Q10. What is the minimum distance (in metres) required to hear an echo?
[SSC CGL Tier-I — 16 Aug 2017, Shift 1]
(A) 10
(B) 13
(C) 17
(D) 21
Answer: (C) 17
Explanation: A distinct echo normally requires a delay of at least 0.1 second. With sound speed near 340 m/s, the round-trip path is around 34 metres. The reflecting surface is half that distance away, approximately 17 metres.
Q9. The speed of sound waves is 300 m/s. At a place, one person hears a sound after 5 seconds while another hears the same sound after 6 seconds. What are the maximum and minimum possible distances between them?
[CGPSC Prelims Aptitude Test — 2016]
(A) 1.8 km and 0.15 km
(B) 2.2 km and 0.20 km
(C) 2.8 km and 0.25 km
(D) 3.3 km and 0.3 km
(E) None of these
Answer: (D) 3.3 km and 0.3 km
Explanation: The two listeners are 1,500 m and 1,800 m from the source, respectively. Their maximum separation, when on opposite sides, is 3,300 m. Their minimum separation, when along the same direction, is the difference: 300 m.
Q8. Fathometer is used to measure:
[MPPSC State Service (General Studies) — 2016]
(A) Earthquakes
(B) Rainfall
(C) Ocean depth
(D) Sound intensity
Answer: (C) Ocean depth
Explanation: A fathometer employs echo sounding to determine water depth. It transmits a sound pulse toward the seabed and measures the return time of the echo. Depth equals half the product of this time and sound speed in water.
Q7. Consider these properties of electromagnetic waves:
1. They need no material medium to propagate.
2. Their electric and magnetic fields are parallel to each other.
3. Their energy is shared equally between the electric and magnetic fields.
4. Both fields attain maxima and minima together.
Which are correct?
[UPSC Engineering Services Examination (Electronics) — 2015, Paper I]
(A) 1, 2 and 3 only
(B) 1, 3 and 4 only
(C) 2, 3 and 4 only
(D) 1, 2, 3 and 4
Answer: (B) 1, 3 and 4 only
Explanation: Electromagnetic waves can cross a vacuum, and in a plane wave electric and magnetic fields carry equal average energy. Their variations are in phase. However, the fields are perpendicular, not parallel, so statement 2 is false.
Q6. Which one of the following statements is correct?
[RPSC RAS Prelims — 2013]
(A) Nanoear can detect sound levels as low as −120 dB
(B) Nanoear can detect sound as low as −60 dB
(C) Nanoear consists of a single silica nanoparticle
(D) Nanoear consists of a single silver nanoparticle
Answer: (B) Nanoear can detect sound as low as −60 dB
Explanation: The nanoear is an experimental technique for sensing extremely weak mechanical vibrations, using optically trapped nanoparticles. In the cited examination, its reported detection sensitivity was around −60 dB relative to the stated reference, much weaker than typical human hearing.
Q5. Which one of the following types of waves is used in a night-vision apparatus?
[UPSC Civil Services Prelims — 2009]
(A) Radio waves
(B) Microwaves
(C) Infrared waves
(D) None of the above
Answer: (C) Infrared waves
Explanation: Thermal night-vision devices detect infrared radiation emitted by objects according to their temperature. Electronics convert this invisible electromagnetic radiation into an image. This technique differs from image intensification, which amplifies faint visible or near-infrared light.
Q4. Which one of the following is not an electromagnetic wave?
[WBCS Prelims — 2008]
(A) Radio wave
(B) X-ray
(C) Visible light
(D) Sound wave
Answer: (D) Sound wave
Explanation: Radio waves, X-rays and visible light are all electromagnetic radiation, consisting of oscillating electric and magnetic fields. Sound is a mechanical disturbance of matter, so it requires a material medium. Unlike electromagnetic radiation, it cannot cross a vacuum.
Q3. Consider the following statements:
1. A flute of smaller length produces waves of lower frequency.
2. Sound travels in rocks in the form of longitudinal elastic waves only.
Which statement(s) is/are correct?
[UPSC Civil Services Prelims — 2007]
(A) 1 only
(B) 2 only
(C) Both 1 and 2
(D) Neither 1 nor 2
Answer: (D) Neither 1 nor 2
Explanation: A shorter flute usually produces a higher fundamental frequency, not a lower one, because its vibrating air column is shorter. In rocks, elastic disturbances may propagate as both longitudinal compressional waves and transverse shear waves. Both statements are incorrect.
Q2. Which one of the following statements is NOT correct?
[UPSC Civil Services Prelims — 2003]
(A) The velocity of sound in air increases with increasing temperature
(B) The velocity of sound in air is independent of pressure at constant temperature
(C) The velocity of sound in air decreases as humidity increases
(D) The velocity of sound in air is not affected by changes in amplitude and frequency
Answer: (C) The velocity of sound in air decreases as humidity increases
Explanation: Adding water vapour to air reduces its average molecular mass, so sound generally travels faster, not slower, when humidity rises at the same temperature. Speed also increases with temperature and is nearly independent of frequency in ordinary air.
Q1. Consider these statements:
1. Longer-wavelength light is scattered much more than shorter-wavelength light.
2. Visible light moves through water at 0.95 times its vacuum speed.
3. Rapidly oscillating electric currents produce radio waves.
4. Police speed-detection devices use Doppler shifts of reflected short radio waves.
Which are correct?
[UPSC Civil Services Prelims — 2002]
(A) 1 and 2
(B) 1 and 3
(C) 2 and 4
(D) 3 and 4
Answer: (D) 3 and 4
Explanation: Short wavelengths generally scatter more strongly, so statement 1 is false. Visible light travels through water at roughly 0.75 of its vacuum speed, disproving 2. Oscillating charges generate radio waves, and radar speed guns exploit Doppler shifts.
