Which one of the following waves does not belong to the category of the other three ?
- (a)X-rays
- (b)Microwaves
- (c)Radio waves
- (d)Sound waves
Correct — D, sound waves. X-rays, microwaves and radio waves are all electromagnetic radiation: they are oscillating electric and magnetic fields, they are transverse, they travel at 3 x 10^8 m/s in vacuum, and they differ only in wavelength and frequency along one continuous spectrum. Sound is something else entirely — a mechanical wave, a travelling pattern of compression and rarefaction in a material medium, longitudinal in a gas or liquid, and moving at only about 340 m/s in air. It is the odd one out because it cannot exist without matter to carry it.
- (a)X-rays — X-rays sit at the short-wavelength, high-frequency end of the electromagnetic spectrum, beyond ultraviolet. They belong squarely to the same family as microwaves and radio waves, so they are not the exception.
- (b)Microwaves — Microwaves are electromagnetic waves lying between radio waves and infrared. Same family, so they cannot be the odd one out.
- (c)Radio waves — Radio waves are the longest-wavelength members of the electromagnetic spectrum. Again the same family — the wavelength differs by many powers of ten from an X-ray, but the physical nature is identical.
Waves split into two great classes. Mechanical waves — sound, water ripples, waves on a string, seismic waves — need a medium, and they carry energy by making the particles of that medium oscillate. Electromagnetic waves need no medium at all; they are self-sustaining oscillations of electric and magnetic fields and cross vacuum at the speed of light. The electromagnetic spectrum runs, in order of increasing frequency, from radio waves through microwaves, infrared, visible light, ultraviolet and X-rays to gamma rays.
The examiner has chosen three widely separated members of one family and one member of the other, betting that a student sorts waves by familiarity rather than by physics. The reliable test is the vacuum test — ask whether the wave could cross empty space. Radio signals reach us from spacecraft, and X-rays from distant stars, so those are electromagnetic. No sound reaches us from the Sun. A second useful check is speed: everything electromagnetic moves at 3 x 10^8 m/s in vacuum, while sound in air is about a million times slower.
- X-rays, microwaves and radio waves are all electromagnetic; only their wavelength and frequency differ.
- Electromagnetic waves are transverse and travel through vacuum at about 3 x 10^8 m/s.
- Sound is a mechanical wave and cannot travel through vacuum.
- Sound in a gas or liquid is longitudinal — the particles oscillate along the direction of travel.
- The order of the electromagnetic spectrum by increasing frequency is radio, microwave, infrared, visible, ultraviolet, X-ray, gamma ray.
Three of the four are electromagnetic; sound is mechanical, so option (d) is the odd one out.
- Assuming any wave used in communication must be electromagnetic — SONAR and ultrasound scanners use sound.
- Thinking X-rays are particles rather than short-wavelength electromagnetic radiation.
- Believing sound travels faster in air than in water or steel; it is in fact slowest in gases.
This concept comes up as an odd-one-out item, as an ordering of the electromagnetic spectrum, or as a statement pair on whether a given wave can cross vacuum.
Assertion (A): Radio waves bend in a magnetic field. Reason (R): Radio waves are electromagnetic in nature.
- (a) Both A and R are individually true and R is the correct explanation of A
- (b) Both A and R are individually true but R is not the correct explanation of A
- (c) A is true but R is false
- (d) A is false but R is true
Answer(d) A is false but R is true
Rests on the same classification — radio waves are electromagnetic — while catching the further error that a static magnetic field would bend them.
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 longitudinal-versus-transverse distinction that separates sound from light, applied to mechanical waves inside the Earth.
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
States in one line the reason sound is the odd one out here.
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
Answer(a) 1, 2 and 3
Isolates the single property that divides the families, which is the discriminator this question turns on.
- practice — not a real PYQ
Which one of the following cannot travel through vacuum?
- (a)Ultraviolet rays
- (b)Gamma rays
- (c)Sound waves
- (d)Infrared rays
Answer(c) Sound waves — sound is mechanical and needs a material medium; the other three are electromagnetic.
- practice — not a real PYQ
Arranged in order of increasing frequency, the correct sequence is
- (a)X-rays, ultraviolet, visible light, radio waves
- (b)Radio waves, microwaves, visible light, X-rays
- (c)Microwaves, radio waves, X-rays, visible light
- (d)Visible light, X-rays, microwaves, radio waves
Answer(b) Radio waves, microwaves, visible light, X-rays — frequency rises across the electromagnetic spectrum in that order.