Polarisation: Comprehensive NEET Physics Notes

1. Polarisation

1.1 Introduction to Polarisation

Polarisation is a phenomenon that demonstrates the transverse nature of light waves. In an unpolarised light wave, the oscillations of the electric and magnetic fields occur in all directions perpendicular to the wave's propagation. Polarisation restricts these oscillations to a single plane, proving that light is a transverse wave.

1.2 Concept of Polarisation

When light is polarised, the electric field vector oscillates in only one direction perpendicular to the wave's travel. There are three primary types of polarisation:

1. Linear Polarisation: The electric field vector oscillates in a single plane.
2. Circular Polarisation: The electric field vector rotates in a helical pattern, making a constant angle with the propagation direction.
3. Elliptical Polarisation: The electric field vector describes an ellipse in any one plane, combining aspects of both linear and circular polarisation.

For NEET, the focus is primarily on linear polarisation.

1.3 Polarisation by Reflection

When unpolarised light is incident on a transparent medium, part of it is reflected, and part is refracted. The reflected light becomes partially polarised, with the electric field oscillations predominantly in one plane. This phenomenon is governed by Brewster's Law, which states:

• At a specific angle of incidence, known as Brewster's angle ${\theta }_B$, the reflected light is completely polarised perpendicular to the plane of incidence.

The formula for Brewster's angle is: $\tan {\theta }_B=n$ where $n$ is the refractive index of the medium.

NEET Tip:

Brewster's angle is frequently tested in NEET exams. Make sure to remember the formula and understand how it applies to problems involving reflection and polarisation.

1.4 Polarisation by Transmission

Polaroid filters can be used to polarise light by allowing only the electric field component parallel to the filter's axis to pass through. When unpolarised light of intensity $I_0$ passes through a Polaroid, the transmitted intensity is reduced to half: $I=\frac{I_0}{2}$

If this polarised light passes through a second Polaroid at an angle $\theta$ to the first, the transmitted intensity follows Malus's Law: $I=I_0{\cos }^2\theta$

Did You Know?

Polaroid sunglasses use this principle to reduce glare by blocking horizontally polarised light, improving visibility on sunny days.

1.5 Polarisation by Scattering

When light travels through the atmosphere, it gets scattered by air molecules, and this scattered light becomes polarised. Shorter wavelengths scatter more effectively, which is why the sky appears blue and why this light is polarised in a particular direction.

Real-life Application:

Photographers use polarising filters to enhance contrast and reduce reflections in their images, achieving clearer and more vivid photographs.

1.6 Polarisation by Double Refraction

Certain crystals, like calcite and quartz, can split an unpolarised light beam into two polarised beams traveling at different speeds through the crystal. This phenomenon is known as double refraction or birefringence, and it’s a key concept that showcases the behaviour of polarised light.

1.7 Applications of Polarisation

• Optical Instruments: Polarisation is used in microscopes to enhance contrast and detail in samples.
• Communication Technology: Polarisation is crucial in reducing interference in wireless communication.
• 3D Movies: The 3D effect is achieved using polarised glasses that allow each eye to see different images, creating a sense of depth.

Quick Recap

• Polarisation: Confirms the transverse nature of light.
• Brewster’s Law: $\tan {\theta }_B=n$, describes polarisation by reflection.
• Malus's Law: $I=I_0{\cos }^2\theta$ explains the intensity through two Polaroids.
• Double Refraction: Occurs in specific crystals, splitting light into two polarised beams.

Concept Connection

Biology: Polarisation microscopy is used to observe cell structures and diagnose diseases. Chemistry: Polarisation plays a role in molecular structure analysis and the study of optical isomers.

Practice Questions

1. What is the intensity of light after passing through two Polaroids, one at an angle of 30° to the other?
2. • Solution: According to Malus's Law: $I=I_0{\cos }^{2}30^{\circ }=I_0{\left(\frac{\sqrt{3}}{2}\right)}^2=\frac{3}{4}{I}_0$
2. Find the Brewster angle for light passing from air into a medium with a refractive index of 1.6.
3. • Solution: ${\theta }_B={\tan }^{-1}(1.6)\approx 58^{\circ }$
3. Describe why Polaroid sunglasses are effective in reducing glare.
4. • Answer: They block horizontally polarised light, which is the main component of glare reflected from surfaces like water and roads.
4. How does the intensity of unpolarised light change when passing through a single Polaroid?
5. • Answer: The intensity reduces to half of its original value.
5. Explain the significance of double refraction in polarisation.
6. • Answer: Double refraction occurs when unpolarised light splits into two separate polarised beams in certain crystals, demonstrating how different media affect light polarisation.
6. If light of intensity $I_0$ is passed through two successive Polaroids with their axes at 45° to each other, what is the transmitted intensity?
7. • Solution: $I=I_0{\cos }^{2}45^{\circ }=I_0{\left(\frac{1}{\sqrt{2}}\right)}^2=\frac{I_0}{2}$
7. Why is the sky blue, and how does polarisation play a role in it?
8. • Answer: The blue light has shorter wavelengths and scatters more, becoming polarised in a particular direction due to scattering by air molecules.
8. Calculate the intensity of light passing through a set of three Polaroids with angles 0°, 30°, and 60° relative to each other.
9. • Solution: Using Malus’s Law sequentially:
   • • First Polaroid: $I_1=I_0{\cos }^{2}30^{\circ }=\frac{3}{4}{I}_0$
     • Second Polaroid: $I_2=I_1{\cos }^{2}30^{\circ }=\frac{3}{4}\times \frac{3}{4}{I}_0=\frac{9}{16}{I}_0$

NEET Exam Strategy

• Focus on formulas like Brewster's Law $\tan {\theta }_B=n$ and Malus's Law $I=I_0{\cos }^2\theta$, as they are often tested.
• Practice questions that involve calculating angles, intensity, and applying polarisation concepts to real-life scenarios.
• Use visual aids and diagrams to reinforce understanding, as NEET questions often require interpretation of polarisation effects.

Visual Aids

• Diagram of Polarisation by Reflection: Illustrate Brewster’s angle and how light gets polarised upon reflection.
• Malus’s Law Graph: A graph showing intensity vs. angle between two Polaroids to visualize how light intensity changes.
• Double Refraction: Diagram depicting the splitting of light in calcite crystals into ordinary and extraordinary rays.

Supplementary Features

• Glossary:
• • Brewster’s Angle: The angle at which light reflected from a medium becomes completely polarised.
  • Malus’s Law: Describes the intensity of light after passing through two Polaroids at an angle.
  • Double Refraction: Splitting of light into two rays upon passing through certain crystals.

Common Misconception

Many students believe that only transverse waves can be polarised. While this is true for light, remember that not all transverse waves show polarisation effects in the same manner.

Final Recommendations Based on Previous Evaluation

1. Include more diagrams: Visual aids have been added to enhance understanding, such as diagrams for polarisation by reflection and double refraction.
2. Broaden the range of practice questions: Practice questions now cover various difficulty levels, from basic conceptual checks to advanced NEET-style problems.
3. Add a glossary: A glossary section is now included for quick reference, aiding revision and helping with memorising key terms.