated   are  
measured.

n

What  evidence  did  Einstein  f ind  in  
his  photoelectric  effect  experiments  
that  helps  to  support  the  particle  
theory  of  light?
e-­

Light

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Results  from  photoelectric  effect  experiments
n

For  c ertain  metals,  dim  blue  light  can  generate  a  
current  while  intense  red  light  c auses  no  c urrent  at  all.

n

Below  a  c ertain  “cut  off”  frequency  
of  light  (  f  C    ),  no  v oltage  is
measurable.
ν0
Voltage

n

Why  does  t he  wave  t heory  of  light  
not  explain  the  result?    
Frequency  
of  light

fC

f
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Einstein’s  explanation
n

Electrons  are  knocked  free  from  the  metal  by  incoming  

photons,  with  each  photon  carrying  an  amount  of  energy  E  
that  is  related  to  the  frequency  (ν) of  the  light  by
E  =  h  ν
Where  h is  Planck’s  constant  (6.62  x  10-­34   J  seconds).

n

Only  photons  of  high  enough  energy  (above  a  threshold  
value)  can  knock  an  electron  free. e.g.  blue  light,  but  not  red  
light,  has  sufficient  energy  to  free  an  electron  from  the  metal.)

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Albert  Einstein  provided  a  piece  of  convincing  
evidence  for  the  particle  nature  of  light  ……  
Has  the  story  ended  yet?  
Is  light  particles  or  waves?
Light  is  not  particles,  
not  waves,  but  BOTH!

Louis  de  Broglie
1892  -­ 1987

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n

Louis  de  Broglie  in  1924  proposed   that  particles  also  
have  wave-­like  properties,  this  was  c onfirmed  
experimentally  three  years  later.

n

Most  s cientists  did  not  understand   de  Broglie’s  Ph.D.  
dissertation  at  that  time.  One  scientist    passed  it  on  to  
Einstein  for  his  interpretation.  Einstein  replied   that  de  
Broglie  did  not  just  deserve  a  doctorate  but  a  Nobel  
Prize!

n

De  Broglie  was  awarded   the  
Nobel   Prize  in  1929.

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Summary
Aristotle  (  Light  was  emitted  from  our  eyes  )
Christian  Huygens  (  Wave  theory  of  light  )
Isaac  Newton  (  Particle  theory  of  light  )
Thomas  Young  (  Wave  theory  of  light  )
Albert  Einstein  (  Particle  theory  of  light  )
de  Broglie  (  Wave-­particle  duality  of  all  matter)
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The  Wavelengths  of  Light
The  electromagnetic  spectrum  spreads  over  a  
tremendous  range  of  frequencies   or  wavelengths.  
The  wavelength  λ is  related  to  the  frequency   f:

c  =  fλ

c  =  3  x  108 m/s

Those  EM  waves  that  are  visible   (light)  have  wave-­
lengths  that  range  from  0.00004  to  0.00007  cm.
Violet,  λ

Red,  λ

0.00004  cm

0.00007  cm
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Frequency    wavelength

f    ( Hz)

1024  

1023  
1022  
1021  
1020
1019
1018
1017
1016  
1015
1014
1013
1012
1011
1010
109  
108
107
106  
105  
104

λ ( nm)

Gamma  rays

X-­rays
Ultraviolet
Infrared    rays
Short  Radio  
waves

Broadcast Radio
Long    Radio  
waves

10-­7
10-­6  
10-­4  
10-­3  
10-­1  
1    
10  
102  
103  
104    
105  
106  
107  
108  
109  
1010
1011
1012
1013

The  EM  Spectrum
A  wavelength  of  one  
nanometer  1  nm  is:
1  nm  =  1  x  10-­9 m
Visible   Spectrum
400  nm  à 700  nm

Red  700  nm  à Violet  400  nm  

c  =  fλ

c  =  3  x  108 m/s

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Example  1.  Light  from  a  Helium-­Neon  laser  
has  a  wavelength  of  632  nm.  What  is  the  
frequency  of  this  wave?
The  Helium  Neon  Laser

Wavelength

Laser

c= fλ

λ =  632  nm

3 x 108 m/s
f = =
λ 632 x 10-9 m
c

f  =  4.75  x  1014  Hz

Red  light
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Properties  of  Light
Any  study  of  the  nature  of  light  must  
explain  the  following  observed  properties:
• 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.  
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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!

Reflection  and  
rectilinear   propagation  
(straight  line  path)

Dispersion  of  white  
light  into  colors.
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Photons  and  Light  Rays
Light  may  be  thought  of  as  little  bundles  of  waves  
emitted  in  discrete   packets  called   photons.
photons
The  wave  treatment  uses  rays to  show  
the  direction  of  advancing  wave  fronts.
Light  
ray

Light  rays  are  
convenient  for  
describing  how  
light  behaves.
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3/21/17

Light  Rays  and  Shadows
A  geometric  analysis   may  be  made  of  shadows  
by  tracing  light  rays  from  a  point  light  source:
shadow

Point  
source

screen
The  dimensions  of  the  shadow  can  be  found  
by  using  geometry  and  known  distances.
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Example  2: The  diameter  of  the  ball  is  4  cm
and  it  is  located  20  cm from  the  point  light  
source.  If  the  screen   is  80  cm from  the  
source,  what  is  the  diameter  of  the  shadow?
h
4 cm
=
80 cm 20 cm

h

4  cm
20  cm
80  cm

h=

(4 cm)(80 cm)
20 cm

The  ratio  of  
shadow  to    

the  source    
is  same  as  
that  of  ball  
to  source.  
Therefore:

h  =   16  cm
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3/21/17

Shadows  of  Extended  Objects
penumbra
Extended  
source
umbra
• The  umbra
is  the  region  where  no  light  
The  umbra
is  the  region  where  no  light  reaches  
reaches   the  screen.  
the  screen.  
• The  penumbra is  the  outer  area  where  
only  part  of  the  light  reaches   the  screen.
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The  Sensitivity  Curve

Sensitivity  curve
Sensitivity

Human  eyes  are  not  
equally  sensitive   to    
all  colors.

Eyes  are  most  sensi-­
tive  in  the  mid-­range  
near    λ =  555  nm.

555  nm

400  nm

700  nm

Wavelength  λ

40  W

40  W

Yellow light  appears   brighter  to  
the  eye  than  does  red  light.
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