Polarisation MCQ Quiz - Objective Question with Answer for Polarisation - Download Free PDF

CONCEPT:

• Total Internal reflection: The total reflection of light from a higher refractive index medium to back in its medium at an angle more than a critical angle known as total internal reflection.
  • It always happens when light travels from denser medium to rarer medium.

• Optical fibre: It is a very thin fibre made of glass and plastic having a radius of the order of a micrometre (10-6 m).
  • It works on the principle of Total Internal Reflection.
  • It is lightweight, fast, and useful for long-distance data and light transmission with very few losses.
  • A bundle of such thin fibres forms a light pipe.

• Interference: The combination of two or more electromagnetic waveforms to form a resultant wave that may have greater, lower, or the same amplitude is called interference.
• There is an experimental observation for measurement of the flatness of the surface by using the interference phenomenon.
• Diffraction: It is defined as bending of waves around the corner when a wave encounters an obstacle.
  • For the structure of a crystal, we use the diffraction phenomenon to see how much the light is diffracted by it. 
• Polarization: It is a property of transverse waves. The direction of oscillations becomes perpendicular to the direction of motion
  • Light passing through a calcite crystal is split into two rays. The two rays of light are each plane-polarized by the calcite such that the planes of polarization are mutually perpendicular.

The optical activity of the above crystal is measured by the polarization phenomenon.

EXPLANATION:

From the above discussion, 

we can match: a-h, b-g, c-f, d-e

So option 2 is correct.

Calculation:

Where I : distance from point source

⇒ 

⇒ 

Also, due to polaroids:

⇒ 

⇒ Ratio becomes 4 times.

⇒ r_new = 8

where is the amplitude and is the intensity.

For unpolarised light, it can be resolved into two components of equal magnitude in any pair of perpendicular directions. So the amplitude that is along the plane of polarisation of the polaroid will always be .

So, the intensity which will pass through it will always be .

Now when this polarised light falls on polaroid B whose plane of polarisation makes an angle with the plane of polarisation of incident light, the component of amplitude along the plane of polarisation of the polaroid is .

Therefore, intensity of light that passes through is where is the intensity of incident light.

Here, and .

So intensity of light emerging through B is

No light is emitted from the second polariod, so and are perpendicular to each other.

Let the initial intensity of light is . So Intensity of light after transmission from first polaroid

Intensity of light emitted from

Intensity of light transmitted from last polaroid i.e. from

The correct answer is 90°.

If no refraction occurs when a ray of light passes from medium X to medium Y, it means that the light ray is incident at an angle such that it does not bend upon entering the new medium. For this to happen, the light must strike the boundary between the two media perpendicularly.

Explanation:

• If a light is incident at an angle of 90°, it will go straight again, making an angle of 90° with the normal; the medium has no effect.
• As a result, the angle of incidence is 90°. 
• If an angle of incidence is 90°, it means the light is propagating along the interface separating the two media.
• In this case, the light does not enter the second medium and so we can not talk about the angle of refraction.
• The angle of refraction can also be regarded as 90°.

Thus, A ray of light passes from medium X to another medium Y. If no refraction occurs then the angle between incident light and surface of medium Y should be 90°.

CONCEPT:

Sound: 

• A sound is a form of energy that produces a sensation of hearing in our ears.
• It is a kind of disturbance that travels through a medium due to repeated vibrations of the particles of the medium about their mean positions.
• The disturbance being handed over from one particle to the next.

Light:

• Light is a form of electromagnetic radiation that can be detected by the human eye.
• When light falls on an object, some part of the light is reflected on our eyes and we are able to see.

EXPLANATION:

• Sound waves are longitudinal waves and light waves are transverse waves. 
• As we know sound waves are longitudinal waves and travel through compression and refractions in the medium on the other hand light waves are transverse in nature and do not require a medium to travel. 
• As we know that, the sound wave and lightwave travels with different velocity and have a different frequency, therefore the wavelength (λ = v/f) is different for both. 
• Both sound and light waves follow the law of reflection, refraction, and interference of waves.
• Polarization occurs with light waves only.

When ordinary light is allowed to pass through calcite or quartz, it splits into two refracted beams(O-ray & E –ray) and both are plane-polarized lights.

Polarisation:

• The phenomenon due to which vibrations of light waves are restricted in a particular plane is called polarisation.
• In an ordinary beam of light from a source, the vibrations occur normal to the direction of propagation in all possible planes. Such a beam of light called unpolarised light.
• If by some methods (reflection, refraction, or scattering) a beam of light is produced in which vibrations are confined to only one plane, then it is called plane polarised light.
•  Hence, polarization is the phenomenon of producing plane polarised light from unpolarised light.

Additional Information

There are three types of polarized light

• Plane Polarized Light
• Circularly Polarized Light (O-ray)
• Elliptically Polarized Light (E-ray)

Concept:

The angle of incidence at which a beam of unpolarized light falling on a transparent surface is reflected as a beam of completely plane polarised light is called polarising or Brewster angle. It is denoted by ip.

Brewster's law: It states that when a ray is passed through some transparent medium having refractive index μ at any particular angle of incidence, reflected ray is completely polarized; and the angle between reflected and refracted ray is 90⁰.

​

 μ = tan θB 

Where μ = refractive index and θB is Brewster's angle or polarizing angle (i_p).

Critical angle: When a ray of light is going from a denser medium to a rare medium then the angle of incidence at which the refraction angle is 90° is called the critical angle.

If θc is the critical angle, the refractive index (μ) is given by:

​Calculation:

If θ_c is the critical angle, the refractive index (μ) is given by:

From Brewster’s law, the polarization angle,

When the ray incident at the polarization angle, the angle between the reflected and refracted rays are 90^o.

So, the angle of refraction (r) + angle of reflection (i_p) = 90° 

⇒ r = 90° - 60° = 30°

Hence option 1 is correct.

CONCEPT:

• Light: It is an electromagnetic wave.
  • Light shows dual nature i.e. wave and particle.

Phenomenon explaining the wave nature of light is/are:

1. Interference
2. Diffraction
3. Polarisation

Phenomenons explaining the particle nature of light are:

1. Photoelectric effect
2. Compton Scattering

• Photons associated with light shows that the energy is Quantised and occurs in discrete levels.

EXPLANATION:

• From the above discussion, the light has a dual nature. So option 4 is correct.

Concept:

A polarimeter consists of a light source, two polarizing prisms or filters, a liquid cell placed between the two polarizers, an eyepiece and a protractor for measuring the angular rotation of light by the solution.

Specific rotation α:

θ = measured rotation in angular degrees, c = concentration in grams per millilitre of solution, l = cell length in decimetres

Calculation:

20 cm = 2 decimetres

Concept:

If μ₀ and μ_e are refractive index of O-ray and E-ray respectively, λ is the wavelength of light and t is the thickness of retardation plate Then;

Path difference between O-ray and E-ray is given by,

Δ = t(μ₀ - μ_e)    ---(1)

And, phase difference between O-ray and E-ray is given as;

       ---(2)

Calculation:

Given: t = 30 micron, μ₀ = 1.554, μ_e = 1.547, λ = 6000 Å

From equation (1) ;

Path difference can be calculated as;

Δ = 30 × 10^-6 × (1.554 – 1.547)

Δ = 30 × 0.007 × 10^-6

Δ = 21 × 10^-8 m

From equation (2) ;

δ = 0.7π

Important Point:

1. Negative crystals are crystals in which refractive index of E-ray (μ_e) is less than that O-ray (μ₀) ⋅ [μ_e 0]

2. Positive crystals are crystals in which refractive index of Oray is less than that for E ray
    [μ₀ e]

Concept:

• The angle of incidence at which a beam of unpolarized light falling on a transparent surface is reflected as a beam of completely plane polarised light is called polarising or Brewster angle. It is denoted by ip.
• Brewster's law: It states that when a ray is passed through some transparent medium having refractive index μ at any particular angle of incidence, the reflected ray is completely polarized; and the angle between reflected and refracted ray is 900.

​

 μ = tan θB 

Where,

μ = refractive index and 

θ_B is Brewster's angle or polarizing angle (i_p).

​Calculation:

Given that:

The angle of incidence (i_p) = 60°

Refractive index (μ) = tan θB = tan ip =  tan 60° = √3 = 1.7

Hence option 3 is correct.

A light wave that has vibrations in more than one plane is known as unpolarized light.

• Polarized waves are light waves in which the vibrations occur in a single plane.
• A Nicol prism is made up of two prisms of calcite (CaCO3).
• Nicol prism is designed in such a way that it eliminates the ordinary rays by total internal reflection and only allow the extraordinary rays to transmit through it.

​Notes:

• Nicol prism is used for producing a polarised beam of light from an un-polarised beam. Hence, option 2 is correct.
• It is based on the principle of action which involves refraction as it passes into the lower half of the prism.
• It leaves the prism as polarised light after undergoing another refraction as it exits the far right side of the prism. Thus its action is based on double refraction.