Introduction to light

1Hanisah/EO301/JKE/PTSS/Dis'13

What is Light?
Hanisah/EO301/JKE/PTSS/Dis'13
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Waves? Particles?
LIGHT enables us to
see Objects. The sun,
a lighted candle and
electric bulb give out
the light. They are
called luminous
bodies.
or

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Isaac Newton
1643 - 1727
Christian Huygens
1629 - 1695
In the17th century, two scientists had different views
about the nature of light ……
Light is
particles
No! Light
is waves

 In Late 1600, Sir Isaac Newton believed that light travels in
the form of particles which named as “corpuscles”.
 Then, in 1678 Christian Huygens argued that light might be
some sort of a wave motion. Huygens came up with
Huygens's Principle that explain the light is a wave
traveling through a medium called “aether”.
 Huygens deduced the Laws of Reflection and Refraction
and could explain Double Refraction to prove the Theory of
Wave.
 However, if light behaves as waves, diffraction and
interference should be seen using light.
Hanisah/EO301/JKE/PTSS/Dis'13 4
WAVE or PARTICLES?

 Huygens failed provide any strong evidence to show that
diffraction and interference of light occurred.
 Furthermore, Huygens could not explain why light has different
colours at all (He did not know that different colors of light have
different “wavelengths”)
 Therefore, Newton’s Particle Theory is acceptable since his
strong evidence about Particle nature of light.
 However, In 1801, Thomas Young’s Double Slit Experiment
showed that light diffracts and produces an interference pattern.
 Therefore, Thomas Young successful provide evidence that light has
“WAVE” properties.
 He showed that light rays interfere with each other; such behavior
could not be explained by particles.
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WAVE or PARTICLES?

 In the 1860’s, Maxwell developed a mathematical model of
electromagnetism.
 He was able to show that these electromagnetic waves travel at
the speed of light.
 Therefore, he asserted that light was a form of high-frequency
electromagnetic wave.
 In 1900, Max Planck was able to explain the spectrum of a
“blackbody” radiator by assuming that light energy is quantized.
That quantum of light energy was later named a PHOTON.
 A few years later, in 1905, Albert Einstein used Planck’s idea to
explain the photoelectric effect to support the particle behavior of
light and came out with a QUANTUM THEORY.
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WAVE or PARTICLES?

Isaac Newton (Particle theory of light)
Christian Huygens (Wave theory of light)
Thomas Young (Wave theory of light)
James Clerk Maxwell (Wave theory of light)
Max Planck (Particle theory of light)
Albert Einstein (Particle theory of light )
Louis de Broglie ( Wave-particle duality)
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WAVE or PARTICLES?

 Experiment to proof
the Dual Nature of
light
• Double slit exp. for
particles
waves
electrons
particle-wave
duality
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 A gun sprays bullets towards a target
 The resulting pattern is a map of the likelihood of a bullet
landing at each point
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Double Slit
Experiment for
Particles

 With waves, however, the result is very different, because of
interference.
 If a slit is opened one at a time, the pattern would resemble that for
bullets: two distinct peaks.
 But when both slits are open, the waves pass through both slits at once
and interfere with each other
Double Slit
Experiment for
Waves

 Now the quantum paradox: Electrons like bullets, strike the
target one at a time.
 Yet, like waves, they create an interference pattern
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Double Slit
Experiment for
Electrons/photons

 If each electron passes individually through one slit, with what does it
"interfere?" Although each electron arrives at the target at a single
place and time, it seems that each has passed through.
 Thus, the electron is understood in terms of a wave-particle duality
(Quantum-Mechanic Theory)
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Double Slit
Experiment for Light
Wave-Particle Duality

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On macroscopic scales, we can treat a large number of photons
as a wave.
When dealing with subatomic phenomenon, we are often dealing
with a single photon, or a few.

 As a conclusion, the scientists have observed that light
energy can behave;
• Either like a wave as it moves through space, OR
• It can behave like a discrete particles with a discrete
amount of energy(quantum) that can be absorbed and
emitted.
 When light traveling through space, they act like waves.
 When light interacts with atoms and molecules, they act
like a stream of energy called photons or quanta.
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Theory of Light
Particle Theory Wave Theory
 Therefore, there are two THEORY OF LIGHT which explain
the nature of light:
1) Wave Theory – Light as a wave
2) Particle Theory – Light as a particle (photon)
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Quantum Mechanic Theory ( Wave-Particle Duality)

 Supported by;
Christian
Huygens
Thomas Young James Clerk
Maxwell
Reflection
and Refraction
Diffraction
and Interference
(Young’s Double Slit
Experiment)
Electromagnetism

 Wave Theory explain that the WAVE as a nature of
light.
 In Wave Theory, light is considered as an
Electromagnetic (EM) Wave.
 This EM wave consists two components which
are Electric field (E) and Magnetic field (H)
which oscillate and perpendicular to each other
as well as to the direction of wave propagation
as shown in Figure 1.
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 According to the Wave Theory proposed by Christian
Huygens, light is considered to be emitted as a series of
waves (wave front) in all directions.
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 A wave has a wavelength (λ) , a frequency (f ) and a velocity
(ν).
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1 cycle
1 wavelength
1 period
A
Figure 1.2 : Waveform
 Therefore, following properties can be defined for light by
considering the wave nature in Figure 1.2

PROPERTIES OF LIGHT WAVES
1. Wavelength (λ) - is the length that one cycle OR
Distance between 2 crests. (Unit: meter, m)
2. Frequency (f) - How often cycle of wave repeats in one
second OR number of cycles per sec. (Unit: Hertz, Hz)
3. Velocity (v) – the distance covered by the wave in one
second. (Unit: m/s)
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According to Figure 1.2…

PROPERTIES OF LIGHT WAVES
4. Period (T) - the duration of one cycle . It is reciprocal of
frequency. (Unit: second, s)
5. Wave Number ( ⱱ ) - the number of waves spread in a
length of one meter . It is reciprocal of wavelength.
(unit: m-1)
6. Amplitude (A) = the distance from the midline to the peak
of wave. Amplitude is a measure of the intensity or
brightness of light radiation.
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f
T
1


1
v

CHARACTERISTICS OF LIGHT WAVES
 The velocity of light wave is not constant. It depends on
type of medium the wave travels through.
 Velocity/speed of light wave (v) in vacuum is denoted by
c. c = 3 x 108 m/s
 The relationship among frequency(f), light velocity (c),
and wavelength (λ) is expressed mathematically as:
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f
c

….. equation 1.1

 From equation 1.1, it can be seen that wavelength (λ) is
inversely proportional to the frequency (f).
 high frequency = short wavelength
 low frequency = long wavelength
 Light wave have different colors of dispersion depends
on the frequency (f) or wavelength (λ).
 Different frequency, or wavelength of wave will give
different color of light as shown in Table1.1.
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 Light wave could diffract and interfere as shown in
Thomas Young’s Double-Slit Experiment.
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Diffraction Interference

 The propagation of light through space can be
described in term of a traveling wave motion.
 The light wave moves energy , without moving mass,
from one place to another at a speed independent of its
intensity or wavelength.
 The light wave could moves in three different
polarization;
 Linear Polarization
 Circular Polarization
 Elliptical Polarization
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Linear Circular
Elliptical

 Supported by;
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Isaac Newton Max Plank Albert Einstein
Reflection
and Refraction,
phenomena of colors
Black Body Radiator
(Quantum Theory)
Photoelectric Effect
(Quantum Theory)

 Isaac Newton (1704) proposed that light consists of a
stream of small particles, because it
• travels in straight lines at great speeds
• is reflected from mirrors in a predictable way
Newton observed that the reflection of light from a mirror
resembles the rebound of a steel ball from a steel plate

 Particle Theory explain that the particle PHOTON as a
nature of light.
 From quantum perspective, light consist of particles
called photon.
So, What is PHOTON?
 Photon is a very tiny little particle that has energy and
movement (momentum) but it has no mass or
electrical charge.
 According to Einstein, Photon is considered as
discrete Packet of Energy (Quantum).

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Photon is a very tiny small
particle that couldn’t see by
eyes
Photon = Packet of
Energy = Wave Packet

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CHARACTERISTICS OF LIGHT PHOTON
 Photon has no mass and electrical charge.
 Photon carries electromagnetic energy, E and
momentum, p as well as intrinsic angular momentum (or
spin) associated with its polarization properties.
 Photon travels at the speed of light in vacuum;
c = 3 x 108 m/s

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 Photon has a wavelike character that determines its
localization properties in space and time, and the rules
by which it interferes and diffracts.
 Photon are always in motion.
 Photons can produces :-
 Infrared Light
 Visible Light (e.g: sunlight)
 Ultraviolet (UV) Light - UVa & UVb rays that give you
sunburns
 X – rays

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 According to quantum theory, a photon has an energy, E
given by;
E = hf = hc/λ (unit: Joule,J)
Where, h = Planck’s Constant = 6.625 x 10-34 J/s
c = velocity of light = 3 x 108 m/s
λ = wavelength of light (in meter)
 A photon also carry momentum, p. The momentum is
related to the energy by;
p = E/c = h/ (unit: Js/m)

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 Question 1
Photon in a pale blue light have a wavelength of 500nm. What
is the energy of this photon? Then, calculate the momentum
of photon.
(answ: E = 3.97 x 10-19 J , p = 1.32 x 10-27 Js/m)
 Question 2
Find the energy of a photon travelling with 200 THz frequency
and its momentum.
(answ: E = 1.325 x 10-19 J , p = 4.42 x 10-28
Js/m)
 Question 3
Given the momentum of photon is 6.84 x 10-28. Find the
frequency of photon.
(answ: f = 309.7 THz)

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 The Energy (E) of the light photon is proportional to the
frequency (f) and inversely proportional to the wavelength (λ).
 The higher the frequency (OR lower the wavelength) the
higher the energy of the photon.
- Higher frequency  photon gains more energy
- Lower frequency  photon gains less energy
 For example, BLUE ray has more energy than RED ray
because BLUE ray has higher frequency and shorter
wavelength. (please refer to Table 1.1)

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 Photon can interacts with other particles such as electrons,
protons, neutrons etc.
 When photons bump into another atoms, some of their energy
can get the electrons in those atoms moving faster than they
were before - that's what we call heat. That's why you get
hot sitting in the sun.
 A Photoelectric Experiment by Einstein shows that a very
energetic photons of BLUE light (has very high frequency)
could knocked the electrons out from metal surface to produce
a current as shown in below Figure 1.3.

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 The increasing of frequency will increase the energy of photon.
Therefore, the photons of BLUE light could eject the electrons
compare to RED light because the BLUE light has higher frequency.
Figure 1.3: Photoelectric Effect

The range of frequencies of electromagnetic radiation is
called the Electromagnetic Spectrum.

 Visible light is a small part of the energy range of
Electromagnetic waves.
 The whole range is called the Electromagnetic
spectrum and visible light is in the middle of it.
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E = hf = hc/
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 The higher the frequency of the light, the higher the energy
of the wave.
 Since color is related to frequency and wavelength, there is
also a direct relation between color and energy.
Table 1.1: Spectrum of Visible Light
Shorter
Wavelength
Higher
Frequency
Increasing
Energy

Dispersion of light by Glass Prism
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Decreasing
Wavelength
Increasing
Frequency
Increasing
Energy

 Definition: LIGHT is a special kind of
electromagnetic energy with a wavelength
range from 380nm to 740nm (visible light).
 This electromagnetic energy consists two
components which are electric field, E and the
magnetic field, H which oscillate and
perpendicular each other.
 This electromagnetic radiation are produced by
the vibrations of a charged particles called
photons.
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1. Light travels in a straight line.
For example, the light from candle through pin hole lies in a
straight line.
 This straight line is called a ray of light.
2. Light travels at a high speed. The speed of light in vacuum
is expressed as, c = 3x108 m/s
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3. A bundle of rays is called a “beam of light”. A beam of
light may be parallel, converging or diverging as
shown in below Figure 1.4
(a) Parallel (b) Converging (c) Diverging
 Light travels in a vacuum at a constant speed.
 However when light travels in non-vacuum media
such as air, glass, water, the speed of light will
decrease (air – 0.03% slower, glass – 30% slower)

4. Light consist of different types of colours.
 These colours are differentiated on the basis of their
wavelengths in the visible light spectrum (see Table 1.1)
5. Light has no mass but carries energy and momentum, p
where the energy of light is proportional to the frequency but
reciprocal to the wavelength.
 When the frequency of light increase, the energy of light
also increase and vice versa.
 Different colors of light has different energy because it
has different frequencies.
6. Light is emitted and absorbed in the form of
Quanta(Photons) but propagated in the form of waves
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Pass Through - The rays of light can pass through
the object
Absorption - The rays of light can be absorbed by
the object.
Reflection – The rays of light can be reflected off the
object.
Scattering - The rays of light can be scattered off the
object.
Refraction - The rays of light can be refracted
through the object.
6. Light has different phenomena when it interact with other
objects such as;
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 The sources of light are many and varied.
 Usually there are 2 categories of source;
• Natural Source – (Sun, Star, radio star, lightning, or any
“Body” that exists at a temperature over absolute zero).
• Man-made Source – (Incandescent light, Fluorescent light,
heater, lasers, antennas, radars, and X-ray tubes).
 All materials with temperature above absolute zero emit
electromagnetic radiation (light).
 For example, atoms and molecules which has their own
characteristics set of spectral lines.
 In this case, we are study two THEORY of light source;
i. Atom Equilibrium
ii. Blackbody Radiation
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Atom absorb energy, unstable
condition
Atom is stable or equilibrium by
released energy as a light
 Light is emitted when the atoms is changed from one
form to another form of energy.
 Extra energy is released as a light.
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Energy from
outside
atom atom Light energy
is released

 We model the energy of an atom with the electrons.
 A nucleus of an atom is surrounded by electrons that in
their orbits/shells.
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 When all the electrons are in “unexcited” or ground
state, the atom is assumed at the lowest energy level
(atom is stable).
 When the atom absorbs energy (heat it up),
electrons will “excited” and jumped to higher-energy
shells (atom is unstable).
 As electrons jump from one shell to another, an
amount of energies (Quanta) are emitted.
 This is how an atom can emit the LIGHT.
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 Each electron that jumped emits one photon of
light.
What color is this light?
 Depends on how big the jump between orbits was.
 The bigger the jump, the higher the energy.
 The energy determines color; a blue photon has
more energy than a red photon.
 Shine all the colors together, you get white light!
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 The term "black body" was introduced by Gustav
Kirchhoff in 1860. Then, have been analyzed by Max Planck
(1900) he came out his Planck’s Quantum Theory.
 Definition: Blackbody is an idealized physical body which
absorbs and emits the radiation of energy completely.
 No electromagnetic radiation passes through it and none
is reflected.
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A hollow metallic sphere
coated inside with
platinum black with a
small aperture in its wall
can act as a near
Blackbody.

 As we know, any heated body (blackbody included)
could radiates light over the whole spectrum of
frequencies.
 When the black body is heated to different high
temperatures, it emits radiations of light at different
wavelengths/frequencies/energies/colors.
 A black body emits a temperature-dependent spectrum
of light.
 This thermal radiation from a black body is
called blackbody radiation.
 The hotter the emitter (the bodies), the more energy
emitted and the shorter the wavelength.

 Total power radiated by Blackbody; WB
WB = εσAT4 (unit: watts)
 ε = emissivity
 A = surface area, m2
 T = temperature in unit Kelvin (K)
 σ = Stefan-Boltzmann’s constant = 5.67 x 10-8 Wm-2 K-4
Noted:
 Graybody, WG is one that does not emit as a perfect “blackbody” but emit at
fraction of the theoretical maximum of a blackbody.
 Emissivity for Blackbody usually is 1(ε = 1.0) but for Graybody the ε is
always less than 1 (ε < 1.0).
 Unit conversion of Temperature;
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Fahrenheit(°F) to Celsius(°C)  °C = 5/9(°F – 32)
Where,
Celsius(°C) to Kelvin (K)  K = °C + 273.15
The area, A for sphere is;
A = 4πr2

 Question 1
Calculate the power radiated by a blackbody at room
temperature of 82°F and surface area is 1m2.
(answer: WB = 464.991 Watts)
 Question 2
Calculate the power radiated by a blackbody at room
temperature of 82°F, emissivity = 0.7 and surface area is
1m2.
(answer: WG = 325.494 Watts)
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 There are two types of System International (SI) unit of light;
• Radiometry
• Photometry
 Radiometry
• Radiometry is a field of detection and measurement of light
energy.
• It uses a standardized system for characterizing the radiant
energy.
 Photometry
• Photometry is the astronomical measurement of brightness or
intensity and colour.
• It measured as the amount of light energy that strikes a certain
surface area on the earth over a certain period of time.
 Below Table 1.2 shows the differences between unit of Radiometry
and Photometry.
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58
Symbol (SI units)
Radiometric term
and units
Photometric term
and units
Definition
Q Radiant Energy (J)
Luminous Energy
(talbot)
Quantity of energy
Ф Radiant Power (W) Luminous Power (lm)
Total power/flux emitted in
all direction.
I(E) Irradiance (W/m2)
Illuminance (lm/m2)
or (lux, lx)
Total power falling on unit
area (Flux density)
I(I)
Radiant Intensity
(W/sr)
Luminous Intensity
(lm/sr) or (candela. cd)
Total power emitted by a
point source into unit solid
angle
L
Radiance
(W/sr.m2)
Luminance/Brightness
(lm/sr.m2) or (cd/m2)
Intensity per unit area in a
given direction.
W
Radiant emittance
(W/m2)
Total power radiated in all
direction from unit area
Table 1.2: Difference between Radiometry and Photometry unit

In this chapter, we only discuss Photometric standard unit.
 Luminous Energy (Q)
• Luminous energy is the measure of the perceived energy of light
• also called the quantity of light
• Unit SI : Lumen second or talbot
 Luminous Power/ Flux (Φ)
• Luminous flux/power is the measure of the perceived power of
light.
• Unit SI : Lumen (lm)
• 1 lumen = the luminous power of light produced by a light source
that emits 1 candela of luminous intensity.
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 Illuminance I(E)
• Illuminance is the total luminous power incident on a
surface, per unit area
• It is a measure of the intensity of the incident light.
• Unit SI : Lux (lx) OR lm/m2
 Luminous Intensity I(I)
• Luminous intensity is a measure of the wavelength-
weighted power emitted by a light source in a particular
direction per unit solid angle.
• Unit SI : Candela (cd) OR lm/sr
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 Luminance (L)
• Luminance is a measure of the luminous intensity per unit
area of light travelling in a given direction.
• It describes the amount of light is emitted from a particular
area, and falls within a given solid angle.
• Unit SI : cd/m2 or lm/sr.m2
• Luminance is often used to characterize reflection of light
from flat or diffuse surfaces.
• The luminance indicates how much luminous power will be
perceived by an eye looking at the surface from a particular
angle of view.
• Luminance is an indicator of how bright the surface will
appear (the measurement of Brightness).

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Introduction to light