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Explain the following giving reason for ...

Explain the following giving reason for each:
(a) How does a polaroid work to produce a linearly polarised light from an unpolarised beam of light ?
(b) Why is it that light waves can be polarised but sound waves cannot be ?
(c) Why are sun goggles made of polaroids preferred over those using coloured glasses ?

Text Solution

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(a) A polaroid consists of long chain molecules aligned in a particular direction. Iff an unpolarised light is incident on such a polaroid, electric vectors of light waves along the direction of the aligned molecules get absorbed. Thus, in the emergent light there are electric vectors oscillating along a direction perpendicular to the aligned molecules and the light is therefore linearly polarised.
(b) Sound waves cannot be polarised because sound waves are longitudinal waves and longitudinal waves cannot undergo polarisation. light waves being transverse waves can be polarised.
(c) Sun goggles made of polaroids are preffered. when ordinary sunlight is incident on sun goggles, the transmitted light is plane polarised and its intensity is redused by half. sun goggles made of coloured glasses allow total light of a particular colour to pass.
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Light waves can be polarised but sound waves cannot be. Why?

Assertion: Light waves can be polarised but sound waves cannot be polarised. Reason: Sound waves in air are longitudinal in nature.

Knowledge Check

  • Assertion : Sound waves cannot propagate through vacuum but light waves can. Reason : Sound waves cannot be polarised but light waves can.

    A
    both, Assertion and Reason are true and Reason is the correct explanation of the Assertion.
    B
    both, Assertion and Reason are true but Reason is not a correct explanation of the Assertion.
    C
    Assertion is true but the Reason is false.
    D
    both,Assertion and Reason are false.

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    how can one distinguish between an unpolarised light beam and a linearly polarised light beam using a polaroid ?

    Distinguish between unpolarised light and linearly polarised light. How does one get linearly polarised light with the help of a polaroid ?

    Show using a proper diagram how unpolarised light can be linearly polarised by reflection from a transparent glass surface.

    Show using a proper diagram how unpolarised light can be linearly polarised by reflection from a transparent glass surface.

    (a) What is linearly polarised light ? Describe briefly using a diagram how sunlight is polarised (b) Unpolarised light is incident on a polaroid. How would the intensity of transmitted light change when the polaroid is rotated ?

    Light waves are electromagnetic waves and hence transverse in nature. The electric field in a light wave propagating in free space is perpendicular to the direction of propagation. But there are infinite number of directions perpendicular to the direction of propagation. but there are infinite number of directions perpendicular to the direction of propagation of light. for example if light propagates along x-axis, the electric field may be along y-axis or along the z-axis or along any direction in y-z plane in the ordinary light. However, if electric field vecE remains parallel to a fixed direction (say y-axis) then such light is called linearly polarised light. There are several methods to produce polarised light from the unpolarised light. now-a-days polaroid sheets are commonly used to produce linearly polarised light. a polaroid has long chain of hydrocarbons which become conducting at optical frequencies. when light falls normally on the polaroid sheet, the vecE parallel to the chains is absorbed in setting up electric currents in the chains but vecE perpendicular to the chain gets transmitted. so, light on passing through the polaroid i.e., the transmitted light become linearly polarised with vecE parallel to the transission (pass) axis of polaroid. If linearly polarised light of intensity 'I' is incident on another polaroid whose pass axis is inclined at an angle theta from pass axis of first polaroid (or transmission axis of vecE of linearly polarised light) the intensity of transmitted light I_(t) is given as: I_(t)=Icos^(2)theta . Q. Can sound waves be polarised ? Give reason for your answer.

    Light waves are electromagnetic waves and hence transverse in nature. The electric field in a light wave propagating in free space is perpendicular to the direction of propagation. But there are infinite number of directions perpendicular to the direction of propagation. but there are infinite number of directions perpendicular to the direction of propagation of light. for example if light propagates along x-axis, the electric field may be along y-axis or along the z-axis or along any direction in y-z plane in the ordinary light. However, if electric field vecE remains parallel to a fixed direction (say y-axis) then such light is called linearly polarised light. There are several methods to produce polarised light from the unpolarised light. now-a-days polaroid sheets are commonly used to produce linearly polarised light. a polaroid has long chain of hydrocarbons which become conducting at optical frequencies. when light falls normally on the polaroid sheet, the vecE parallel to the chains is absorbed in setting up electric currents in the chains but vecE perpendicular to the chain gets transmitted. so, light on passing through the polaroid i.e., the transmitted light become linearly polarised with vecE parallel to the transission (pass) axis of polaroid. If linearly polarised light of intensity 'I' is incident on another polaroid whose pass axis is inclined at an angle theta from pass axis of first polaroid (or transmission axis of vecE of linearly polarised light) the intensity of transmitted light I_(t) is given as: I_(t)=Icos^(2)theta . Q. What is linearly polarised light ?

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