PhysicsChapter 9: Waves
Polarization
Explanation level
Transverse nature of light
Polarization is associated with transverse waves. In a transverse wave, the vibration is perpendicular to the direction in which the wave travels, so there are several possible sideways vibration directions.
Light is an electromagnetic wave. Its electric field and magnetic field oscillate perpendicular to each other, and both are perpendicular to the direction of propagation.
| Quantity | Direction |
|---|---|
| Direction of propagation | Direction in which light travels |
| Electric-field vibration | Perpendicular to propagation |
| Magnetic-field vibration | Perpendicular to propagation and to the electric field |
Unpolarized light
A beam from an ordinary source contains a very large number of light waves. Their electric fields vibrate in different directions around the direction of propagation, although every vibration remains perpendicular to that direction. Such light is unpolarized.
Common unpolarized sources
- The Sun
- An electric lamp
- A candle flame
Plane-polarized light
If a suitable method restricts the electric-field vibrations to one direction, the light is plane-polarized. All permitted vibrations then lie in one plane containing the direction of propagation.
Vibrating-string model
A string stretched along a fixed direction can be shaken sideways in many possible planes. These motions are all transverse because they are perpendicular to the string. If the string passes through a narrow vertical slot, only vertical vibration can continue; the slot models a polarizing device.
| String model | Light-wave meaning |
|---|---|
| String vibrates in many transverse directions | Unpolarized light |
| Slot permits one vibration direction | Polarizing action |
| String emerges vibrating in one plane | Plane-polarized light |
Why longitudinal waves cannot be polarized
In a longitudinal wave, particles vibrate parallel to the direction of propagation. There is no range of sideways vibration directions to select from, so the vibration cannot be restricted to one transverse plane. Sound waves in air therefore cannot be plane-polarized.
What polarization proves
Because light can be polarized and only transverse waves can show polarization, the observation of polarization provides evidence that light waves are transverse.
Reasoning from observation
Observation: Light can be transformed from an unpolarized beam into a plane-polarized beam.
Experiment: A suitable polarizing method restricts the allowed electric-field vibration direction.
Result: The emerging light vibrates in one transverse direction.
Conclusion: Light has transverse wave behaviour.
Important distinctions
| Term | Meaning |
|---|---|
| Direction of propagation | The direction in which wave energy travels |
| Direction of vibration | The direction in which the field oscillates |
| Unpolarized light | Many transverse vibration directions |
| Plane-polarized light | One selected transverse vibration direction |
Conceptual example
A rope passing through a narrow gap
Situation: A horizontal rope is shaken in many sideways directions and then passed through a narrow vertical slot.
Explanation: The slot suppresses horizontal and diagonal vibrations but permits vertical vibration. The emerging rope wave is confined to one plane, which models plane polarization.
Common mistakes
Check your understanding
Q. What is unpolarized light?
A. It is light whose electric-field vibrations occur in many directions perpendicular to the direction of propagation.
Q. What is plane-polarized light?
A. It is light whose electric-field vibrations are restricted to one direction or one plane.
Q. Why can light be polarized?
A. Light is a transverse electromagnetic wave, so its vibration direction can be selected.
Q. What does polarization demonstrate about light?
A. It demonstrates that light has transverse wave behaviour.
Quick revision
- Polarization is a phenomenon associated with transverse waves.
- Light is an electromagnetic transverse wave with electric and magnetic fields perpendicular to each other and to the direction of propagation.
- Ordinary light from the Sun, a lamp or a candle is generally unpolarized.
- In unpolarized light, electric-field vibrations occur in many directions perpendicular to the direction of propagation.
- Plane-polarized light has its electric-field vibrations restricted to one direction or one plane.
- Polarization transforms unpolarized light into polarized light.
- A vibrating string provides a useful model: its transverse vibration can be unrestricted or confined to a single plane.
- Longitudinal waves cannot be plane-polarized because their vibrations are parallel to the direction of propagation.
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Exam preparation
Exam Focus
Important points
- Definition of polarization.
- Unpolarized versus plane-polarized light.
- Light is a transverse electromagnetic wave.
- String model of polarization.
- Polarization proves the transverse nature of light.
Common mistakes
- Confusing propagation direction with vibration direction.
- Saying unpolarized light has no direction.
- Claiming longitudinal sound waves can be plane-polarized.
Quick revision
Revision Zone
Review the key facts, formulas and checks before you move on.Polarization
Sticky Notes · Quick Revision
Key Point 1
- Polarization is a phenomenon associated with transverse waves.
Key Point 2
- Ordinary light from the Sun, a lamp or a candle is generally unpolarized.
Key Point 3
- Plane-polarized light has its electric-field vibrations restricted to one direction or one plane.
Key Point 4
- Longitudinal waves cannot be plane-polarized because their vibrations are parallel to the direction of propagation.
Ready to test yourself?
You've completed the learning section.
- 8 concept questions
- Easy → Medium → Hard
- Explanation after each answer
Exam Practice
Important Question
Q1. Define polarization and plane-polarized light.
Important Question
Q2. Differentiate between unpolarized and plane-polarized light.
Important Question
Q3. Explain polarization using a vibrating string.
Reasoning
Q4. How does polarization show that light is a transverse wave?
Concept
Q5. Why can sound waves in air not be plane-polarized?
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