Are you a Particle or a Wave?
1
Nono, Janel
2
Lapiz, Edmarl
3
Lomongo, Aster
4
Invento, Accel
5
Minoza, Baby Joy
6
Erojo, Jerrick
7
Vito, Airah
MEMBE
Group
5
RS
Objectives of the Topic:
Describe how the propagation
of light, reflection, and
refraction are explained by the
wave model and the particle
model of light.
What is Light?
It is the form of energy that makes it possible to see things.
When you see your face clearly in a
mirror, you have learned the concept
of reflection.
Light bounces off when it strikes the
smooth surface of a mirror and learned
that the angle of incidence is equal to the
angle of reflection.
What is Light?
It is the form of energy that makes it possible to see things.
When you see your face clearly in a
mirror, you have learned the concept
of reflection.
Light bounces off when it strikes the
smooth surface of a mirror and learned
that the angle of incidence is equal to the
angle of reflection.
What is Light?
However, this is no longer the case when light enters a
transparent medium like water.
Light bends as it enters the water's surface
from the air. To illustrate, a pencil
submerged in a glass of water appears
broken when observed from the side of the
glass where the portion of the pencil is
submerged.
This concept is called refraction.
A look back in history
The concept of light started way,
way back in the 4th century BC.
A look back in history
a well-respected Athenian
natural philosopher
proposed that light consisted
of rays emitted by the eyes.
The bouncing off of rays on an
object allows us to perceive its
color, size, and shape.
(
)
A look back in history
he explained that people can see their
surroundings when light reflects from the
objects and passes through their eyes.
He said that light does not emanate from
our eyes, but it is emitted by objects such
as lamps.
He concluded that light travels from the
source in straight lines.
(
)
A look back in history
From then on, several schools of thought
on the nature of light emerged and two
of the biggest rival theories on the nature
of light are the corpuscular theory and
the wave theory.
A look back in history
Corpuscular theory of light was
pioneered by the English mathematician,
astronomer, and physicist in 1672.
He believed that light was composed
of a stream of corpuscles (small
particles) that travel in a straight line
at a certain speed.
A look back in history
These corpuscles are weightless, rigid, and perfectly elastic
that they can bounce off from an opaque medium. Although
this theory failed to explain certain light phenomena and
properties such as diffraction, polarization, and interference,
Newton was so greatly revered as a scientist that it was nearly
impossible for anyone to dispute his theory.
A look back in history
in 1801 he presented a serious
challenge to Newton’s ideas.
Young saw some problems with Newton’s
corpuscular theory. For Young, light had
too many characteristics of a classic
waveform like diffraction and refraction.
These two basic phenomena could not be
explained by Newton’s theory.
A look back in history
Young’s experiment provided support for the wave theory of light.
Figure 1
Figure 2
A look back in history
Figure 1
A thin card held in
front of a beam of
light.
A look back in history
Figure 1.1
A thin card held in
front of a beam of
light.
A look back in history
Figure 2
Interference of light
as seen in Young’s
experiment.
A look back in history
Figure 2.1
Interference of light
as seen in Young’s
experiment.
light
A look back in history
After almost a century, issues in the wave theory
began to appear even though Newton’s corpuscular
theory of light had been discarded. The most
significant of these was the photoelectric effect.
A look back in history
Photoelectric effect –
a phenomenon in which light illuminating
a metal surface causes electrons to be
ejected.
A look back in history
A look back in history
Einstein suggested that the photoelectric effect
could be readily and easily explained if one takes
light as a packet of energy–which may be
considered as a stream of particles.
Einstein thought that if energy comes in packets, then light could
come in packets too! So, in 1905, Einstein suggested that the
experiment could be readily and easily explained if one imagined
that light in fact consists of a Figure 1: A thin card held in front of a
beam of light Figure 2: Interference of light as seen in Young’s
experiment stream of particles.
These particles, which he later named photons, exhibit wave-like
properties. This was the start of the idea of wave-particle duality.
This implies that Electromagnetic radiations (EM waves), must
possess both wave-like and particle-like properties.
The Dual Property of
Light
Propagation, Reflection, and Refraction of Light
In science, models are used to make predictions about the behavior of
phenomena in the physical world.
If the model fails to account for or explain key observations, we modify the
model or use an entirely different model.
Models are usually created to explain observable natural phenomenon.
Model of a particular natural phenomenon may have contradictions
in their explanations.
A good example of competing models is the understanding of the nature
of light.
How does light behave as a particle?
Propagation of Light
Isaac Newton used the analogy of
a ball to explain the rectilinear
motion of light. Just like when a
ball is thrown up into the air, the
ball follows a parabolic path
because of the effect of gravity
(see Figure 3).
Figure 3
How does light behave as a particle?
Propagation of Light
However, when the ball is
thrown horizontally very
quickly, the ball follows a
straight-line path (see Figure 4).
He proposed that particles of
light follow a straight path until
they are reflected, refracted, or
disturbed in some other
manner.
Figure 4
How does light behave as a particle?
Propagation of Light
You may also observe this
phenomenon when light beams
pass through jalousie windows
where light follows a straight path
(see Figure 5).
Figure 5
How does light behave as a particle?
Reflection of light
Newton found that when light strikes a mirror,
the angle of incidence is equal to the angle of
reflection. He demonstrated this with hard
spheres against a hard surface. He was able to
show that the initial velocity of the sphere is
equal to its final velocity in an elastic collision
between hard spheres. He assumed that light
particles display elastic collision since no
energy is lost when light hits a surface.
Figure 6
How does light behave as a particle?
Refraction of light
Newton explained refraction by comparing the
movement of the particles of light with that of a ball
descending an inclined plane (Figure 7). According to
Newton, the particles of light will accelerate as they
pass from air to water. Newton claimed that water
attracted the particles of light, predicting that the
speed of light would be faster in water than in air.
However, scientific experiments showed a different
result. Light actually travels faster in air than in water
because of the higher index of refraction of water. The
higher the index of refraction of a medium, the slower
is the speed of light.
Figure 7
Partial Reflection and Refraction of Light
Newton’s particle model of light failed to thoroughly explain partial
reflection and refraction of light, and diffraction. Questions also arose
about the propagation of light. If light were a particle, how could light
beams pass through each other without scattering? See for example
Figure 8. As you rotate the glass block, the light ray will partially reflect
and partially refract. But when the incidentangle exceeds the critical
angle, the light ray will totally reflect.
Newton tried to explain this by suggesting a theory of “fits”. According
to him when light arrives at the border of two media, light rays hit
either an access to easy reflection or access to easy refraction. In other
words, the particles of light must somehow alternately reflect and
refract. However, how, in partial reflection, does a particle know if it
should reflect or refract? Even Newton acknowledged that his
explanation of partial reflection and refraction was insufficient.
What is light according to Christian Huygens? (Light
as a Wave)
A Dutch physicist
believed that light was made up of waves
vibrating up and down perpendicular to
the direction of the wave propagation.
He formulated a way of visualizing wave
propagation. This became known as the
“Huygens’ Principle”.
he is considered the pioneer of the wave
theory of light.
His wave theory of light has stood the test of
time and today, it is considered the backbone
of optics (BYJU's The Learning App 2020).
What is light according to Christian Huygens? (Light
as a Wave)
Propagation of Light
Christian Huygens thought of light rays as
the direction of travel of the wave (wave ray).
As shown in Figure 9A, every point on a
wavefront can be considered as a source of
tiny wavelets that spread out in the forward
direction of the wave itself. The direction of
light is dictated by the direction of these
wavelets.
Figure 9A
What is light according to Christian Huygens? (Light
as a Wave)
Reflection of light
This can be observed when using water waves. The
angle of incidence of the wave is equal to its angle of
reflection. As the wave front hits the surface, wavelets
hit the surface at different times (see Figure 9B). The
new wavelets will now travel in the direction tangent
to the wavelets that hit the surface. If we make a line
perpendicular to the surface of the mirror, we can
observe that the angle between the normal line and
the reflected ray is equal to the angle between the
normal line and the incident ray
What is light according to Christian Huygens? (Light
as a Wave)
Refraction of Light
When a beam of light travels between two
media having different refractive indices, the
beam undergoes refraction and changes
direction when it passes from the first
medium into the second. To determine
whether the light beam is composed of
waves or particles, a model for each can be
devised to explain the phenomenon
(Figure 8).
What is light according to Christian Huygens? (Light
as a Wave)
Refraction of light
According to Huygens' wave theory, a small portion
of each angled wave front should impact the
second medium before the rest of the front reaches
the interface. The portion that is now travelling in
the new medium will slow down due to the higher
refractive index, while the portion that is still in the
first medium is still moving at the same speed.
Because of the difference in speed of the wavefront
passing through two different media, the direction
of the wavefront will bend towards the normal line –
the line perpendicular to the surface of the second
medium where the wavefront enters.
Photoelectric Effect and the Particle Nature of
Light
In 1905, Albert Einstein proposed that electromagnetic waves,
including light, consist of discrete packets of energy called
photons. A photon is massless and has no electric charge. Its
speed is exactly equivalent to the speed of light, c, in a
vacuum. Each photon has a fixed amount of energy which
depends on the frequency, not on the intensity as previously
believed by James Clerk Maxwell, the proponent of the
Electromagnetic Wave Theory of Light. Red light has lower
frequency compared to violet light, hence the photons that
make up red light has lesser energy.
Photoelectric Effect and the Particle Nature of
Light
The dependence of a photon’s energy on frequency
was first observed in the photoelectric effect
experiment. Heinrich Hertz, a German physicist,
helped established the photoelectric effect while he
was conducting experiments on the production and
reception of electromagnetic waves. However, he did
not conduct further investigations nor make further
attempts to explain his observations.
Photoelectric Effect and the Particle Nature of
Light
In Einstein’s Theory of Photoelectric Effect, a beam of light is not a wave
propagating through space. A beam of light is made of a group of discrete
packets of energy. When a certain beam of light shines on a metal plate, the
Figure 8.A Figure 8.B emission of electrons from the metal plate will not depend
on the intensity of light nor on the duration of exposure. If you have a lowfrequency light and you increase its intensity in your attempt to dislodge a
certain electron from a metal plate, you are merely increasing the number of
low-energy photons. None of these low-energy photons would be able to dislodge
the electron unless the photons carry an energy higher than the electron’s
binding energy. Thus, the particle model predicts that only photons with enough
energy (above the threshold frequency) can knock out electrons from a metal
plate. An electron within a metal plate will likely be emitted or rejected when it
absorbs an energy from a photon that is higher than its binding energy.
Photoelectric Effect and the Particle Nature of
Light
The energy of a photon of light of a specific
frequency, f, is given by the equation: 𝑬 = 𝒉𝒇 where
h is the Planck’s constant. Light is then believed to
be quantized. For his discovery of the photoelectric
effect, Albert Einstein was awarded the Nobel Prize
in Physics in 1921.
Photoelectric Effect and the Particle Nature of
Light
The classical notion of the particle nature of light provided by
Isaac Newton - which can be used to explain some properties of
light such as light reflection and refraction, and even light
propagation - has since then been replaced by this modern
framework of the particle nature of light. Light should no
longer be perceived to be composed of corpuscles, but of
discrete packets of energy called photons. When we talk about
light as a particle, we now think about photons. When we talk
about light as a wave, we now think about electromagnetic
waves.
The Modern Framework – The Dual Nature of
Light
The wave-particle duality states that light may be described as
either a particle or a wave. This was suggested by Louis de
Broglie, a French physicist, in 1923. The wave-particle duality is a
feature that is shared by light and all matter. These days, we
believe that the behavior of light cannot be fully accounted for
by a classical particle model of Isaac Newton or by the classical
wave model of Christiaan Huygens. Light has both particle-like
and wave-like properties. It can behave simultaneously as a
particle and as a wave. In some phenomena we see its wave-like
properties, while in other situations we observe its particle-like
properties.
The Modern Framework – The Dual Nature of
Light
This new theoretical and mathematical framework that began to
arise in the 20th century paved way to what we now know as
quantum mechanics. It is a theory in physics that aims to account
for observations that could not be explained or reconciled with
classical physics. We must remember though that this modern
framework on the nature of light continues to evolve as
technology advances. In 2015, the first ever snapshot of light as
both a particle and a wave was observed by Fabrizio Carbone and
his team at the Swiss Federal institute of Technology in Lausanne,
Switzerland through an ultrafast energy-filtered transmission
electron microscope.
Quiz Time
Quiz Time!
1.
Which property of light is observed
when it bounces off the surface of
object?
A. diffraction
B. interference
C. reflection
D. refraction
Quiz Time!
2.
Which of the following describes light
as it is propagated from a luminous
object?
A. light is bent
B. light is bounced off
C. it travels in a straight line
D. it curves around the object
Quiz Time!
3.
Which property of light is observed
when it bends as it passes from one
medium to another?
A. diffraction
B. interference
C. reflection
D. refraction
Quiz Time!
4.
What would happen to the light of a laser
pointer as it enters a rectangular glass
block at an angle of 15°?
A. It would bounce off the surface.
B. It would travel in a straight line.
C. It would bend at a certain angle.
Quiz Time!
5.
Which of the following best explains
why we can see an object placed in
front of us?
A. it can reflect light
B. it can refract light
C. it can absorb light
D. it can transmit light
Quiz Time!
6-20.
Create a Venn diagram and write the
similarities and differences of how the
Huygens' wave model and Newton’s particle
model of light describe the propagation of
light, reflection, and refraction of light.