Waves – Lesson 1 – The Nature of a Wave
From Physics Classroom
Categories of Waves
Waves come in many shapes and forms. While all waves share some basic characteristic properties and behaviors, some waves can be distinguished from others based on some observable (and some non-observable) characteristics. It is common to categorize waves based on these distinguishing characteristics.
Longitudinal versus Transverse Waves versus Surface Waves
One way to categorize waves is on the basis of the direction of movement of the individual particles of the medium relative to the direction that the waves travel. Categorizing waves on this basis leads to three notable categories: transverse waves, longitudinal waves, and surface waves. Continue reading
Does Military Sonar Kill Marine Wildlife?
From Scientific American, June 10, 2009
Dear EarthTalk: Is it true that military sonar exercises actually kill marine wildlife? — John Slocum, Newport, RI
Unfortunately for many whales, dolphins and other marine life, the use of underwater sonar (short for sound navigation and ranging) can lead to injury and even death. Sonar systems—first developed by the U.S. Navy to detect enemy submarines—generate slow-rolling sound waves topping out at around 235 decibels; the world’s loudest rock bands top out at only 130. These sound waves can travel for hundreds of miles under water, and can retain an intensity of 140 decibels as far as 300 miles from their source.
These rolling walls of noise are no doubt too much for some marine wildlife. While little is known about any direct physiological effects of sonar waves on marine species, evidence shows that whales will swim hundreds of miles, rapidly change their depth (sometime leading to bleeding from the eyes and ears), and even beach themselves to get away from the sounds of sonar. Continue reading
How does sonar work?
From Science Wire
Sonar is simply making use of an echo. When an animal or machine makes a noise, it sends sound waves into the environment around it. Those waves bounce off nearby objects, and some of them reflect back to the object that made the noise. It’s those reflected sound waves that you hear when your voice echoes back to you from a canyon. Whales and specialized machines can use reflected waves to locate distant objects and sense their shape and movement.
The range of low-frequency sonar is remarkable. Dolphins and whales can tell the difference between objects as small as a BB pellet from 50 feet (15 meters) away, and they use sonar much more than sight to find their food, families, and direction. Continue reading
Quantum Dots: Applications for Artificial Atoms Expanding
By Lynn Savage, Feb 2011
Quantum dots, also known as semiconductor artificial atoms, have existed for several years now, and their use keeps on expanding. Primarily used as substitutes for dyes in fluorescence imaging, their propensity for being highly tunable emitters that resist photobleaching represents only a small fraction of their benefits.
Commonly but not always comprising cores of cadmium selenide with a coating of zinc sulfide, quantum dots are perhaps best known for the tunability of their emission wavelength, once excited by an external light source: Change the size of the particle, get a specific wavelength in return. Early research into the properties and potential uses for quantum dots kept them in solutions, and then later brought into proximity with a variety of other particles. This led to their nearly ubiquitous use as donors in Förster’s resonance energy transfer (FRET), a renowned method for near-diffraction-limit imaging of cells and other subjects. Continue reading
Dynamics of Protoplanetary Disks
By Philip J. Armitage, 2011
Protoplanetary disks are the observational manifestation of the initial conditions for planet formation. They can be defined as rotationally supported structures of gas (invariably containing dust) that surround young, normally pre-main-sequence stars. Although most observed disks have inferred masses that are a small fraction of the stellar mass, no meaningful distinction can be drawn between physical processes in protoplanetary disks and those that occur in the earlier, protostellar phase, in which both star and disk are accreting rapidly. Similarly, a common set of processes operate, albeit to varying degrees, in disks around brown dwarfs, Classical T Tauri stars (low-mass pre-main-sequence stars that are actively accreting), and massive stars. A clear demarcation does separate protoplanetary disks from debris disks: dusty gas-poor structures around older stars whose properties reflect the collisional evolution of a population of small bodies (Wyatt, 2008).
Around low-mass stars, protoplanetary disks are persistent; the typical lifetime of ∼106 years (Haisch, Continue reading
“Cartesian theater” is a derisive term coined by scientist Daniel Dennett to refer pointedly to a defining aspect of what he calls Cartesian materialism.
Descartes originally claimed that consciousness requires an immaterial soul, which interacts with the body via the pineal gland of the brain. Dennett says that, when the dualism is removed, what remains of Descartes’ original model amounts to imagining a tiny theater in the brain where a homunculus (small person), now physical, performs the task of observing all the sensory data projected on a screen at a particular instant, making the decisions and sending out commands (cf. the homunculus argument).
The term Cartesian Theater was brought up in the context of the multiple drafts model that Dennett posits in Consciousness Explained (1991):
Cartesian materialism is the view that there is a crucial finish line or boundary somewhere in the brain, marking a place where the order of arrival equals the order of “presentation” in experience because what happens there is what you are conscious of. Continue reading