The Doppler effect describes the change in frequency or wavelength of a wave in relation to an observer who is moving relative to the wave source. We need to identify the condition that directly causes this observed frequency change.
Correct Option: D) The source or observer is moving
The Doppler effect is fundamentally about the change in observed frequency (or wavelength) due to the relative motion between the source of the waves and the observer. When the source and observer are moving towards each other, the observed frequency increases. When they are moving away from each other, the observed frequency decreases. The formula for the observed frequency \(f'\) when the source and observer are moving is given by:
\[ f' = f \left( \frac{v \pm v_o}{v \mp v_s} \right) \]where \(f\) is the actual frequency of the source, \(v\) is the speed of the wave in the medium, \(v_o\) is the speed of the observer, and \(v_s\) is the speed of the source. The signs depend on the direction of motion. This formula clearly shows that the observed frequency \(f'\) changes if either \(v_o\) or \(v_s\) (or both) are non-zero, i.e., if there is relative motion.
The question asks where sound cannot travel. To answer this, we need to understand the nature of sound waves and their requirement for a medium.
Correct Option: A) Vacuum — Sound is a mechanical wave and requires a medium for its propagation. A vacuum is a space entirely devoid of matter, meaning there are no particles to vibrate and transmit the sound energy. Therefore, sound cannot travel through a vacuum.
The question asks for the definition of a node in a standing wave. We need to recall the characteristics of standing waves, specifically the points where the amplitude is minimal or maximal.
B) A point where the wave has zero amplitude
This correctly defines a node in a standing wave. At these points, the destructive interference between the two waves is complete, leading to no displacement from the equilibrium position.
The question asks for the approximate speed of sound in air at 0 degrees Celsius. This is a standard physical constant that candidates are expected to know or be able to recall within a reasonable approximation.
Correct Option: A) 331 m/s is the widely accepted approximate value for the speed of sound in air at 0 degrees Celsius.
The question asks for the unit used to measure the loudness of sound. We need to identify the correct physical unit from the given options that quantifies the perceived intensity or amplitude of sound.
Correct Option: C) Decibels is the standard unit used to measure the loudness or sound intensity level. It is a logarithmic scale that reflects how the human ear perceives sound.
The question asks for the alternative name for the first harmonic in a resonating tube. This is a fundamental concept in wave physics, specifically related to standing waves in pipes or strings.
A) The fundamental frequency — The first harmonic is indeed synonymous with the fundamental frequency. It represents the lowest possible frequency at which a system can resonate.