Polarization of Light
Electromagnetic Waves
Learning Goal:
By the end of today’s lesson, I will be able to:
- describe and explain the diffraction, refraction, polarization, and
interference of light waves (e.g., reduced resolution caused by
diffraction, mirages caused by refraction, polarization caused by
reflection and filters, thin-film interference in soap films and air
wedges, interference of light on CDs) (E3.2 )
- use the concepts of refraction, diffraction, polarization, and wave
interference to explain the separation of light into colours in various
situations (e.g., light travelling through a prism; light contacting
thin film, soap film, stressed plastic between two polarizing filters)
(E3.3 )
Electromagnetic waves
As you know from Unit 3 that charged
particle generates both electric and
magnetic fields due to the fundamental
principles of electromagnetism. Here's why
and how these fields arise:
• A magnetic field (𝐵) is generated by
moving electric charges (electric currents)
- When a charge moves, it creates a
magnetic field around it.
• An electric field (ɛ) is created by any
charged particle, whether it is stationary
or in motion. The presence of an electric
charge produces an electric field.
Sources of Electromagnetic(EM) radiation
A variety of sources can
produce electromagnetic
radiation such as sun,
microwave oven, X-ray
machines, light bulbs, etc.
A source of light creates
oscillating electric and magnetic
fields, perpendicular to each
other, that moves away from the
source.
Electromagnetic Wave
What is Polarization?
Light normally is unpolarized, partially polarized or polarized.
When it is unpolarized it vibrates in many directions.
Polarization means making light vibrate in only one direction.
After passing through a special filter (called a polarizer), the
light is changed so it only vibrates in one direction. Polarizer
only allows light with an electric field to pass through.
A polarizer is an optical device that allows
light waves of a specific polarization to
pass through while blocking waves of other
How Light Becomes Polarized
Selective Absorption: Polarizing filters block all but one
orientation of electric field.
Reflection: Light reflecting off surfaces (like water or roads)
becomes partially polarized.
Scattering: Light scattered by air molecules becomes
polarized, explaining why the sky appears blue.
Polarization by selective absorption
Polarization by selective absorption is a method of producing
polarized light by using a material that absorbs light waves
vibrating in certain directions and transmits others.
How Polarization by selective absorption
works
Unpolarized light consists of waves vibrating in all
directions perpendicular to the direction of travel.
When this light passes through a polarizing filter (often
made of Polaroid material), the filter only allows the
component of light vibrating in a specific direction (the
transmission axis) to pass through.
The other components those vibrating at angles not
aligned with the transmission axis are absorbed by the
material.
Polarization by Reflection
Polarization by reflection is a phenomenon where light becomes
partially or completely polarized upon reflecting off a non-metallic
surface such as glass, water, or asphalt.
How Polarization by reflection works
When unpolarized light (with electric field vibrations in all
directions perpendicular to its path) strikes a surface at an
angle, the reflected light can become polarized.
At a specific angle called the Brewster angle, the reflected
light is completely polarized perpendicular to the plane of
incidence (meaning the electric field oscillates only in one
direction, parallel to the surface).
Uses:
• It is used in glare reduction, photography, optics, and scientific
instruments.
Application of Polarization by reflection
Sunglasses – Glare Reduction
Anti-Glare Lenses:
These sunglasses are designed to reduce glare from horizontal
surfaces, such as roads and bodies of water. Light reflected from
these flat surfaces is generally horizontally polarized. The lenses
contain a special polarizing filter with a vertical orientation that
blocks this horizontally polarized light, which is the primary cause
of glare. As a result, polarized sunglasses are especially useful for
activities like driving, fishing, and other outdoor activities.
View with Polarizing Lenses:
Without Polarizing lenses
With Polarizing lenses
How Do Polarized Sunglasses Work?
Polarization by Scatterings
Polarization by scattering is a phenomenon where light becomes
polarized when it is scattered by small particles in the atmosphere or
other media.
Polarized Light by Scattering
The degree of polarization depends on the scattering angle — it is
maximum at a 90° angle from the source.
How Polarization by scattering works
1. Sunlight enters the atmosphere:
Sunlight is unpolarized, meaning its electric field vibrates
in all directions perpendicular to the direction of travel.
2. Light hits air molecules:
When sunlight strikes tiny molecules in the air, the
electric field in the light wave causes the charges (mainly
electrons) in the molecules to move or oscillate.
3. Light is absorbed and re-emitted:
The molecules briefly absorb the light energy and then reemit it in different directions. This process is called
scattering.
4. Re-emitted light becomes polarized:
Because light is a transverse wave, the electric field of the
scattered light lies in a specific direction — it is always
perpendicular to the direction in which the light is
scattered.
5. Resulting light is polarized:
The scattered light, especially at 90° to the original
direction of sunlight, becomes completely planepolarized. Its electric field vibrates in a single direction.
Blue Sky and Red Sunset
Question: Why is the sky blue in the day and red at sunsets?
The sky is blue because . . .
• The tiny particles in the
atmosphere (dust, clumps of
air molecules, microscopic
water droplets) are better at
scattering shorter wavelength
blue light than the longer
wavelength red light.
• As sunlight passes through
the atmosphere, the scattered
blue light give the
atmosphere an overall blue
glow.
The sunset is red because . . .
• At sunrise and sunset, sunlight enters our atmosphere at a shallow
angle (small angle) and travels a long distance before reaching
our eyes.
• During this long passage, most of the blue light is scattered away
and virtually all that we see coming to us from the sun is its red
and orange wavelengths.
Example 1: Polarization by selective absorption
What will happen if a horizontally oriented slit is
placed immediately after the vertically oriented slit?
None of the light will make it through.
Example 2: Polarization by selective absorption
What properties of waves does this demonstrate?
It demonstrates that the waves are transvers and not
longitudinal. (otherwise, polarization by selective
absorption would not work)
Example 3: Application of Polarization
How do polarized lenses exclusively absorb glare from the
light that is reflected from the surface of a stream or the
hood of a car?
Polarized lenses exclusively absorb glare from the light that
is reflected from the surface of a stream or the hood of a
car because the reflected light is horizontally oriented, and
the sunglasses are vertically polarized.
Example 4: Application of Polarization
Just before the sunset, a driver encounters sunlight
reflecting off the side of a building. Will polarized
sunglasses stop this glare? Explain.
No, because the reflected light is vertically oriented and
so too are the sunglasses.
Textbook Practice Questions:
Page 537 # 1 – 3, 5 – 7, 10 and 11