Unit 4 - Day 12: Polarization of Light
Course/Unit: SPH4U / Wave Nature of Light Date: To be decided
Lesson Big Idea(s):

The behavior of light as a wave can be described mathematically.

Technologies that use the principles of the wave nature of light can have societal and environmental implications.
Ministry Expectations:
E3 demonstrate an understanding of the properties of waves and light in relation to diffraction, refraction, interference, and polarization.
Specific Expectations:
E3 analyze, on the basis of research, some of the social, ethical, and legal implications of biotechnology
Learning Goals:
By the end of the class students will understand:

The difference between polarized and unpolarized light.

How to determine the intensity of light that passes through various linear polarizers.
Expected Prior Knowledge:

SPH4U: Fields Unit - Electric and Magnetic field components.

Wave properties of light.
Before: Minds On
Success Criteria:
I know I have achieved the learning goals when I can:

Explain why Polaroid sunglasses reduce glare.

Explain how different configurations of linear polarizers can partially block and fully block out light.

State two methods of generating polarized light.
Time: Description Materials
10 min Minds On: Linear Polarizers

Perform a demonstration involving two linear polarizers or two pairs of Polaroid sunglasses.

Hold up one polarizer in front of a light source. Ask students to observe what to the light as it passes through the linear polarizer. They can write observations on the white board.

Still holding the one linear polarizer, ask students to predict what happens if you rotate it. They can write predictions on white boards.

Now, hold a second linear polarizer adjacent to the first one. Ask students to
 predict what happens as you begin to rotate one of the polarizers. Have students hold up their white boards.
Perform demo several times: keeping one polarizer fixed while the second is rotated, rotating both simultaneously, and keeping both stationary.
 Linear polarizers or 2 pairs of
Polaroid sunglasses
 Light source
 White boards
 Markers
Transition from Minds On to Action:

Polarized sunglasses eliminate the glare of reflected light while other sunglasses do not. What is unique about polarized lenses? The answer is based on a specific property of electromagnetic radiation.
During: Action
Time: Description
10 min Action1: “Chalk-and-talk” style lesson on the polarization of light

EM waves have electric (E) and magnetic (B) field components.

The E and B fields are always perpendicular to each other and the direction of propagation.
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But, the orientation of the E and B fields can take on many different directions as the EM waves propagate through a medium.

The polarization of light is by definition defined to be the direction that the E field
Materials
 White board
 Markers

Unit 4 - Day 12: Polarization of Light oscillates in.

Light coming from the sun for example can have an E field component that is any random direction. Such light is said to be unpolarized light.

Light that has all the E field components oscillating in the same direction (e.g. y direction) is said to be linearly polarized light.
20 min Action 2: “Chalk-and-talk” lesson on the different ways to polarize light.
There are different ways that we can generate polarized light from unpolarized light.
1. Polarization by Selective Absorption

Because an E field component can take on any random direction in a 2dimensional plane, we have to assume that some of the E field will be oriented along both the x and y-axes.
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In order to generate linearly polarized light, we need to selectively absorb the E field along either the x or y-axis.

This is what linear polarizers do. They are made of a material called Polaroid, which contains sheets of molecules, which are arranged linearly. These molecules selectively absorb the E-field components that are parallel to the axis of the molecules, and allow the components that are perpendicular to pass.

The axis through which the Polarois molecules are aligned is called the
transmission axis.

In this manner, linearly polarized light can be generated from randomly polarized light.

Draw schematic showing this.

When two linear polarizers are placed adjacent to one another, such that the orientation of the Polaroid molecules are perpendicular to one another, there is no resulting light that passes through both polarizers. The reason is that both components of the E field are absorbed, which explains why we completely blocked out the light source in the demonstration.

Draw schematic showing this.
2. Polarization by Reflection

When light strikes a surface, the E field components that are perpendicular to the surface are absorbed, and the E field components that are parallel to the surface are reflected.
10 min Action3: “Chalk-and-talk” style lesson on Malus’s Law

Malus’s Law, which is named after Etienne-Louis Malus, says that when a perfect polarizer is placed in a polarized beam of light, the intensity I, of the light that passes through is given by:
𝑰 = 𝑰 𝒐 𝒄𝒐𝒔 𝟐 𝜽

Note: If unpolarized light passes through a polarizer, then half the intensity passes through.
Example 1: Unpolarized light of initial intensity 100 W/m 2 passes through three linear polarizers. The second linear polarizer has its transmission axis oriented 45 degrees relative to the first linear polarizer. Lastly, the third linear polarizer has its transmission axis oriented 90 degrees relative to the first linear polarizer. Determine the intensity of the light that passes through the third linear polarizer.
After: Consolidation
Time Description
 White board
 Markers
 White board
 Markers

Unit 4 - Day 12: Polarization of Light
20 min Consolidation: Homework

Assign homework (see unit outline), allow students remaining time to begin working on problems.

Can work individually or in groups.

Walk around and assist students where needed.