Understanding of the production and detection of plane polarized light is the first step to dive deeper into the world of polarization.

Introduction

Polarization is a fundamental property of light that describes the orientation of its electric field vibrations. The basics of this idea have already been discussed; click here. To dive deeper into the ideas of polarization, you first need to explore production, detection, and the effects of polarized light.

Production, Detection, and the Effects of Polarized Light _ Illustration

Let us start our discussion with simple concepts.

Difference between Unpolarized Light and Polarized Light

Here is a quick recall of the differences between unpolarized light and polarized light.

Unpolarized Light

Ordinary light sources emit vibrations in all possible planes, making the light unpolarized.

Examples

Firelight
Sunlight
Candlelight
Incandescent or fluorescent bulbs

Polarized Light

If the vibrations of the electric field are confined to one plane, the light becomes polarized.

Example

Laser light
LCDs
Light passing through birefringent crystals

Feature	Unpolarized Light	Polarized Light
Vibration Direction	Multiple random planes	Single plane
Examples	Sunlight, bulbs	Polarized sunglasses, LCD light
Production	Natural sources	Filters, reflection, scattering
Glare Reduction	No	Yes
Uses in Tech	General illumination	Photography, displays, optics
Transmission Through Polarizer	50%	Depends on angle (Malus’ Law)

Production and Detection of Plane Polarized Light

Light emitted from an ordinary incandescent bulb is unpolarized, meaning its electric field vibrations are randomly oriented in space.

Illustration of Unpolarized Light Emitted from Incandescent Bulb

Polarization Using Polaroids

When unpolarized light passes through a Polaroid sheet (polarizer), the transmitted light becomes plane polarized.

Adding a second Polaroid sheet (analyzer) allows control over transmitted light intensity.

Rotation of Analyzer w.r.t Polarizer

If the transmission axes of the polarizer and analyzer are parallel, light passes through both sheets.

Illustration of Polarization Using Polaroids _ Parallel Polarizer and Analyzer

If the analyzer is slowly rotated w.r.t the polarizer, the following may happen:

The transmitted intensity decreases.
It reaches zero when the axes are perpendicular.
Further rotation restores the light as axes realign.

Illustration of Polarization Using Polaroids _ Perpendicular Polarizer and Analyzer

Key Insight
This activity validates that light is a transverse wave. If it were longitudinal, the intensity would never fall to zero, even with crossed Polaroids.

Polarization of Light by the Method of Reflection

Malus’s Discovery

In 1808, Malus discovered that ordinary light becomes polarized when reflected from a plane glass surface.

Ray Diagram _ Polarization of Light by the Method of Reflection

Observing the reflected light through a rotating Polaroid shows that light almost disappears at a specific orientation.
On glass, this occurs at an angle of incidence ≈ of 57°.

Surfaces like tables also partially polarize reflected light, as seen by their dimming under a rotating Polaroid.

Brewster’s Law

Brewster’s Law states that:

“The angle of incidence at which reflected light is completely plane polarized is such that the reflected and refracted rays make a right angle with each other”.

Brewster Angle

The polarizing angle, or Brewster angle, is the angle of incidence where reflected light is completely plane polarized.

Relation between Angles

Let!

$i_p = \text{polarizing angle}$
$r = \text{angle of refraction}$

By geometry:

$i_p + r = 90^\circ \quad \Rightarrow \quad r = 90^\circ - i_p$

Snell’s Law

From Snell Law:

$n_1 \sin i_p = n_2 \sin r$

$n_1 \sin i_p = n_2 \sin \left(90^\circ-i_p\right)$

$n_1 \sin i_p = n_2 \cos i_p$

The mathematical form of Brewster’s Law is:

$\dfrac{n_2}{n_1} = \tan i_p$

For air $\left(n_1 = 1\right)$ , so, it can also be written as:

$\tan i_p = n$

The tangent of the Brewster angle equals the refractive index of the reflecting medium.

Optical Activity

Optical activity is the ability of certain substances to rotate the plane of polarization of light passing through them.

Examples of Optically Active Substances

Cinnabar (HgS)
Sugar solution
Collagen
Insulin
Quartz

Factors Affecting Rotation

Optical activity is influenced by multiple factors, including:

Path length of light through the substance
Wavelength of light
Type of substance
Concentration

Cause

Optical activity arises from asymmetric (often helical) molecular structures.

Application

Measuring the rotation of polarized light through a sugar solution allows for determining its concentration.

Conclusion

Polarization is a powerful concept that reveals the vector nature of light. By using Polaroids, reflection at Brewster’s angle, or optically active substances, you can manipulate plane-polarized light.

These foundational ideas pave the way for advanced study in optics, imaging, communication, and material science.

Frequently Asked Questions (FAQs)

What is the key purpose of an analyzer in a polarization experiment?

The key purpose of an analyzer is:

“To detect and measure the presence and orientation of plane polarized light”.

It helps to determine:

whether the light is polarized, and
the direction of its plane of polarization.

If the analyzer is rotated, changes in transmitted intensity reveal the polarization state.

What is Brewster’s Angle?

Brewster’s angle is the angle of incidence at which light reflected from a surface becomes completely plane polarized.

At this angle:

The reflected and refracted rays are perpendicular (90°) to each other.
The reflected light is fully polarized in a direction perpendicular to the plane of incidence.

It is given by Brewster’s law:

$\tan i_p = n$

Where

$i_p = \text{Brewster's angle}$
$n = \text{refractive index of the medium the light enters}$

What are the applications of Brewster’s angle?

Brewster’s angle is used in:

Photography
Laser technology
Anti-glare sunglasses
Optical coatings & design
Glare reduction in displays and screens

Would it be possible to use a polarizer as an analyzer? If yes, give at least two examples.

Yes!

A polarizer and analyzer are physically the same device. Their job depends on how you use them.

Examples

1. Checking the polarization of reflected light

A Polaroid acting as an analyzer will dim the reflected light at specific angles.

2. Determining the polarization direction of laser light

Rotating a second polarizer (used as analyzer) shows maximum or minimum intensity.

What do you know about the production and detection of plane polarized light?

Plane polarized light can be produced using:

Scattering
Reflection at Brewster’s angle
Polaroid sheets (selective absorption)
Birefringent crystals (double refraction)

It can be detected using:

An analyzer, which changes intensity as it rotates
Optical activity, where the plane of polarization rotates in a solution
Polarization by reflection, where brightness changes under a Polaroid

What do you know about production and detection of plane polarized light?
OR
How can the plane polarized light be produced and detected? What does it prove?

Production

Passing unpolarized light through a polarizer
Using reflection at Brewster’s angle
Passing through a birefringent crystal

Detection

Observing intensity changes with a rotating analyzer

What it proves

These experiments prove that light is a transverse wave. It is because only transverse waves can have different vibration directions filtered or eliminated.

How can polarized light be obtained by the method of reflection? Explain.

Polarized light is obtained by reflection when light strikes a surface at a specific angle called the Brewster angle.

At this angle:

The reflected and refracted rays are at 90° to each other.
The reflected light becomes completely plane polarized perpendicular to the plane of incidence.

This is described by Brewster’s law:

$\tan i_p = n$

Where

$i_p = \text{Brewster's angle}$
$n = \text{refractive index of the medium the light enters}$

Common examples:

Glare from water, glass, or shiny surfaces
Polarization in photography and sunglasses

What is meant by optical activity? Discuss it.

Optical activity is the property of certain substances to rotate the plane of polarization of light passing through them.

Causes

Asymmetry or chirality in molecules
Helical or non-superimposable structures

Examples

Cinnabar (HgS)
Sugar solution
Collagen
Quartz
Insulin

Factors affecting rotation

Path length
Wavelength
Nature of substance
Concentration

Applications

Measuring concentration of sugar solutions (polarimetry)
Studying biological molecules
Pharmaceutical quality control

Master Production and Detection of Plane Polarized Light | Beginner 101

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