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Circular Polarisation

Light in the form of a plane wave in a vacuum or air is called linearly polarised. Light is the best example of a transverse wave. Natural light is typically unpolarised; every plane of propagation is equally probable. When light is made up of two plane waves of identical amplitude by varying in phase by 90°, then the light is considered to be circularly polarised. When two plane waves of varying amplitude are correlated in phase by 90° or when the relative phase is other than 90°, the given light is considered to be elliptically polarised.

The video explains the basic concepts of polarisation of waves

Polarisation

Polarisation is a characteristic of transverse waves that determines the geometrical orientation of their oscillations. A basic example of a polarised transverse wave is vibrations moving along a guitar string (taut string). Depending on how strings are plucked, the vibration can be in a horizontal direction, vertical direction, or at any angle perpendicular to the strings. On the other hand, in longitudinal waves, like sound waves in a gas or liquid, the particles’ displacement in the oscillation is every time in the propagation’s direction. Therefore, these waves do not show polarisation. Transverse waves that show polarisation include light, radio waves, shear waves, etc.

Circular Polarisation

Circularly polarised light is made up of dual perpendicular electromagnetic plane waves of identical amplitude and 90° variation in phase.

In the field of electrodynamics, the direction and the strength of an electric field are represented by the electric field vector.

In the instance of a circularly polarised wave, the tip of the field vector, at a particular point in space, relates to the light’s phase as it propagates through space and time. At any point in time, the electric vector of the wave shows a point on a helix aligned along the propagation’s direction. A circularly polarised wave can turn into one of two attainable senses. One is right-handed or clockwise circular polarisation in which the field vector turns in a right-hand sense relative to the propagation’s direction, and left-handed or counter-clockwise circular polarisation in which the electric field vector turns in a left-hand sense. Circular polarisation is a special case of elliptical polarisation.

Relationship between Circular Polarisation and other Polarisation

The characteristics of circular polarisation and its relation to other polarisations are usually understood by taking the electric field as being split into two components that are mutually perpendicular.

Circularly polarised light can be transformed into linearly polarised light by strictly passing it through a quarter-wave plate. Passing linearly polarised light through a quarter-wave plate with the axes at 45° to its polarisation axis converts it to circular polarisation. This is the most common method of generating circular polarisation in practice. Remember that passing linearly polarised light waves through a quarter-wave plate at an angle except 45° will usually create elliptical polarisation.

Difference between Linear, Circular and Elliptical Polarisation

The fundamental difference between linear, elliptical and circular polarisation is:

In linear polarisation, the electric field of the light is restricted to a single plane where the propagation happens. In circular polarisation, there are two linear planes in the light’s electric field that are strictly perpendicular to each other. In elliptical polarisation, the light’s electric field is at an elliptical propagation. Elliptically polarised light is made up of two strictly perpendicular waves of varied amplitude, which vary in phase by 90°.

Advantages and Disadvantages of Circular Polarisation

The main advantages of circular polarisation are:

  • In circular polarisation, there is much less chance for cross-pol interference.
  • RFID readers do not need to know the tag direction, unlike linear polarised RFID readers. So they are used for applications where one does not know the direction of RFID tags while scanning the data.
  • RFID readers do not have to be on the exact plane and at identical heights as RFID tags, unlike linear polarisation RFID readers.
  • No polarisation adjustment is needed as it is fixed using an Orthomode Transducer.
  • Circularly polarised antennas give a much higher probability of an active link as it is transmitted on every plane.

The main disadvantages of circular polarisation are:

  • Circular polarisation provides a shorter read range for RFID readers compared to linear polarisation.
  • The technology behind the circular polarisation makes the cost of the production antenna system much higher.
  • It provides lower cross-pol isolation.

Frequently Asked Questions – FAQs

What is meant by polarisation?

Polarisation is a characteristic of transverse waves that determines the geometrical orientation of their oscillations. A basic example of a polarised transverse wave is vibrations moving along a guitar string (taut string). Depending on how strings are plucked, the vibration can be in a horizontal direction, vertical direction, or at any angle perpendicular to the strings.

What is circular polarisation?

When light is made up of two plane waves of identical amplitude by varying in phase by 90°, then the light is considered to be circularly polarised.

What is plane polarisation?

Light in the form of a plane wave in a vacuum or air is called linearly polarised.

What is elliptical polarisation?

When two plane waves of varying amplitude are correlated in phase by 90° or when the relative phase is other than 90°, then the given light is considered to be elliptically polarised.

What are the uses of polarisation?

Polarisation is used to differentiate between longitudinal waves and transverse waves.
They are used in sunglasses to filter unwanted reflected light.
They are used to analyse the chirality of organic compounds.
They are typically used in 3D glasses and 3D cameras.

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