Why is it Dark at Night?
Total Internal Reflection
The above image is an example of Total Internal Reflection (TIR). Total internal reflection is an optical phenomenon where light traveling from a denser medium to a rarer one is reflected back into the denser medium when it strikes the boundary between those two media. This occurs when the angle of incidence (see diagram below) is greater than what is known as the Critical Angle. The critical angle is the angle of incidence that results in an angle of refraction (see diagram below) of 90° . If the angle of incidence is greater than the critical angle, the angle of refraction must be greater than 90° . This means that the refracted ray is actually reflected when it hits the boundary between the denser and rarer media.
Angle i = angle of incidence. In above diagram (which depicts total internal reflection) the angle i is greater than the critical angle. Angle r = angle of reflection. This angle is equal in value to angle i. The angle of refraction (pointed to in diagram) is greater than 90°
TIR is taken advantage of in Optical Fibres. Light and other electromagnetic waves are transmitted though the denser core of the glass fibre. They are then totally internally reflected when they meet the boundary between the denser core and the rarer glass cladding. This TIR occurs continually though the whole fibre, trapping the light inside. Optical Fibres are very important in telecommunications and have uses in medicine.
Researchers Use Lasers to Control Nematode Brains, Delivering commands and Manipulating Senses
Using genetic tools, researchers engineered worms whose neurons gave off fluorescent light, allowing them to be tracked during experiments.
Researchers also altered genes in the worms that made neurons sensitive to light, meaning they could be activated with pulses of laser light (using optogenetics).
They discovered that controlling the dynamics of activity in just one interneuron pair (AIY) was sufficient to force the animal to locate, turn towards, and track virtual light gradients.
The largest challenges, though, came in developing the hardware necessary to track the worms and target the correct neuron in a fraction of a second.
“The goal is to activate only one neuron,” he explained. “That’s challenging because the animal is moving, and the neurons are densely packed near its head, so the challenge is to acquire an image of the animal, process that image, identify the neuron, track the animal, position your laser and shoot the particularly neuron — and do it all in 20 milliseconds, or about 50 times a second.
…The end result, he said, was a system capable of not only controlling the worms’ behavior, but their senses as well. In one test described in the paper, researchers were able to use the system to trick a worm’s brain into believing food was nearby, causing it to make a beeline toward the imaginary meal.
The experiment entailed balancing the downward gravitational force with the upward drag and electric forces on tiny charged droplets of oil suspended between two metal electrodes. Since the density of the oil was known, the droplets’ masses, and therefore their gravitational and buoyant forces, could be determined from their observed radii. Using a known electric field, Millikan and Fletcher could determine even the charge on oil droplets in mechanical equilibrium. By repeating the experiment for many droplets, they confirmed that the charges were all multiples of some fundamental value, and calculated it to be 1.5924(17)×10−19 C, within 1% of the currently accepted value of 1.602176487(40)×10−19 C. They proposed that this was the charge of a single electron.
(Oil droplets from an atomizer enter the apparatus through a tiny hole. Some droplets acquire a frictional electric charge as they escape from the atomizer. A source of ionizing radiation, such as X rays, also produces ions. The absorption of these ions sporadically changes the electric charges on the droplets. The droplets are observed through a telescope with a measuring scale in the eyepiece. )
James Watt’s work on developing the steam engine lead to the discovery of what are now called Watt’s curves and linkages. The animation above shows how they are constructed from linking a fixed radius to another with a rod. I tweaked the lengths here to make a lovely heart. With different lengths it is possible to make sections of the red curve almost exactly straight. Watt was able to use this to double the power of a beam engine, and nowadays this is used in the suspension systems of some cars. [more] [more2] [code]
This stunning National Geographic photo contest winner shows an F-15 banking at an airshow and a array of great fluid dynamics. A vapor cloud has formed over the wings of the plane due to the acceleration of air over the top of the plane. The acceleration has dropped the local pressure enough that the moisture of the air condenses. Some of this condensation has been caught by the wingtip vortices, highlighting those as well. Finally, the twin exhausts have a wake full of shock diamonds, formed by a series of shock waves and expansion fans that adjust the exhaust’s pressure to match that of the ambient atmosphere. (Photo credit: Darryl Skinner/National Geographic; via In Focus; submitted by jshoer)
So the other day, I was talking to a scientist here at the lab about how electrons move through graphene (a wonder material you should really go read about if you haven’t heard of it). We got onto the topic of how elements bond by sharing electrons and he asked if I could picture the Periodic Table in my head. I didn’t have to imagine it because I pulled out the rolled up Periodic Table from my awesome pen (it actually came in handy!) and he started talking to me about about salt. Sodium (Na) has an extra electron it’s not using and Chlorine (Cl) has space for one, so they click together and become salt! Well, I’m sure they don’t actually “click” but that’s what it sounds like in my head. Chemistry is more fun with sound effects.
Anyway, after this little tangent, we got back to our actual conversation, but I couldn’t stop thinking about salt. I looked it up on Wikipedia, which showed me a cool picture of copper sulfate, a bright blue salt, and then I decided to go looking for pictures of other colored salts, and I stumbled upon the set of photos above.
These are from photographer Fabian Oefner, who put colored salt crystals on a speaker, attached a microphone to a flash trigger, and got these incredibly gorgeous shots as a result. I would love to have these printed on huge canvases to hang in my home.
(PS: When people ask me what I do without a TV, I never know what to say, but THIS IS WHAT I DO. I spend my evenings Googling salt and really absolutely enjoying it.)