SHS

Physical Science
Quarter 2- Module 3
Week 3
NATURE AND PROPAGATION OF LIGHT
AND PHOTONS

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Physical Science– Grade 11
Quarter 2 – Module 3, Week 3: Nature and Propagation of Light and Photons

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Regional Director: Gilbert T. Sadsad

Assistant Regional Director: Jessie L. Amin

Development Team of the Module

Writer: Rommel Carl R. Peralta

Editors: Lesson 1 - Jocelyn P. Navera and Brenly B. Mendoza

Lesson 2 - Jocelyn P. Navera, Brenly B. Mendoza, Kristina Nieves, Aster
Malto, Jeanine Cristobal and Kennette De la Fuente

Reviewers: Kristina N. Nieves and Aster Malto, Jeanine Cristobal and

Kenneth De la Fuente

Illustrator: Ray Daniel G. Peralta

Layout Artist: Jose P. Gamas, Jr.

Language Editor: Diana Desuyo

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 Self-Learning Module for Senior High School Learners

LESSON
NATURE AND PROPAGATION OF LIGHT
AND PHOTONS

In the previous lessons, you understood that light

originates from the accelerated motion of electrons. It is an
electromagnetic phenomenon and only a tiny part of a larger
whole wide range of electromagnetic waves called the
electromagnetic spectrum. Today, you will know about the
two forms of a light – a particle and a wave and will understand how these two forms
explained the propagation, reflection, and refraction of light. Moreover, you will be
given activities to enhance your understanding towards the relationship of energy of
the photon and photon frequency.
In this module, you will investigate about light as a wave and a particle. This
module contains two lessons.
Lesson 1 – Nature and Propagation of Light

Lesson 2 – Photons

1. Describe how the propagation of light, reflection, and refraction are explained
by the wave model and particle model of light. (S11/12PS-IVf-59)
2. Explain how photon concept and the fact that the energy of a photon is
directly proportional to its frequency can be used to explain why red light is
used in photographic dark rooms, why we get easily sunburned in ultraviolet
light but not in visible light and how we see colors. (S11/12PS-ivg-64)

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 Knowing the properties of light is very essential in
understanding the complexity of matter and energy. Hence,
this knowledge will give you the brightness to the gray area
of unknown. Do you want to want to see this brightness?
Read on and accomplish the tasks prepared for you.
At the end of the module, you should be to:

 Explain the wave-particle duality of light.

 Use the wave and particle model of light to explain how light is being propagated,
reflected and refracted.
 Identify and describe the properties of photons.
 Explain how the relationship between the energy of a photon and its frequency can
be used to explain why red light is used in photographic dark rooms, why we get easily
sunburned in ultraviolet light but not in visible light and how see colors.

Directions: Read and understand each item carefully.

Choose the letter of the correct answer. Write your answer
on your answer sheet.

1. Which of the following concepts refers to the angle that is equal to the angle of
incidence for all wavelengths and for any pair of materials?
a. Reflection
b. Refraction
c. Polarization
d. Interference

2. Which of the following statements TRUE about the dual theory of light?
a. It states that light acts as a wave when it moves through space and as a particle
when it interacts with matter.
b. It states that light acts as a particle when it moves through space and as a wave
when it interacts with matter.
c. A and B
d. None of the choices

3. What is the mass of a photon?

a. Zero b. 2.23 X 10-15 kg c. 2.99 X 10-8 kg d. 4.00 X 10-20kg

4. Which of the following is CORRECT?

a. The higher the frequency, the more energy the photon has.
b. The lower the frequency, the more energy the photon has.
c. The higher the frequency, the lower energy the photon has.
d. The higher the frequency, the less energy the photon has.

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 5. What is the speed of a photon when it travels in an empty space?
a. 2 X 108 m/s b. 3 X 108 m/s c. 4 X 108 m/s d. 5 X 108 m/s

Hi! How did you find the test?

Please check your answers at the answer key
section and see how you did. Don’t worry if you got
a low score, this just means that there are more
things that you can learn from this module. So, hop
on!

Lesson 1 – Nature and Propagation of Light

CROSSWORD PUZZLE
Directions: Complete the crossword puzzle by identifying
the concepts being introduced by the definitions below.
Write your answer in your answer sheet.

1. It is an energy-carrying wave emitted by a vibrating charge (often electrons) that

is composed of oscillating electric and magnetic fields that regenerate one
another.

2. It is an electromagnetic wave. When emitted or absorbed, it also shows particle

properties. It is emitted by accelerated electric charges.

3. This refers to the bending of an oblique ray of light when it passes from one
transparent medium to another. This is caused by a difference in the speed of
light in the transparent media.

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 4. It pertains to the return of light rays from a surface in such a way that the angle at
which a given ray is returned is equal to the angle at which it strikes the surface.
.
5. It is the surface of constant phase that moves with a speed equal to the
propagation speed of the wave.

Good job in finishing the activity! Take note of the key concepts you had written. These
words might appear on the next activities.

WAVE-PARTICLE DUALITY

In 1905, a German physicist named Albert Einstein

developed a novel theory about electromagnetic (EM) radiation which is called the wave-
particle duality theory. It explains the behavior of electromagnetic radiation as a wave and
a particle. Einstein explained that when an electron returns to a lower energy level and gives
off electromagnetic energy, the energy is emitted as a discrete “packet” of energy. This
packet of energy is called photon. According to Einstein, a photon is in a form of particle but
moves like a wave (see Figure 1). The theory suggests that waves of photons traveling
through matter or space make up electromagnetic radiation.

Energy of a Photon

Photon energy is the energy carried by

a photon. The photon energy is directly proportional
to its electromagnetic frequency and thus, inversely
proportional to the wavelength, the higher amount of
photon's frequency, the higher its energy and the
longer the wavelength of the photons, the lower its
energy.
There are different units that can be used in Source: Wave-Particle Theory. https://flexbooks.ck12.org/cbook/ck-12-
photon energy; these are electronvolt (eV) and physics-flexbook-2.0/section/13.4/primary/lesson/wave-particle-theory-ms-ps

the joule. One joule is equivalent to 6.24 × 1018 eV, Figure 1.Wave of photon travelling through
the large units can be used for representing the matter or space
energy of photons with higher frequency
and higher energy like gamma rays, as
contrasting to photons with low energy
photons, like those in the radio
frequency region of the electromagnetic
spectrum.

Evidence for the Wave-Particle Theory

Evidences were discovered and

showed after Einstein proposed his wave-
particle duality theory. Scientists shone
laser light using two slits in a barrier that
blocked the light (see Figure 2). They took Source: Wave-Particle Theory. https://flexbooks.ck12.org/cbook/ck-12-physics-
flexbook-2.0/section/13.4/primary/lesson/wave-particle-theory-ms-ps
photos of the light that passed through the Figure 2. Experiment showing that light is both a particle and
slits using a very sensitive camera. The wave

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photos showed small pinpoints of light passing through the double slits. This seemed to
reveal that light consists of very small particles. However, when the camera was exposed to
the light for a long time, the pinpoints accrued in bands that look like interfering waves.
Therefore, the experiment exhibited that light appears to consist of particles that act like
waves.

LIGHT MYSTERY
Part A – Directions: Refer to the picture below. Write your answer
in your answer sheet.

Picture A shows the image of the students on water while Picture B shows a glass with
refracted light. How do these phenomena happen?

A B
Photocredits: Cogasa, delos Angeles & Photocredits: Olmeda, Olavides, & Oloya (2020),
Gonzales (2020), Ligao National High Ligao National High School, Ligao City, Albay
School, Ligao City, Albay

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Part B - Directions: The concepts below are connected to your explanation in the Light
Mystery Activity. Let us deepen your understanding by explaining the phenomena being
exhibited by the picture below.

LIGHT
BEAM

REFLECTED
RAY

AIR
WATER
REFRACTED

Photocredit: Peralta, Rommel Carl (2020). Physical Science Module

Figure 3: Light Ray Diagram

Guide Questions:
1. How is light propagated?
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2. What two important aspects of propagation of light are being exhibited by Figure 3?
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Good job in finishing the activity! Take note of the key concepts you had written. These
words might appear on the next activities.

When a light wave strikes a smooth interface

separating two transparent materials (such as air, glass or
water), the light wave is somewhat reflected and partly
refracted into the second medium or material, as shown in
Figure 3. For example, when you look into a glass window
in the mall from the street, you see a reflection of the street
scene, but a person inside the mall can look out through
the glass window at the same scene as light reaches him
or her by refraction.
Now, try to describe REFLECTION and REFRACTION
of light by illustrating the particle and wave model of light.
Describe how light is being propagated in each model.

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 Particle and Wave Refraction
When a light beam travels between two media having different refractive indices, the
light beam undergoes refraction, and changes its direction when passes through different
medium (see Figure 3 &4) just like in
picture B which shows that the light was
bended when it passes through a glass of
water. On the other hand, a model for
each can be devised to explain and to
determine whether the light beam is
composed of waves or particles,
phenomenon (see Figure 4). Huygens'
wave theory explains that a small portion
of each angle wavefront should hit the
second medium before the rest of the
front reaches the interface. Light beam
will start to move through the second
Figure 4. Refraction of Particles and Waves
medium while the other part of the wave is
still traveling in the first medium, but will
move slower due to the higher refractive index of the second medium. The light beam will
bend into the second medium since the wavefront is traveling at two different speeds,
therefore changing its angle of propagation.

Particle and Wave Reflection

Wave theory speculates that a
source of light releases light waves that
spread in various directions. The light
waves are reflected according to the
arrival angles when the light beam
touches a mirror, but with each wave
turned back to front that yielded a
reversed image (Figure 3). The shape of
arriving waves depends upon the Particles Waves
distance of the light source from the
mirror. Light that came from a close Figure 5. Particles and Waves Reflected by a Mirror
source maintains a spherical, highly
curved wavefront. On the other hand, the light beam being emitted from a distance source
will spread more and impact the mirror with wavefronts that are almost planar.
In the case for a particle nature of light, light emitted by a source, regardless of the
distance, light arrives at the mirror surface as a stream of particles, which bounce away or
are reflected from the smooth surface (see Figure 5). Since the particles are very small, a
great number of particles are involved in a propagating light beam. Upon touching the mirror,
the particles bounce from various points, so their order in the light beam is reversed upon
reflection to produce a reversed image. Both the particle and wave theories explain reflection
from a smooth surface. However, the particle theory also suggests that if light beam touches
a very rough surface, its particles bounce away at different angles which resulted to the
scattering the light.
SOURCE & PHOTOCREDITS: Robert T. Sutter, Matthew J. Parry-Hill and Michael W. Davidson
Accessed October 19, 2020.
https://micro.magnet.fsu.edu/primer/java/particleorwave/refraction/index.html

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 Directions: Describe and explain the property of light
depicted in each photo below. Write your answer in your
answer sheet.

Photo credits: Oraa, Penafiel & Peralta (2020), Ligao National High School, Ligao City, Albay

Photo credits: Oraa, Penafiel & Peralta (2020), Ligao National High School, Ligao City, Albay

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Lesson 2 – Photons
JUMBLED LETTERS
Directions: Below are the persons behind the development of
concept of photons. Arrange the letters and match it to their
accomplishment.

1. He discovered that light travelled in straight lines.

2. He argued that light is made of pulses that propagate instantaneously when contacting
„balls‟ in a medium.

3. He showed how to make reflected, refracted, and screened waves of light and also
explained double refraction

4. He predicted an „electromagnetic wave‟, which can self-sustain, even in a vacuum, in the

absence of conventional currents

5. He used the term Lichtquant, or quantum of light

CHOICES
SCDEARTES ELIDUC HGENSUY STEEIINN NTONWE XWELMAL

Good job in finishing the activity! Take note of the key concepts you had written. These
words might appear on the next activities.

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 PHOTON

Photon is a light‟s particle which essentially a packet

of electromagnetic radiation. The energy of the photon is dependent on
its frequency . When the frequency is higher, the photon has more energy. Hence, a beam of
light has plenty of photons. With these, the red light (with plenty of photons, with low energy)
can carry high power energy to a given area than blue light (fewer photons with high
energy).
The speed of light (c) in a vacuum has a constant speed of 3x108 m/s. This is
because of more active and with high frequency photons like X-rays and gamma rays travel
at exactly the same speed as photons with lower energy and low frequency, like those in
the infrared. When the frequency of a photon rises, the wavelength (λ) falls, and as the
frequency falls, the wavelength rises. The equation that is connected to these three
quantities for photons is:
c=λf
where: c = speed of light (3 x108 m/s)
λ = photon‟s wavelength
f = photon‟s frequency

Since wavelength and frequency are determined by each other, the formula for the
photons containing energy can be written in two different forms:

E=hf or E=hc
f
where: E = photon‟s energy
h = Planck's constant (6.62606957(29)×10-34 J·s )
f = photon‟s frequency
λ = photon‟s wavelength
c = speed of light (3x108 m/s)

One of the oddest discoveries of quantum mechanics is that light and other particles,
like photons, different waves or particles depends on the kind experiment that evaluates
them. Light spreads out according to wavelength when it passes through a prism. In
contrast, bombard metal with light, and displays a particle side, where only photons that
have greater amount of energy release electrons. This experiment, called the photoelectric
effect. This was one of the reasons why won Einstein his Nobel Prize. Photons with
inadequate energy can hit metal, however, won't hit any loose electrons. Photons that
surpass a verge of energy usually do hit the loose electrons, but, as the photon's energy
becomes greater than necessary the probability that it ejects an electron reduces. Hence, a
low summation of energy beam of violet light might emit electrons from a specific metal,
where a high energy red beam fails to emit one. Even though each photon in the red beam
has lower energy, it has more energy. Thus, this discovery is the reason for the birth of
quantum revolution.
Source:https://energyeducation.ca/encyclopedia/Photon#:~:text=A%20photon%20is%20a%20particle,more%
20energy%20the%20photon%20has.&text=The%20speed%20of%20light%20(c)%20in%20a%20vac
uum%20is%20constant.
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Photon Properties
Photons have basic properties, these are:

 Photons have zero mass and rest energy. It only exists as particles in motion.
 Photons are elementary particles even if it has lack of rest mass.
 Photons have zero electric charge.
 Photons are stable.
 Photons are spin-1 particles which converts them bosons.
 Photons have energy and momentum which are dependent on its frequency.
 Photons can interact with other particles like electrons in the Compton Effect.
 Photons can be destroyed or created by different natural means, for example when
radiation is absorbed or ejected.
 Photons travel as fast as speed of light when in empty space.

History

Light has a nature either as a particle or a wave. For a long time, philosophers and
scientists have debated about the matter that was barely resolved a century ago. Around the
sixth century BC, the branch of Hindu philosophy called Vaisheshika had an astonishing
physical perception about light. Like the ancient Greeks, believed that the world was based
on the elements of fire, earth, air, and water. Light was thought to be made of tejas, a very
fast-moving particle. It similar to the modern theory of light which is light is composed of
photons, a term that was in 1926 by Gilbert Lewis and Frithiof Wolfers.
In 300 BC, the ancient Greek philosopher Euclid underwent a huge scientific
breakthrough when he postulated that light travelled in straight path. He also described the
laws of reflection. A century later, Ptolemy complemented the laws of reflection by the
concepts of refraction. In 1021, the laws of refraction were formally established by Ibn al-
Haytham in his book Kitab al-Manazir, or Book of Optics.

During the Renaissance period, in the essay of René Descartes entitled La

dioptrique, he discussed that light is made of beats that propagate promptly when contacting
„balls‟ in a medium. Later in 1690, Christian Huygens in his writing entitled Traité de la
lumière discussed that light as compressible waves in a flexible medium which is similar to
sound waves. Huygens exhibited the reflection, refraction, and light‟s screened waves and
elucidated the double refraction of light.
With these, scientists formulated two entrenched camps. One side believed that light
was in a form of waves while the other side believed that light is in a form of particles or
corpuscles. The corpuscularists Isaac Newton wasn‟t fond at wave theory since that light
would be able to lose too far into the shadow.

In 18th century, corpuscular theory dominated the arguments about the nature of
light. But May 1801, Thomas Young introduced the two-slit experiment where he exhibited
the interference of light waves. In the first version of his experiment, he actually didn‟t utilize
two slits, but rather used a single thin card. He simply covered a window by a piece of paper
with a very small hole which served to funnel a very thin light beam. Using the card in his
hand, Thomas Young witnessed how the single beam splits into two. The light that passed
on the one side of the card interfered with light from the other card‟s side to generate fringes,
which could be seen on the opposite wall. Later, Thomas Young used this data to compute
for the wavelengths of the different light colors and came close to its modern values. The
demonstration showed a solid evidence that light was a in a form of a wave.
On the other hand, the corpuscularist movement was gaining steam after the recent
developments attributed to the polarization of light to some kind of asymmetry among the
light corpuscles. In 1821, the corpuscularist movement suffered a great defeat at the hand of

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Augustin Fresnel who showed that polarization could be explained if light were a transverse
wave in form with no longitudinal vibration. Fresnel also came up with a detailed wave theory
of diffraction.

By this point, there was little stable status for the followers of Newton to continue the
debate. It appeared that light is in a form of a wave and that‟s that. However, the problem
was that the fabled aether which is a mysterious medium required to support
electromagnetic fields and to result to laws of propagation of Fresnel which was lost despite
best efforts of everyone in order to find it.

Years later, a huge breakthrough came around 1861 when James Clerk Maxwell
summarized experimental and theoretical knowledge about electricity and magnetism in 20
different formulas. Maxwell predicted an electromagnetic wave, which can self-sustain, even
in a vacuum or in the absence of conventional currents. Hence, no aether is required for light
to propagate. He also predicted that the speed of this wave can be 310,740,000 m/s which
were just a few percentage of the exact value of the speed of light.

With this agreement, it seems that light and magnetism have affections of the same
substance, and light is an electromagnetic disturbance spread through the field. From that
day forward, the concept of light was joined for the first time with those of electricity and
magnetism.

On December 14, 1900, Max Planck exhibited that heat radiation was emitted and
absorbed in distinct packets of energy which is called quanta. In 1905, Albert Einstein
explained that quantum is also applied to light. Einstein used the terminology Lichtquant, or
quantum of light. Now, at the dawn of the 20thcentury, a new revolution in physics would
once again hinge on the nature of light. This moment, it is not about what light is if it is a
crepuscule or wave. It is whether it is both or not.

Modern Theory of Light and Photons

According to Albert Einstein, light is a particle in a form of photon which flow through
a wave. He was convinced that light had a particle form following his discovery about
photoelectric effect, in which electrons were emitted in the metal surface exposed to light.
That scenario could not have happened when light was a wave. Another mystifying matter is
the process on how photoelectrons multiply and generate and when strong light is applied.
Einstein discussed that photoelectric effect through mentioning that “light itself is a particle”.

The Einstein‟s theory of light quantum explained that energy of light is related to its
oscillation frequency. He further explained that photons have energy which is equal to the
“Planck‟s constant multiplied to oscillation frequency,” and this photon energy is the altitude
of the oscillation frequency while the light‟s intensity corresponds to the amount of photons.
The properties of light, which is a type of electromagnetic wave, are due to the behaviour of
very small particles called photons that cannot be seen using the naked eye.

According to Einstein, when electrons within matter collides the photons, the former
receives the energy of latter and flies out and that the higher the oscillation frequency of the
photons that collide, the higher amount of electron energy will come flying out. A practical
example of this is the solar panels. Thus, light is a flow of photons, the energy of these
photons is the altitude of their oscillation frequency, and the light intensity of is related to the
amount of photons.

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 Source: https://lco.global/spacebook/light/light-
particle/#:~:text=Light%20behaves%20mainly%20like%20a,shorter%20
wavelength%20photons%20have%20more.

Albert Einstein was able to prove his theory through deriving Planck‟s constant from
his experiments on the photoelectric effect. His computation provided 6.6260755 x 10-34 as
Planck‟s constant value which is exactly what Max Planck obtained in 1900.Clearly, this
pointed to a close relationship between the properties and the light‟s oscillation frequency
and the properties of light particle. In 1920s, Erwin Schrödinger discussed these ideas with
using his equation for the quantum wave function to show the appearance of the wave.

More than a century since Einstein exhibited the double nature of light, Swiss
physicists at the École Polytechnique Fédérale de Lausanne showed the snapshot of the
dual behavior of light for the first time. In 2015, the team of Fabrizio Carbone performed an
experiment in which a laser was utilized to fire onto a nanowire. This caused the electrons to
vibrate. Light travels along the tiny wire in two directions. When the light waves travelling in
opposite directions to meet each other, they form a new wave that is standing in place. Here,
this standing wave serves the light source for the experiment that radiates around the
nanowire. The result of this experiment acts as a fingerprint of the wave-nature of light.

Facts about Photons

 Not only is light made up of photons, but all electromagnetic energy (i.e.
microwaves, radio waves, X-rays) is made up of photons.
 The original concept of the photon was developed by Albert Einstein. However, it
was scientist Gilbert N. Lewis who first used the word “photon” to describe it.
 The theory that states that light behaves both like a wave and a particle is called the
wave-particle duality theory.
 Photons are always electrically neutral. They have no electrical charge.
 Photons do not decay on their own.

Source:Puiu, Tibi (2007), What exactly is a photon, https://www.zmescience.com/science/what-is-photon-

definition04322/#:~:text=A%20photon%20is%20the%20smallest,denoted%20by%20the%20letter%20c.

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 Directions: Write TRUE if the statement is correct and write
FALSE if the statement is incorrect.

1. Photons have zero mass and rest energy. They only exist as moving particles.
2. Photons are elementary particles despite lacking rest mass.
3. Photons have positive electric charge.
4. Photons are unstable.
5. Photons are spin-1 particle which makes them electrons.
6. Photons carry energy and momentum which are dependent on the frequency.
7. They can have interactions with other particles such as electrons.
8. They can be destroyed or created by many natural processes, for instance when
radiation is absorbed or emitted.
9. When in empty space, photon travels at the speed of light.
10. Proton is a particle of light which essentially is a packet of electromagnetic radiation.

Directions: Answer the following questions. Write your answers

in your answer sheet.

1. What is the wave-particle duality of light?

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2. How does light propagated, reflected and refracted? Draw the wave and particle
diagram to answer this question.

3. Why does photographers used red light in producing pictures in the dark room?
___________________________________________________________________
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__________________________________________________________________

4. Why do people get sunburned by ultraviolet light?

___________________________________________________________________
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 The following terms used in this module are defined as
follows:

Lesson 1

 WAVE-PARTICLE DUALITY THEORY OF LIGHT states that light acts as a wave

when it moves through space and as a particle when it interacts with matter.
 ELECTROMAGENTIC WAVE is an energy-carrying wave emitted by a vibrating
charge (often electrons) that is composed of oscillating electric and magnetic fields
that regenerate one another.
 HUYGEN‟S PRINCIPLE states that if the position of a wave front at one instant is
known, then the position of the front at a later time can be constructed by imagining
the front as a source of secondary wavelets. Huygens‟s principle can be used to
derive the laws of reflection and refraction.
 INDEX OF REFRACTION OF A MATERIAL is the ratio of the speed of light in
vacuum to the speed in the material. If is the wavelength in vacuum, the same wave
has a shorter wavelength in a medium with index of refraction n.
 LAW OF REFLECTION states that the angle of reflection equals the angle of
incidence.
 LAW OF REFRACTION relates the angles of incidence and refraction to the indexes
of refraction of the materials.

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  LIGHT is an electromagnetic wave. When emitted or absorbed, it also shows particle
properties. It is emitted by accelerated electric charges.
 PLANE OF INCIDENT is a single plane where all incident, reflected, and refracted
rays and the normal lie.
 REFLECTION is the return of light rays from a surface in such a way that the angle at
which a given ray is returned is equal to the angle at which it strikes the surface.
When the reflecting surface is irregular, the light is returned in irregular directions;
this is diffuse reflection. In general, the bouncing back of a particle or wave that
strikes the boundary between two media
 REFRACTION is the bending of an oblique ray of light when it passes from one
transparent medium to another. This is caused by a difference in the speed of light in
the transparent media. In general, the change in direction of a wave as it crosses the
boundary between two media in which the wave travels at different speeds.
 WAVEFRONT is a surface of constant phase; wave fronts move with a speed equal
to the propagation speed of the wave.

Lesson 2

 BOSON is a particle that follows Bose–Einstein statistics. Bosons make up one of

two classes of elementary particles, the other being fermions.
 ENERGY is the capacity for doing work. It may exist in potential, kinetic, thermal,
electrical, chemical, nuclear, or other various forms.
 FREQUENCY is the number of waves that pass a fixed point in unit time; also, the
number of cycles or vibrations undergone during one unit of time by a body in
periodic motion.
 LIGHT refers to electromagnetic radiation of any wavelength, whether visible or not
 PHOTON is a bundle of electromagnetic energy. It is the basic unit that makes up all
light.
 PHOTOELECTRIC EFFECT is a phenomenon in which electrically charged particles
are released from or within a material when it absorbs
electromagnetic radiation.

Hurray! you have completed your learning

episodes in this module! Please check your answers
by referring to the answer key.
Congratulations!

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