Correct Option
The correct option is (a).
Explanation
Ultrasound refers to longitudinal mechanical waves with frequencies exceeding the upper limit of human hearing. Due to their high frequency and short wavelength, these waves possess high energy and high directionality, allowing them to travel in straight lines without significant diffraction.
Statement-wise Analysis
- Statement 1 is Correct: The audible frequency range for humans is approximately 20 Hz to 20,000 Hz (20 kHz). Sound waves with frequencies higher than 20 kHz are technically defined as ultrasound.
- Statement 2 is Incorrect: Unlike audible sound, ultrasound waves have very short wavelengths. This allows them to travel along well-defined paths even in the presence of obstacles. They do not scatter immediately; instead, they tend to reflect off surfaces or travel through media with minimal bending (diffraction), making them ideal for scanning and ranging.
- Statement 3 is Incorrect: Ultrasound is widely used in non-destructive testing (NDT) to detect internal cracks and flaws in metal blocks. The waves penetrate the metal and reflect back from any internal discontinuity (flaw). Detectors analyze these reflections to identify defects without damaging the material.
Key Takeaway: Ultrasound (>20 kHz) travels in straight paths, reflects from boundaries of different densities, and is extensively used in medical imaging (sonography) and industrial flaw detection.