The angle of incidence plays a crucial role in ultrasound imaging, particularly in the reflection and refraction of ultrasound waves. When ultrasound waves strike an interface (such as between different tissues) at an angle, they are either reflected or refracted depending on the angle and the acoustic impedance of the tissues. The angle of incidence affects the quality and accuracy of the image. If the angle is too steep, the ultrasound may be entirely reflected, resulting in a loss of signal and poor image quality. Conversely, an optimal angle, typically as close to perpendicular as possible, ensures maximum wave penetration and better image resolution. This principle is vital in medical ultrasound, where accurate imaging is essential for diagnosis. Technicians must be skilled in adjusting the transducer to achieve the optimal angle, ensuring that the waves penetrate effectively and provide a clear, detailed image of the internal structures.

The choice of frequency in ultrasound applications is governed by a trade-off between resolution and penetration depth. In medical ultrasound, high frequencies, typically in the range of 1-15 MHz, are used because they provide greater resolution, which is essential for detailed imaging of small structures within the human body. Higher frequency waves have shorter wavelengths, allowing for finer detail in the images. However, these high-frequency waves are more readily absorbed by the body, limiting their penetration depth.

In contrast, sonar systems, used for underwater navigation and object detection, often operate at lower frequencies, typically ranging from 10 kHz to 1 MHz. Lower frequencies have longer wavelengths, which enables them to travel longer distances underwater and penetrate deeper. They are less absorbed by water and can detect larger structures over greater distances. However, the trade-off is a lower resolution compared to high-frequency medical ultrasound. The choice of frequency in sonar is thus a balance between the need for range and the desired resolution.

Ultrasound waves, especially those used in medical imaging, are generally considered safe and do not cause physical harm to tissues. The primary reason is the low power output of diagnostic ultrasound equipment, which is insufficient to cause significant heating or mechanical damage under normal conditions. The mechanical effects of ultrasound are related to the phenomenon of cavitation, where small bubbles in a liquid can form and collapse rapidly. However, the energy levels used in medical ultrasound are typically too low to induce such effects in tissues.

However, at higher intensities, like those used in therapeutic ultrasound, ultrasound waves can have biological effects. These effects are used beneficially for therapeutic purposes, such as in physiotherapy for tissue healing and pain relief. In such cases, the ultrasound is applied in a controlled manner to avoid any potential harm. It's essential to distinguish between the low-intensity ultrasound used for imaging and the higher intensity used for therapeutic purposes. The safety of ultrasound largely depends on its use within established guidelines and the skill and knowledge of the operator.

Ultrasound has unique applications in cleaning and various industrial processes, primarily due to the phenomenon of cavitation. In ultrasonic cleaning, high-frequency sound waves are passed through a liquid to create rapid, high-energy collisions between the liquid and the object being cleaned. These collisions produce tiny vacuum bubbles that collapse with great force, a process known as cavitation. This intense force dislodges dirt, oil, and other contaminants from surfaces, even in hard-to-reach places like crevices and joints.

In industrial processes, ultrasound is used for a range of applications, including speeding up chemical reactions (sonochemistry), improving the dispersion and mixing of materials, and aiding in the extraction of compounds. Ultrasonic waves can alter physical and chemical properties of materials, making them more reactive or easier to process. For instance, in the food industry, ultrasonication is used to extract compounds from plants efficiently or to homogenise mixtures. The use of ultrasound in these applications is valued for its efficiency, effectiveness, and often reduced environmental impact compared to traditional methods.

Ultrasound, like all sound waves, requires a medium to travel through because it is a mechanical wave, not an electromagnetic wave. Mechanical waves propagate through the vibration of particles in a medium, such as air, water, or solid materials. In a vacuum, where there are no particles to vibrate, there is no medium for the sound waves to travel through, and therefore, they cannot propagate.

This distinction is crucial in understanding why sound cannot travel through space, which is a vacuum, while electromagnetic waves like light can. The requirement for a medium means that ultrasound technology is limited to environments where there is a medium to carry the waves. This fundamental property of sound waves is essential in understanding their applications and limitations. For example, in space exploration, radio waves (a form of electromagnetic radiation) are used for communication instead of sound waves, because radio waves can travel through the vacuum of space.