What is Polarisation of Light? Meaning, Differences, and Examples
Polarisation of Light plays a key role in understanding the wave nature of optics for JEE Main. It describes how light waves can vibrate in specific directions, leading to phenomena and applications important for competitive exams and experiment-based questions.
In its natural form, light emitted from most sources is unpolarised—it vibrates in many planes perpendicular to its direction of propagation. Through polarisation, this vibration is restricted to a single plane, demonstrating that light is a transverse wave. Many conceptual and numerical questions in JEE Main rely on grasping this core distinction.
Polarisation affects how we interpret experiments, solve MCQs on optics, and apply concepts to real-life scenarios like sunglasses, photography, and even atmospheric effects. Mastering the formulas and differences between types of polarisation is essential for exams.
Introduction and Meaning: Polarisation of Light
Polarisation of Light is the process by which oscillations of light waves are confined to a single plane perpendicular to the direction of propagation. It essentially distinguishes between polarised (single direction vibration) and unpolarised (random direction vibration) light, which proves the transverse nature of light waves.
This topic is fundamental in JEE Main because it appears in direct concept-based questions, as well as in optics numericals and lab experiments. Understanding it builds your foundation for advanced optics problems.
Types of Polarisation of Light
| Type | Description | Diagram |
|---|---|---|
| Linear Polarisation | Oscillations confined to a single, straight plane | Slit passing vertical vibrations only |
| Circular Polarisation | Tip of electric field vector traces a circle | Combination of two linear, phase-shifted vibrations |
| Elliptical Polarisation | Tip traces an ellipse—general case | Two linears, unequal amplitude/phase |
Linear polarisation is most frequently asked in JEE Main, but recognising all three with labelled diagrams helps distinguish MCQ choices and diagrams in exam questions.
Polarised vs Unpolarised Light: Core Distinction
A common exam mistake is confusing polarised and unpolarised light in diagrams or concept questions. The following table helps clarify identification for JEE.
| Property | Polarised Light | Unpolarised Light |
|---|---|---|
| Vibration Direction | Single plane | Random planes |
| Diagram | Straight lines in one direction | Arrows in many orientations |
| Source | Reflection, polaroids | Bulb, sun, candle |
| Wave Nature Proven | Transverse only | Not proved |
To identify diagrams: polarised light shows all vectors along one direction; unpolarised shows a mix. Master this for quick MCQ elimination.
Methods of Producing Polarisation: Reflection, Scattering, Experiment
Polarisation can be produced by several physical processes crucial in JEE numericals and lab setups:
- Polarisation by Reflection: When unpolarised light hits a medium at a special angle (Brewster’s angle), the reflected ray becomes completely polarised—in a direction perpendicular to the plane of incidence.
- Polarisation by Scattering: Light scattered by small particles (e.g., sky) is partially polarised. This is why the sky appears blue and polarised when observed at 90° from the sun.
- Polarisation by Transmission (Polaroids): Passing light through a polarising filter (polaroid) blocks all but one plane of vibration, giving fully polarised light.
For Brewster’s Law: tan θB = n2/n1, where θB is Brewster’s angle, and n2, n1 are refractive indices.
Polarisation of Light Experiment: Step-wise JEE Approach
A common practical involves using two polaroids and observing intensity changes as one is rotated. This experiment forms the basis for understanding Malus’ Law and for answering JEE practicals or viva.
- Arrange a light source and place the first polaroid (P1) in its path.
- Add a second polaroid (P2) beyond P1, both initially aligned.
- Rotate P2 slowly and observe how light intensity changes—maximum when axes are parallel.
- At 90° (crossed), almost no light passes through.
- This verifies Malus' Law: I = I0cos2θ.
Always mention the inference: maximum intensity for aligned polaroids, zero for crossed.
Formula Guide: Polarisation of Light in JEE
- Malus' Law: I = I0cos2θ (I: transmitted intensity, I0: incident intensity, θ: angle between transmission axes).
- Brewster’s Angle: tan θB = n2/n1 (where n are refractive indices).
- Degree of Polarisation: P = (Imax – Imin)/(Imax + Imin).
Example: Light is reflected from glass (n = 1.5). Find Brewster's angle.
tan θB = 1.5/1 ⇒ θB = arctan(1.5) ≈ 56.3°
Use SI units and check angle conventions carefully in exam problems.
Applications and Importance: Polarisation of Light
- Polarised sunglasses block glare by filtering horizontal light.
- LCD and camera filters use polarising films for display clarity.
- Proof of wave nature: Only transverse waves can be polarised.
- Rainbow colours and blue sky are partly due to polarised scattering.
- Used in stress analysis in plastics/glass via polarised light.
In exams, applications questions often connect to real-world devices or conceptual physics reasoning. Master at least 3 examples for safety.
Common pitfalls for JEE include forgetting that only transverse (not longitudinal) waves can be polarised, confusing axes alignment in Malus’ Law questions, or misusing angle conventions (degrees vs radians). Always mark axes and directions clearly in diagrams and calculations.
For deeper context, see Huygens Principle for wavefront discussion, or reference Biot-Savart Law for advanced wave phenomena if relevant. For related optics concepts, visit Vedantu's Optics page.
FAQs on Polarisation of Light: Concept, Types, and Applications
1. What is meant by polarisation of light?
Polarisation of light is the process in which the vibrations of light waves are restricted to a single plane. This property proves that light is a transverse wave.
- Unpolarised light vibrates in many planes.
- Polarised light vibrates only in one plane.
- This is crucial for physics exams and real-world optical applications.
2. How can we explain polarization in simple terms?
In simple terms, polarization means arranging the light waves so they all vibrate in just one direction.
- Think of unpolarised light as a rope shaken in all directions.
- If you pass it through a narrow slit or use a polarizer, only one direction (up and down, for example) gets through.
- This is like combing messy hair into a straight line.
3. What is the main difference between polarised and unpolarised light?
The main difference is that polarised light vibrates in one direction, while unpolarised light vibrates in multiple directions.
- Polarised: All wave vibrations are in a single plane.
- Unpolarised: Vibrations are random, in all possible planes perpendicular to the direction of travel.
- This difference is important for identifying wave properties in exams.
4. What are some real-life examples of polarisation?
Real-life examples of polarisation show its practical importance in daily life.
- Sunglasses use polarised lenses to reduce glare from roads or water surfaces.
- Camera filters enhance contrast and reduce reflections.
- 3D movies use polarised glasses to separate left and right eye images.
- LCD screens rely on controlled polarisation for display.
5. How is light polarised by reflection?
When unpolarised light strikes a surface (like water or glass) at a specific angle called Brewster's angle, the reflected light becomes fully polarised in a single plane.
- This is called polarisation by reflection.
- The phenomenon is used in glare-reducing sunglasses and optical instruments.
- Brewster's law helps calculate the polarising angle.
6. What are the types of polarisation of light?
There are three main types of polarisation of light:
- Linear Polarisation: Vibrations restricted to one plane only.
- Circular Polarisation: Electric field rotates in a circle as the wave moves.
- Elliptical Polarisation: Vibrations trace an ellipse.
7. Can sound waves be polarised?
No, sound waves cannot be polarised.
- Sound is a longitudinal wave, where vibrations are parallel to the direction of travel.
- Only transverse waves (like light) can be polarised, as they have vibrations perpendicular to the wave direction.
- This is a key syllabus concept for wave optics and light.
8. Does polarisation prove light is a transverse wave? How?
Yes, polarisation proves that light is a transverse wave.
- Only transverse waves can be polarised, because their vibrations can be restricted to particular planes.
- If light were a longitudinal wave, polarisation would not be possible.
- This point is often asked in exam theory questions.
9. What is Brewster's angle in polarisation of light?
Brewster's angle is the special angle of incidence at which light, when reflected, becomes completely polarised perpendicular to the plane of incidence.
- At this angle, the reflected and refracted rays are perpendicular.
- Formula: tan(θB) = refractive index (μ)
- Used in numericals and experiment setups for polarisation by reflection.
10. What is Malus' law related to polarisation?
Malus' Law quantifies how the intensity of polarised light changes when it passes through a polariser at an angle.
- Formula: I = I₀ cos²θ, where θ is the angle between the light's original polarisation direction and the axis of the analyser.
- Helps solve numerical problems on polarisation intensity.
- Frequently used in board and JEE exam questions.
