Polarisation of Light Notes Class 12 PDF | NCERT, Board, NEET & JEE

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Polarisation of Light Notes Class 12 PDF | NCERT, Board, NEET & JEE

Polarisation

Polarisation is an important property of light that proves light is a transverse wave. It is possible only in transverse waves because their vibrations occur perpendicular to the direction of propagation.

Definition: Polarisation is the phenomenon in which the vibrations of a transverse wave are restricted to one direction (or one plane) perpendicular to the direction of propagation.

Important Points:

  • Only transverse waves can be polarised.
  • Longitudinal waves cannot be polarised.
  • Polarisation proves the transverse nature of light.

Transverse Wave

A transverse wave is a wave in which the particles of the medium vibrate perpendicular to the direction of propagation.

For a wave travelling along the x-axis, the particles may vibrate along the y-axis or z-axis.

Remember: The direction of vibration is always perpendicular to the direction of propagation.

y-Polarised Wave

If a transverse wave travels along the x-axis and all particles vibrate only along the y-axis, it is called a y-polarised wave.

y(x,t) = a sin(kx − ωt)

Where:

  • a = Amplitude
  • k = Wave number
  • ω = Angular frequency

Key Points:

  • Propagation is along the x-axis.
  • Vibration is along the y-axis.
  • It is a transverse wave.
  • It is also linearly polarised.
  • Its plane of vibration is the x-y plane.

Linearly Polarised Wave

Definition: A transverse wave in which every particle vibrates along one fixed straight line is called a linearly polarised wave.

Since a y-polarised wave vibrates only along the y-axis, it is linearly polarised.

Plane Polarised Wave

Definition: A wave whose vibrations remain confined to one fixed plane is called a plane polarised wave.

For a y-polarised wave, the plane of vibration is the x-y plane.

z-Polarised Wave

If the wave propagates along the x-axis and all particles vibrate only along the z-axis, it is called a z-polarised wave.

z(x,t) = a sin(kx − ωt)

Key Points:

  • Propagation is along the x-axis.
  • Vibration is along the z-axis.
  • It is linearly polarised.
  • Its plane of vibration is the x-z plane.

Unpolarised Wave

Definition: A transverse wave in which the direction (or plane) of vibration changes randomly with time is called an unpolarised wave.

Although the vibrations are always perpendicular to the direction of propagation, there is no fixed direction of vibration.

Examples:

  • Sunlight
  • Light from an electric bulb
  • Light from a sodium lamp
Exam Tips
  • Only transverse waves can be polarised.
  • Light is a transverse electromagnetic wave.
  • A y-polarised wave vibrates along the y-axis.
  • A z-polarised wave vibrates along the z-axis.
  • Unpolarised light has vibrations in all possible directions perpendicular to the direction of propagation.

Polarisation of Light

Light emitted from ordinary sources such as the Sun, electric bulbs, and sodium lamps is unpolarised. Its vibrations occur randomly in all directions perpendicular to the direction of propagation. A polaroid is used to convert unpolarised light into plane-polarised light.

Light is a transverse electromagnetic wave. The electric field of light vibrates perpendicular to the direction of propagation. When these vibrations are restricted to only one direction, the light is said to be plane polarised.

Important: The phenomenon of polarisation proves that light is transverse in nature.

Polaroid

Definition: A polaroid is a thin plastic sheet made of long-chain molecules aligned in one particular direction. It allows vibrations in only one direction to pass and absorbs vibrations in the perpendicular direction.

Thus, a polaroid converts unpolarised light into plane-polarised light.

Working of a Polaroid

When unpolarised light falls on a polaroid:

  • The component of the electric field parallel to the aligned molecules is absorbed.
  • The component perpendicular to the aligned molecules is transmitted.
  • The transmitted light vibrates in only one direction.
  • Hence, the transmitted light is plane polarised.

Pass Axis

Definition: The direction along which a polaroid allows light vibrations to pass is called its pass axis (or transmission axis).

Only the component of light parallel to the pass axis is transmitted through the polaroid.

Intensity of Light Through a Single Polaroid

When unpolarised light passes through a single polaroid, only one component of vibration is transmitted.

Therefore, the intensity of the transmitted light becomes

I = I0/2

where

  • I0 = Intensity of incident unpolarised light
  • I = Intensity of transmitted plane-polarised light

Key Points

  • Ordinary light is unpolarised.
  • Light is a transverse electromagnetic wave.
  • A polaroid converts unpolarised light into plane-polarised light.
  • The pass axis determines the direction of transmitted vibrations.
  • A single polaroid transmits only 50% of the incident unpolarised light intensity.
Exam Tips
  • Remember that a polaroid absorbs one component and transmits the other.
  • The transmitted light is always plane polarised.
  • Intensity after one polaroid is I = I0/2.
  • The pass axis is the direction of vibration of the transmitted light.

Polariser

Definition: A polariser is a polaroid that converts unpolarised light into plane-polarised light.

When unpolarised light passes through a polariser, the transmitted light vibrates only along the pass axis of the polariser.

Analyser

Definition: An analyser is a second polaroid used to detect or analyse plane-polarised light.

The intensity of light transmitted by the analyser depends on the angle between the pass axes of the polariser and analyser.

When two polaroids are used together, the first polaroid produces plane-polarised light, while the second polaroid analyses the state of polarisation. These two polaroids are called the polariser and the analyser.

Malus' Law

When plane-polarised light passes through an analyser, the transmitted intensity is given by

I = I0 cos²θ

where

  • I = Intensity of transmitted light
  • I0 = Intensity of incident plane-polarised light
  • θ = Angle between the pass axes of the polariser and analyser

This relation is known as Malus' Law.

Special Cases of Malus' Law

Angle (θ) Intensity Observation
0° I = I0 Maximum intensity is transmitted.
45° I = I0/2 Half of the incident intensity is transmitted.
90° I = 0 No light is transmitted (Crossed Polaroids).

Parallel and Crossed Polaroids

1. Parallel Polaroids

  • Pass axes are parallel (θ = 0°).
  • Maximum light passes through the analyser.

2. Crossed Polaroids

  • Pass axes are perpendicular (θ = 90°).
  • No light is transmitted.
  • The field of view appears dark.

Key Points

  • The first polaroid acts as the polariser.
  • The second polaroid acts as the analyser.
  • The analyser controls the transmitted intensity.
  • Intensity depends on the angle between the two pass axes.
  • Malus' Law is applicable only to plane-polarised light.
Formula Summary
  • After one polaroid: I = I0/2
  • After analyser: I = I0 cos²θ
Exam Tips
  • θ = 0° → Maximum intensity.
  • θ = 90° → Zero intensity (Crossed Polaroids).
  • Remember the formula I = I0 cos²θ.
  • Malus' Law is one of the most important Board and NEET questions.

Applications of Polaroids

  • Polarised Sunglasses: Reduce glare from roads, water, and snow.
  • Photography: Used in camera filters to improve image contrast and reduce reflections.
  • LCD Screens: Mobile phones, calculators, laptops, and TVs use polaroids.
  • 3D Movies: Special polaroid glasses allow each eye to receive a different image, producing a 3D effect.
  • Scientific Instruments: Used in polarimeters, microscopes, and stress analysis.

Important Formulae

Formula Description
I = I0/2 Intensity after passing through one polaroid.
I = I0 cos²θ Malus' Law (Intensity after analyser).
ω = 2πν Angular frequency.
λ = 2π/k Relation between wavelength and wave number.

Chapter Summary

  • Light is a transverse electromagnetic wave.
  • Only transverse waves can be polarised.
  • Polarisation proves the transverse nature of light.
  • A polaroid converts unpolarised light into plane-polarised light.
  • The first polaroid is called the polariser.
  • The second polaroid is called the analyser.
  • The transmitted intensity follows Malus' Law.

Important Points for Board Exams

  • Define polarisation.
  • State why only transverse waves can be polarised.
  • Define linearly polarised and plane-polarised waves.
  • Define pass axis.
  • Differentiate between polariser and analyser.
  • State and apply Malus' Law.
  • List applications of polaroids.

Frequently Asked Questions (FAQs)

1. What is polarisation?

Polarisation is the phenomenon of restricting the vibrations of a transverse wave to one direction perpendicular to its direction of propagation.

2. Why can't longitudinal waves be polarised?

Because their particles vibrate parallel to the direction of propagation, leaving no other direction to restrict.

3. What is a polaroid?

A polaroid is a sheet that converts unpolarised light into plane-polarised light.

4. What is the pass axis?

The pass axis is the direction along which a polaroid transmits light vibrations.

5. State Malus' Law.

The intensity of plane-polarised light after passing through an analyser is given by I = I0 cos²θ.

Quick Quiz

1. Polarisation is a property of:





2. A single polaroid transmits:





3. Malus' Law is:





Revision Box
  • ✓ Polarisation proves that light is transverse.
  • ✓ Only transverse waves can be polarised.
  • ✓ A polaroid converts unpolarised light into plane-polarised light.
  • ✓ After one polaroid: I = I₀/2.
  • ✓ After analyser: I = I₀ cos²θ.
  • ✓ θ = 0° → Maximum intensity.
  • ✓ θ = 90° → No light is transmitted.

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