Polarized light is an electromagnetic wave whose electric field oscillates in a preferred direction (linear polarization) or according to a regular pattern (circular, elliptical polarization). Natural light is not polarized: the orientations of the field are random.
Discover FizziQ
How to measure it in class
With the FizziQ app and polarizing filters, it is possible to explore polarization.
Steps:
- Observe the smartphone screen through a polarizing filter
- Rotate the filter and observe the intensity variations
- Note that the LCD screen emits polarized light
- Cross two polarizing filters: total extinction at 90°
- Use the FizziQ light meter to measure the transmitted intensity as a function of angle
- Verify Malus’s law: I = I₀ × cos²θ
Scientific activities on this topic
Possible extensions with FizziQ: Malus’s law with the light meter and polarization by reflection (Brewster’s angle).
Learn more
Types of polarization:
- Linear: the field oscillates in a fixed plane
- Circular: the field vector rotates at constant speed
- Elliptical: general intermediate case
- Unpolarized: random superposition of all orientations
Polarization methods:
| Method | Principle |
|---|---|
| Polarizing filter | Selective absorption |
| Reflection (Brewster) | Reflection at a particular angle |
| Birefringence | Calcite, quartz |
| Scattering | Sky light |
Applications:
- LCD screens (liquid crystals between polarizers)
- 3D glasses in cinemas (different polarizations)
- Glare reduction (sunglasses, photography)
- Stress analysis (photoelasticity)
Formula
Malus’s law: I = I₀ × cos²θ
where:
- I: transmitted intensity
- I₀: incident polarized intensity
- θ: angle between the polarization direction and the polarizer axis
Brewster’s angle: tan(θ_B) = n₂/n₁
where n₁ and n₂ are the refractive indices of the two media.
Application examples
- LCD screens appear black when viewed with certain polarized glasses
- The sky is partially polarized, perpendicular to the direction of the Sun
- Bees use the polarization of the sky to orient themselves
- Photographers use polarizing filters to eliminate reflections
FAQ
Q: Why does the screen turn black if you rotate the filter by 90°? A: The LCD screen emits polarized light. When the filter is perpendicular to this polarization, Malus’s law gives I = I₀ × cos²(90°) = 0.
Q: Is sunlight polarized? A: No, but it can become polarized by reflection (on water, glass) or by scattering (blue sky).
Q: How to verify Malus’s law with FizziQ? A: Measure the transmitted illuminance through a polarizer at different angles (0°, 30°, 45°, 60°, 90°). Plot I as a function of cos²θ: it is a straight line.
Q: Why do 3D glasses use polarization? A: Each eye receives a differently polarized image. The polarized lenses filter selectively: left eye = left image, right eye = right image.
Related concepts
Visible Spectrum - Wavelength - Interference - Colors