Lesson 5
Light
PHYSICS 2 | STEM 2ND SEMESTER
Objectives
By studying Light, we shall find that a small number of underlying principles describe
all of them and that light is more common than you have ever imagined. These
critical-thinking and problem-solving skills will help us become the individuals we
aspire as reflected in the program educational objectives.
So, at the end of this lesson, you should be able to:
1. analyze how the context of light affects our day-to-day living;
2. actualize the sense of responsibility and honesty in doing experiments involving
light
3. conduct an experiment using the concepts of light to compare experimental and
theoretical data
Essential Question
How does light travel?
Early Concepts of Light
Several early scientists claimed that
light travels in transverse waves
According to Einstein, light consists
of particles called photons
Photon: Massless bundles of
concentrated energy
The Speed of Light
Early scientists used the periods of objects
in our solar system to estimate the speed of
light
Later, experiments using reflected light from
mirrors were used
Through these experiments, the speed of
light through a vacuum was found to be:
300,000 km/s = 186,000 mi/s
Electromagnetic Waves
Light is energy emitted by accelerating electric
charges, usually electrons
This energy travels in a wave that is partially
electric and partially magnetic
These are electromagnetic waves
Visible light is a very small portion of the
electromagnetic spectrum
Electromagnetic Waves
Can be transmitted in a way that they can be deciphered
later
Radio waves
Unlike sound waves
Electromagetic waves do not require a medium
i.e. They can travel through space
Electromagnetic Spectrum
Opaque and Transparent
Materials
Transparent: Materials which allow light to
pass through
Electrons receive energy from the entering light
wave and re-emit this energy as light
Opaque: Materials which do not allow light
to pass through
Electrons receive energy from the entering light
wave and passes it onto neighboring atoms
This energy is converted to random kinetic
energy and the materials become slightly warmer
Shadows
Ray: A thin beam of light
When light shines on an object, some of the rays may
be stopped while others pass
Shadows are formed where light rays cannot reach
Shadows
A shadow usually consists of a dark region on the
inside, and a light region on the outside
Umbra: A total shadow
Penumbra: Partial shadow appearing where some
light is blocked, but other light fills in
Umbra and Penumbra in a Solar
Eclipse
Structure of the Eye
Structure of the Eye
Human Eye
What we see can be distorted
Our mind’s interpretation of an image can be altered
Bell Ringer
1.) What is the difference between opaque and
transparent?
2.) What is the difference between the umbra and
penumbra?
Intensity
The intensity of light depends on the light source
The intensity is also dependent on the distance from the
source
Larger the distance – smaller the intensity
Smaller the distance – greater the intensity
Shadows
Remember…
Umbra: A total shadow
Penumbra: Partial shadow appearing where some
light is blocked, but other light fills in
What happens to each as we increase the distance?
Law of Reflection
Reflection- When a wave reaches a boundary between two
media, some or all of the waves bounce back
Θ1=Θ2
Θ1 is the incident incoming angle
Θ2 is the reflected outgoing angle
Ex: 60° incident 60° reflected
Mirrors
Mirrors use the law of refection to show images
Real image: all of the light rays show where the image actual is
Virtual image: can be seen by an observer but cannot be shown on
a screen because the light rays do not come together
Refraction
Sound waves travel at different speeds depending on the
medium
Light has the same property
Refraction (Snell’s Law)
Light travels at different speed depending on the medium
Index of refraction (n)
N= speed of light in vacum/speed of light in medium
Or n=c/v
Air has an index of refraction of about 1
Snell’s Law
Light’s angle in the medium is dependent on both the index
of refraction and the incident angle
n1sinθ1=n2sinΘ2
Example: A laser light ray enters a cup of water (n=1.33)
from air (n=1) at an angle of 15°. What is the outgoing
angle?
Refraction Demos
Bowl of Water
Test Tube
Pickle Jar
Bell Ringer
A light ray hits a mirror at 15.6°. What is the reflected
angle?
Give an example where there is evidence of the following…
Light being reflected
Light being refracted
Polarization
Some light is polarized, meaning its
waves are confined to a 2-D plane
A single vibrating electron will produce
polarized light
Common light sources emit non-
polarized light since the electrons vibrate
randomly in all directions
Polarization Filters
Polarization
Polarization
Polarization and 3-D Viewing
Vision in three dimensions depends on
the fact that each eye views a scene
from a slightly different angle
Bell Ringer
What does a polarized filter
do?
How are they used to view
3D movies?
Total Internal Reflection
At some angle (dependent on the medium) all of the light
rays will be reflected back into the medium and not leave.
Applications
Fiber optic cable Sparkling Diamonds
Curved Mirrors
Two ways to observe a curved mirror
Converging
concave )
Diverging
convex (
Curved Mirrors
Concave lens – parallel rays converge at the focal point
Focal length – distance from lens to focal point
We can examine an image formed by the lens by only
examining a number of rays
Ray Tracing
Principle rays
1.) Top of Image, then through F
2.) Through F
3.) Through middle vertex
Bell Ringer
What is the difference between a real and a virtual image?
Lens Applications
Binoculars
Telescopes
Microscope
Glasses
The Color Spectrum
Spectrum: A spread of wave frequencies
White Light: The combination of all
colors
Under white light:
White objects appear white
Colored objects appear in their
individual colors
Therefore, white is not a color, but a
combination of all colors
The Color Spectrum
Black is similarly not a color, but
the absence of light
Black objects absorb some light
of all visible frequencies
Not all light is absorbed by black
objects
Color by Reflection
When electrons receive energy, they
become become excited and then
transfer this extra energy outward
Some energy is absorbed into the
material
The rest of the light is reflected outward
giving an the appearance of color
Color by Reflection
Objectscan ONLY reflect frequencies
present in the light shining on them
For example: If a red light shines on a
blue object, the object will appear black
because there is no blue in the light for
the object to reflect back.
Color
by Transmission
The color of a transparent object
depends on the color of light it
transmits
Pigment: The material in the glass that
selectively absorbs colored light
Electrons in the pigment are
responsible for this absorption
Any absorption results in an increase
in temperature of the material
Bell Ringer
Why do red objects appear red?
Draw the ray diagram for an image when an object is
placed inside the focal point in front of a converging lens.
Retinal Fatigue
Retinal Fatigue
Retinal Fatigue
Retinal Fatigue
Retinal Fatigue
Retinal Fatigue
Diffraction
Light behaves as a wave to cause light and dark spots
(constructive and deconstructive) interference
Applet
Ex: Snowman Glasses
Light
Lightof all the visible frequencies
mixed together produces white light
This can be achieved by mixing only
blue, red, and green light of equal
brightness
Mixing Colored Light
Almost any color can be made by
overlapping light of three colors by
varying the brightness of each
Combining red, green, and blue
produces the highest number of
different colors
For this reason, red, green, and blue
are known as additive primary colors
Additive Primary
Colors
Red + Green = Yellow
Red + Blue = Magenta
Blue + Green = Cyan
Complimentary Colors
ComplimentaryColors: Two colors,
which when added together,
produce white
Yellow + Blue = White
Magenta + Green = White
Cyan + Red = White
Atomic
Spectra
Every element has its own color
when made to emit light
In gas form, atoms are far apart
Their vibrations will not be interrupted
by neighboring atoms
Neon – Red
Mercury – Blue-Violet
Helium – Pink
Spectroscope: An instrument which
breaks glowing elements into a
series of colored lines
Atomic Spectra
This
pattern is called a line
spectrum
Each line corresponds to a specific
frequency of light
Linespectrums are the
“fingerprints” of each element
Atomic Spectra
Hydrogen:
Helium:
Carbon:
Continuous:
