Waves¶
The science of Mechanical and Electromagnetic waves support a range of Maritime equipment and concepts, and as such a basic understanding is key to working through other concepts.
Waves carry energy from one place to another. Mechanical waves do do so through a medium of air, liquid, or solid. Electromagnetic waves do not require a medium, can travel in vacuum, and in fact are obstructed by air, liquid, and solid matter to varying degrees.
We can describe waves using graphs that depict high points (crests), mid points (resting position), low points (trough), and how the wave alters over time.
The amplitude is the distance from resting position to the crest.
The period is how long it takes for the wave to go through one cycle (crest to crest or trough to trough).
The frequency is the number of times that a period fits in to 1 second. This is measured in Hertz, so 1Hz means one period per second. 1000Hz (or a kilohertz - 1KHz) would mean one thousand periods per second.
Propagation¶
All the above concepts consider time and amplitude only, but there’s one last major consideration, waves travel through space. This is called propagation, and the speed of propagation is the velocity of the wave.
For mechanical waves, this speed is dependant on the medium the wave is travelling through, as they cannot travel without a medium. For electromagnetic waves, they travel at 300,000,000 metres per second in vacuum, and slow down when travelling through airs, liquids, and solids.
The wavelength is the distance between two crests or troughs, travelling at that speed.
If the frequency and velocity is known you can calculate the wave length.
When waves hit objects¶
Waves behave in a variety of ways when they hit some new object, be it the land, a body of water, the atmosphere, another ship. While the specifics of what waves have what kind of prominent behaviours is not the focus in this section, it’s good to know the terminology, as they do have practical consequences for the technology and techniques we mariners use.
Reflection¶
The wave bounces off an object and continues to travel in the same form it was before. Any mirror demonstrates this with light, but it’s also what is happening when an echo occurs, sound is reflected back towards you.
Absorption¶
An object can absorb the energy from the wave. In the case of absorbing light, this means the object will become hotter as a consequence, but it also means that light will no longer reflect. When waves are absorbed, they are often later radiated in a different form. Light waves will radiate as infrared energy when they are released by the land.
Diffraction¶
Diffraction is the bending and spreading of waves around an obstacle. In the case of visible light, the separation of wavelengths through diffraction results in a rainbow.
Scatter¶
Scattering occurs when light bounces of the object in a variety of directions. This effect is why the sky appears blue: short wavelength light (blue and violet) is scattered as it passes through the atmosphere and illuminates the sky with those particular wavelengths, while the rest of the light passes through.
Refraction¶
Refraction is light waves changing direction as they pass from one medium to another, the change in speed means they bend in direction as well. This effect has a range of consequences throughout Maritime subjects, impacting on the accuracy of Celestial Navigation, and the horizon of RADARs and radio communication.
Doppler Effect¶
When the transmitter of a wave is moving, it effectively pushes the wavelengths smaller in the direction of travel, and enlarges the wavelengths away from the direction of travel. Anything that is moving the same speed as the transmitter won’t see a difference, but everything else will see a shift in frequency - higher when the distance is closing, lower when the distance is opening.
This is heard regularly on the road, especially with sirens from emergency vehicles - the sound as they approach is higher than the sound once they’ve passed.