Which of the following statements about electromagnetic waves, sound waves and water waves is/are correct ? 1. They exhibit reflection 2. They carry energy 3. They exert pressure 4. They can travel in vacuum Select the correct answer using the code given below :
- (a)1, 2 and 3
- (b)2 and 4
- (c)1 and 3 only
- (d)1 only
Correct — A, 1, 2 and 3. Read the stem carefully: it asks what is true of all three kinds of wave together. Reflection is a general property of waves, so light bounces off a mirror, sound returns from a cliff as an echo and water waves rebound from a harbour wall — statement 1 holds for all three. Carrying energy is what a wave is for; it moves energy from place to place without moving matter bodily along with it, which is why a sound wave can shatter glass and an ocean swell can destroy a sea wall — statement 2 holds. Exerting pressure follows from carrying momentum: light exerts radiation pressure, which is what a solar sail uses, sound exerts an acoustic pressure that is how a microphone detects it at all, and water waves press against anything in their path — statement 3 holds. Statement 4 is the one that splits the group, because travelling through vacuum is the privilege of electromagnetic waves alone.
- (b)2 and 4 — Statement 4 is the single false one in the set, because sound and water waves are mechanical and need a material medium — sound cannot cross a vacuum at all, which is why a bell ringing inside an evacuated jar falls silent. Any option containing 4 is therefore wrong.
- (c)1 and 3 only — Reflection and pressure are correctly kept, but dropping the energy statement is a serious error. Transporting energy is the defining function of a wave, and the fact that a wave carries momentum at all — which is what produces the pressure in statement 3 — presupposes it.
- (d)1 only — This concedes only the most obvious property. A candidate picking it has probably assumed that pressure and energy belong exclusively to mechanical waves and that light, being 'weightless', can do neither.
Waves fall into two families. Mechanical waves — sound, water waves, waves on a string, seismic waves — are disturbances of a material medium and cannot exist without one. Electromagnetic waves are oscillating electric and magnetic fields sustaining each other, need no medium and travel through vacuum at about 3 × 10⁸ metres per second. What both families share is the wave behaviour itself: reflection, refraction, diffraction, interference, and the transport of energy and momentum without any bulk transport of matter.
The design of this item is worth naming, because NDA reuses it. A list of properties is given, most of them common to all waves, and one is slipped in that belongs to only one family. The winning move is to identify the odd property first and eliminate every option containing it — here that removes option (b) at once and leaves a choice between three, two and one true statements. Then ask which of the remaining properties are truly universal. Radiation pressure is the one candidates doubt, so it is worth fixing with an example: sunlight pushes on a spacecraft's solar sail, and the pressure of radiation is part of why a comet's tail streams away from the Sun rather than trailing behind the comet's motion. On the electromagnetic side, note the common textbook error that these waves are elastic waves — an NDA paper marked exactly that claim false, because an elastic wave by definition needs a medium to be elastic.
- Reflection, refraction, diffraction and interference are general wave properties shown by mechanical and electromagnetic waves alike.
- Waves transport energy and momentum without transporting matter bodily along with them.
- Radiation pressure is the pressure exerted by electromagnetic waves; it is the principle behind the solar sail.
- Only electromagnetic waves travel through vacuum; sound and water waves are mechanical and need a medium.
Find the property that belongs to one family alone, strike out every option containing it, and the item is nearly solved.
- Assuming light exerts no pressure because photons have no rest mass.
- Treating electromagnetic waves as elastic waves — they are not, since they need no medium.
- Overlooking that a wave carries energy without carrying the medium along with it.
NDA repeatedly gives a property list covering several wave types and hides one property that belongs to a single family.
Consider the following statements : 1. In a seismograph, P waves are recorded earlier than S waves. 2. In P waves, the individual particles vibrate to and fro in the direction of wave propagation whereas in S waves, the particles vibrate up and down at right angles to the direction of wave propagation. Which of the statements given above is/are correct?
- (a) 1 only
- (b) 2 only
- (c) Both 1 and 2
- (d) Neither 1 nor 2
Answer(c) Both 1 and 2
The same wave vocabulary applied to earthquakes — mechanical waves whose behaviour depends entirely on the medium they pass through, which is what separates them from the electromagnetic family.
Which of the following are the characteristics of electromagnetic waves? 1. They are elastic waves. 2. They can also move in vacuum. 3. They have electric and magnetic components which are mutually perpendicular. 4. They move with a speed equal to 3 lakh meters per second. Select the correct answer using the code given below:
- (a) 1, 2, 3 and 4
- (b) 1, 2 and 4 only
- (c) 2 and 3 only
- (d) 3 and 4 only
Answer(c) 2 and 3 only
It certifies the one property that separates the families in this item — electromagnetic waves alone move in vacuum — and marks the 'elastic wave' claim false.
Which one of the following statements is true for sound waves propagating in air ?
- (a) Sound is an electromagnetic wave and transverse in nature
- (b) Sound is a mechanical wave and longitudinal in nature
- (c) Sound is a mechanical wave and transverse in nature
- (d) Sound is an electromagnetic wave and longitudinal in nature
Answer(b) Sound is a mechanical wave and longitudinal in nature
It fixes sound firmly in the mechanical family, which is the reason statement 4 of this item cannot be true of all three wave types.
- practice — not a real PYQ
Which one of the following can travel through a vacuum?
- (a)Sound waves
- (b)Water waves
- (c)X-rays
- (d)Seismic S waves
Answer(c) X-rays — an electromagnetic wave, and the only kind in the list that needs no medium.
- practice — not a real PYQ
A solar sail is propelled through space chiefly by
- (a)the pressure exerted by sunlight
- (b)the solar magnetic field
- (c)the gravitational pull of the Sun
- (d)sound waves from the Sun
Answer(a) the pressure exerted by sunlight — radiation pressure, which is momentum carried by an electromagnetic wave.