Waves Basics — Set 5
Physics · तरंगों की मूल बातें · Questions 41–50 of 60
A minimum distance of about 17 m is often needed for a distinct echo in air because of the 0.1 s condition. This distance corresponds most closely to?
Correct Answer: A. About 17 m one-way to the reflector
About 17 m one-way is correct because sound must travel to the reflector and back in at least 0.1 s. Using v ≈ 340 m/s gives round-trip distance ≈ 34 m and one-way ≈ 17 m. In smaller rooms, echoes merge and sound like reverberation.
Beats are produced when two sound waves of nearly equal frequencies superpose. Beats are best described as?
Correct Answer: A. A periodic variation in loudness
Periodic variation in loudness is correct because the combined amplitude rises and falls. The beat frequency equals the difference between the two frequencies. Beats are used to tune instruments by reducing the beat rate to zero.
Sound intensity level (in decibel) is commonly defined as?
Correct Answer: A. β = 10 log10(I/I0)
β = 10 log10(I/I0) is correct because decibel uses a logarithmic ratio of intensities. I0 is a reference intensity, often taken as 10^−12 W/m² for air. A 10 dB increase corresponds to ten times the intensity.
The speed of sound in air at room temperature is approximately?
Correct Answer: B. 340 m/s
340 m/s is correct as a common approximate value for air at room temperature. Speed of sound depends on temperature and medium properties. In liquids and solids, sound usually travels faster than in air.
Which seismic wave is transverse in nature?
Correct Answer: D. S-wave
S-wave is correct because it involves transverse particle motion. P-waves are longitudinal waves and travel through solids and liquids. S-waves do not travel through liquids effectively.
P-waves in earthquakes are generally?
Correct Answer: B. Longitudinal waves
Longitudinal waves are correct because P-waves compress and expand the medium along travel direction. They usually arrive before S-waves in earthquakes. P-waves can travel through solids and liquids.
In an electromagnetic wave, the electric field (E) and magnetic field (B) are?
Correct Answer: D. Perpendicular to each other and perpendicular to direction of travel
Option D is correct because E and B are mutually perpendicular and also perpendicular to propagation. This makes electromagnetic waves transverse. The direction of propagation is along E × B.
For an electromagnetic wave in vacuum, the relation between field magnitudes is?
Correct Answer: B. E = cB
E = cB is correct for electromagnetic waves in vacuum. Here c is the speed of light in vacuum. This relation connects electric and magnetic field strengths in a traveling wave.
A wave travels 120 m in 0.4 s. What is its speed?
Correct Answer: D. 300 m/s
Option D is correct because speed v = distance/time = 120/0.4 = 300 m/s. Speed is measured in meters per second in SI units. For a given speed, wavelength depends on frequency by v = fλ.
A wave has speed 50 m/s and frequency 25 Hz. What is its wavelength?
Correct Answer: C. 2 m
Option C is correct because λ = v/f = 50/25 = 2 m. Wavelength is the spatial length of one cycle. If frequency increases with the same speed, wavelength decreases.