A Level Physics 7.5 Polarisation MCQs – 30 Practice Questions with Answers (Cambridge 9702)
Free practice MCQs for Cambridge International AS & A Level Physics (9702), Section 7.5 Polarisation. These original questions cover what polarisation means, how polarising filters behave, and the everyday applications of polarised light – with a worked explanation behind every Show Answer button.
Polarisation Basics
Q1. What is meant by a plane-polarised transverse wave?
- (A) Its oscillations are parallel to the direction of wave travel
- (B) Its oscillations are confined to a single plane that contains the direction of travel
- (C) Its oscillations take place in every plane perpendicular to the direction of travel
- (D) Its oscillations have the same amplitude in every direction
Show Answer
Answer: B – A plane-polarised (or linearly polarised) wave oscillates in one plane only, and that plane contains the direction of travel. Oscillations in every plane would be an unpolarised wave, and oscillations parallel to the travel direction describe a longitudinal wave.
Q2. In an unpolarised light wave, the oscillations take place
- (A) in one fixed plane only
- (B) along the direction of travel only
- (C) in all planes perpendicular to the direction of travel
- (D) only when the light passes through a filter
Show Answer
Answer: C – Unpolarised light oscillates in every plane that is perpendicular to the direction of travel, with the mix of planes changing randomly. Restricting the oscillations to just one of those planes is exactly what polarisation does.
Q3. Which type of wave can be polarised?
- (A) A transverse wave only
- (B) A longitudinal wave only
- (C) Both transverse and longitudinal waves
- (D) Neither transverse nor longitudinal waves
Show Answer
Answer: A – Only transverse waves can be polarised, because only transverse waves have oscillations in a plane perpendicular to the travel direction that can be restricted to a single plane.
Q4. Sound waves in air cannot be polarised. This is because sound waves
- (A) travel too slowly
- (B) always need a material medium
- (C) have wavelengths that are too long
- (D) are longitudinal, so their oscillations are along the direction of travel and there is no plane of oscillation to restrict
Show Answer
Answer: D – In a longitudinal wave the oscillations are parallel to the direction of travel, so there is only one possible direction of oscillation. Polarisation – selecting one plane of oscillation – is therefore impossible for sound.
Q5. The fact that light can be polarised is evidence that light travels as
- (A) a longitudinal wave
- (B) a transverse wave
- (C) a stream of particles with no wave behaviour
- (D) a wave that needs a medium
Show Answer
Answer: B – Only transverse waves can be polarised, so the successful polarisation of light demonstrates that light (and every other electromagnetic wave) is transverse: its oscillations are perpendicular to the direction of travel.
Q6. The plane of polarisation of a wave is the plane that contains
- (A) the direction of travel only
- (B) the direction of travel and is perpendicular to the oscillations
- (C) the direction of the oscillations and the direction of travel
- (D) neither the oscillations nor the direction of travel
Show Answer
Answer: C – The plane of polarisation is defined as the plane containing both the direction in which the wave oscillates and the direction in which the wave is travelling.
Q7. A transverse wave travels along a rope towards a barrier that has a narrow vertical slit in it. The wave passes through the slit only when
- (A) its oscillations are horizontal
- (B) its amplitude is large enough
- (C) its frequency is high enough
- (D) its oscillations are vertical, in the plane of the slit
Show Answer
Answer: D – The slit acts like a polarising filter: it allows through only the component of the oscillation that is parallel to the slit. A vertically oscillating wave passes a vertical slit; a horizontally oscillating wave is blocked.
Q8. For an unpolarised electromagnetic wave, the electric field oscillates
- (A) in all directions in the plane perpendicular to the direction of travel
- (B) in one fixed direction only
- (C) along the direction of travel
- (D) only inside a polarising filter
Show Answer
Answer: A – In unpolarised light the electric field vector vibrates in every direction within the plane perpendicular to the travel direction. A polariser selects just one of those directions.
Q9. Which of the following waves cannot be polarised?
- (A) Microwaves
- (B) X-rays
- (C) Ultrasound waves in air
- (D) Visible light waves
Show Answer
Answer: C – Ultrasound is a sound wave and is longitudinal, so it cannot be polarised. Microwaves, X-rays and visible light are all transverse electromagnetic waves and can all be polarised.
Q10. When an unpolarised light wave becomes plane-polarised, what changes?
- (A) Its speed decreases
- (B) Its oscillations become restricted to a single plane
- (C) Its frequency decreases
- (D) It becomes a longitudinal wave
Show Answer
Answer: B – Polarisation changes only the planes in which the wave oscillates. The speed, frequency and wavelength of the light are unaffected, and the wave remains transverse.
Polarising Filters
Q11. A Polaroid filter transmits the part of an incident light wave whose oscillations are
- (A) perpendicular to the transmission axis of the filter
- (B) in every plane at once
- (C) along the direction of travel of the light
- (D) parallel to the transmission axis of the filter
Show Answer
Answer: D – A Polaroid has a transmission axis. The component of the oscillation parallel to this axis is transmitted; the component perpendicular to it is absorbed by the filter.
Q12. Unpolarised light of intensity I0 is incident on a single ideal polarising filter. The intensity of the transmitted light is
- (A) I0 / 2
- (B) I0
- (C) zero
- (D) 2I0
Show Answer
Answer: A – Unpolarised light has, on average, half of its oscillation energy in the plane parallel to the transmission axis and half perpendicular to it. The filter absorbs the perpendicular half, so the transmitted intensity is I0 / 2.
Q13. A single ideal polariser is placed in a beam of unpolarised light and slowly rotated about the direction of the beam. The transmitted intensity
- (A) varies between zero and a maximum
- (B) increases steadily as the filter rotates
- (C) stays constant at half the incident intensity
- (D) falls to zero when the filter has turned through 90°
Show Answer
Answer: C – Unpolarised light has no preferred plane of oscillation, so whichever way the transmission axis points it always passes the same fraction – one half – of the light. Rotating a lone polariser in unpolarised light changes nothing.
Q14. Unpolarised light passes through two ideal polarising filters whose transmission axes are at 90° to each other (crossed polarisers). The intensity transmitted by the second filter is
- (A) half the original intensity
- (B) zero
- (C) equal to the full original intensity
- (D) a quarter of the original intensity
Show Answer
Answer: B – The first filter produces plane-polarised light oscillating parallel to its own axis. This plane is perpendicular to the second filter’s axis, so the second filter absorbs all of it and nothing is transmitted.
Q15. Unpolarised light of intensity I0 passes through two ideal polarising filters whose transmission axes are exactly parallel. The intensity transmitted by the second filter is
- (A) zero
- (B) I0 / 4
- (C) I0
- (D) I0 / 2
Show Answer
Answer: D – The first filter halves the intensity to I0 / 2 and polarises the light parallel to the second filter’s axis. An ideal second filter then transmits all of the light reaching it, so the final intensity is still I0 / 2.
Q16. Plane-polarised light passes through an analyser. The analyser is rotated slowly from the aligned position (0°) to the crossed position (90°). The transmitted intensity
- (A) decreases gradually from a maximum to zero
- (B) stays at its maximum and then drops suddenly to zero at 90°
- (C) increases gradually from zero to a maximum
- (D) stays at zero throughout the rotation
Show Answer
Answer: A – As the analyser turns, a progressively smaller component of the polarised oscillation lies along its transmission axis, so the transmitted intensity falls smoothly from maximum at 0° to zero at 90°.
Q17. Plane-polarised light is incident on an analyser whose transmission axis is at 45° to the plane of polarisation. Compared with the aligned position, the transmitted intensity is
- (A) zero
- (B) reduced, but not zero
- (C) greater than at the aligned position
- (D) exactly the same
Show Answer
Answer: B – At 45° a component of the oscillation still lies along the transmission axis, so some light passes, but less than the maximum transmitted when the axes are aligned.
Q18. In a polariser-analyser arrangement, the job of the second filter (the analyser) is to
- (A) produce polarised light from unpolarised light
- (B) increase the intensity of the light
- (C) test the light and reveal the direction of its plane of polarisation
- (D) change the wavelength of the light
Show Answer
Answer: C – The first filter (polariser) creates plane-polarised light. The second filter (analyser) is rotated to investigate that light: the transmitted intensity is maximum when the analyser’s axis matches the plane of polarisation.
Q19. Plane-polarised light passes through an analyser that is initially aligned with the plane of polarisation. The analyser is then rotated through 180°. The transmitted intensity
- (A) stays at zero for the whole rotation
- (B) increases throughout the rotation
- (C) falls to zero at 90° and stays at zero
- (D) falls to zero at 90° and returns to a maximum at 180°
Show Answer
Answer: D – At 90° the analyser is crossed with the light and blocks it. Turning further brings the axis back into line with the plane of polarisation (an axis has no arrow direction, so 180° is equivalent to 0°), and transmission is maximum again.
Q20. A student shines unpolarised light of intensity 80 units onto a single ideal polarising filter. The transmitted intensity, in units, is
- (A) 40
- (B) 80
- (C) 20
- (D) 0
Show Answer
Answer: A – One ideal polariser always transmits half of the unpolarised light incident on it: 80 / 2 = 40 units.
Applications & Observations
Q21. Polarised sunglasses have lenses with a vertical transmission axis. They are especially good at cutting glare from a wet road because the light reflected from the road surface is
- (A) vertically polarised
- (B) still completely unpolarised
- (C) polarised mainly in the horizontal plane
- (D) a longitudinal wave
Show Answer
Answer: C – Reflection from a horizontal surface makes the reflected light partially plane-polarised with its oscillations mainly horizontal. A lens whose transmission axis is vertical absorbs this horizontal component, so the glare is strongly reduced while ordinary light still gets through.
Q22. Light reflected from the surface of a lake on a bright day tends to be
- (A) completely unpolarised, just like the incident light
- (B) partially plane-polarised, with oscillations mainly horizontal
- (C) converted into a longitudinal wave
- (D) polarised with oscillations vertical only
Show Answer
Answer: B – Reflection from a horizontal surface such as water favours the horizontal component of oscillation, so the reflected light is partially polarised in the horizontal plane. This is why polarised sunglasses are so effective against glare off water.
Q23. In engineering stress analysis, a transparent plastic model of a component is viewed between two crossed polarising filters. Bright coloured patterns are seen in the model because
- (A) the plastic absorbs some colours of light
- (B) the plastic reflects light from its surface
- (C) the crossed filters themselves are coloured
- (D) stressed regions of the plastic rotate the plane of polarisation by different amounts, so some light can then pass through the second filter
Show Answer
Answer: D – With nothing between them, crossed filters transmit no light. Stressed plastic alters the polarisation of light passing through it, by an amount that depends on the local stress, so light leaks through the analyser in patterns that map out the stresses in the model.
Q24. In a liquid crystal display (LCD), each pixel controls how much light it lets through because the liquid crystal layer
- (A) rotates the plane of polarisation of the light by an amount controlled by the applied voltage, so the second polarising filter passes or blocks the light
- (B) emits its own plane-polarised light
- (C) absorbs every plane of oscillation equally
- (D) changes the speed of the light
Show Answer
Answer: A – An LCD pixel is sandwiched between polarising filters. Applying a voltage changes how much the liquid crystal rotates the plane of polarisation, which changes how much light the second filter transmits – producing the light and dark areas of the display.
Q25. In a 3D cinema system based on polarisation, the audience wears glasses with polarising lenses because the projector shows the two images
- (A) in two different colours only
- (B) with exactly the same plane of polarisation
- (C) with different, perpendicular planes of polarisation, and each lens passes the image meant for that eye only
- (D) at two different brightnesses
Show Answer
Answer: C – The left-eye and right-eye images are projected with perpendicular planes of polarisation. Each lens of the glasses is a polariser aligned with one image, so each eye sees only its own image and the brain combines them into a 3D picture.
Q26. In a school experiment, microwaves from a transmitter pass through a grille made of parallel metal bars before reaching a receiver. The received signal falls to zero when the bars are
- (A) perpendicular to the electric field oscillations of the microwaves
- (B) parallel to the electric field oscillations of the microwaves
- (C) removed from between the transmitter and receiver
- (D) made of a non-metallic material
Show Answer
Answer: B – Metal bars parallel to the electric field oscillations absorb that component of the wave, just as a Polaroid absorbs one component of light. Turning the grille so the bars are perpendicular to the oscillations lets the wave pass – a direct demonstration that microwaves are transverse and can be polarised.
Q27. A receiving aerial for radio waves picks up the strongest signal when the aerial
- (A) is aligned parallel to the direction of oscillation of the electric field of the transmitted wave
- (B) is aligned perpendicular to the electric field oscillations of the wave
- (C) is buried below ground level
- (D) is as short as possible
Show Answer
Answer: A – Radio waves are transmitted plane-polarised. The aerial must line up with the plane of the electric field oscillations so the field can drive a maximum current along the aerial; this is why TV and radio aerials have a definite orientation.
Q28. A dipole aerial is aligned for the best possible reception of a plane-polarised radio signal. The aerial is then rotated through 90° in the plane perpendicular to the incoming wave. The received signal becomes
- (A) stronger than before
- (B) completely unchanged
- (C) only slightly weaker
- (D) zero, because the aerial is no longer aligned with the oscillations of the wave
Show Answer
Answer: D – This is the radio-wave equivalent of crossed polarisers. After a 90° rotation the aerial is at right angles to the electric field oscillations, so the field cannot drive a signal current along it.
Q29. At one particular angle of incidence on a glass block, the light reflected from the surface is found to be completely plane-polarised. This observation shows that reflection can
- (A) unpolarise polarised light
- (B) polarise light, because the reflected wave keeps only one plane of oscillation
- (C) turn light into a longitudinal wave
- (D) double the intensity of the incident light
Show Answer
Answer: B – Reflection is one way of polarising light. At a particular angle (Brewster’s angle) the reflected light contains only one plane of oscillation, so it is completely plane-polarised.
Q30. Polarisation provides a test that can distinguish a transverse wave from a longitudinal wave because
- (A) longitudinal waves always travel faster
- (B) transverse waves always have longer wavelengths
- (C) only oscillations perpendicular to the direction of travel can be restricted to a single plane; longitudinal oscillations are along the direction of travel and cannot be restricted in this way
- (D) longitudinal waves cannot be reflected
Show Answer
Answer: C – If a wave can be polarised – for example, blocked by a correctly rotated filter – its oscillations must lie in planes perpendicular to the travel direction, proving it is transverse. A longitudinal wave, whose oscillations are along the travel direction, can never show this effect.
Keep Practising
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