Tag Archives: light

Night vision

The vision of human beings is well-adapted to daylight; the human eye has evolved to see in the range of wavelengths that are brightest in the spectrum of light that the Sun emits.

The intensity of the light the Sun emits by wavelength, with the visible region highlighted.

But humans don’t see particularly well in the dark. The cones that are responsible for colour vision don’t function well at low light intensities, which is why night vision is almost entirely monochromatic – in the dark humans see in black and white.

When moving from bright light into darkness the first thing that happens to the eye is that the pupil dilates to allow in more light. The iris dilator muscle causes the pupil to increase in diameter by a factor of five (from 2 mm to 10 mm), increasing the amount of light entering the eye by about twenty-five (52) times, but this isn’t enough for true night vision.

The chemical rhodopsin that is present in the rod (brightness-sensing) cells is responsible for night vision. When exposed to light, rhodopsin immediately (within 200 femtoseconds*) splits to form a chemical called photorhodopsin, and then soon afterwards (within a few picoseconds) another chemical called bathorhodopsin.

The splitting of rhodopsin is accompanied by the formation of other chemicals called retinals, and during this splitting process a signal is sent down the optic nerve to the brain, registering the detection of light. (Retinal is created from vitamin A, and so people with a diet lacking in vitamin A frequently suffer from night blindness.)

A molecule of rhodopsin (rainbow-coloured) embedded in a lipid bilayer.
A (black) retinal molecule is bound within the rhodopsin.

Over time, and at a consistent rate, the opsins and retinals recombine to form rhodopsin. If the eye is exposed to bright light all the rhodopsin splits at once (a process called photobleaching). When subsequently exposed to darkness there is therefore no rhodopsin to split and the eye cannot detect light properly. The person in question must wait for the rhodopsin to naturally recombine over time before proper vision can return, a process that takes between ten and thirty minutes to occur. When fully accustomed to the dark, the eye is between ten thousand and a million times more sensitive to light than previously.

The rhodopsin in human eyes is less-sensitive to red light than to other colours and therefore night vision is not particularly effected by red light. This is why red light is used in darkrooms and in aircraft before night-time parachute jumps.

Human eyes, unlike the eyes of many animals, do not have the tapetum lucidum which gives those animals superior night vision. The tapetum lucidum sits behind the retina and acts like a mirror, reflecting back photons of light that were not initially absorbed by the retina, giving the retina a “second chance” to detect the light. This improves their night vision and is what gives rise to the phenomenon of “eyeshine” often seen when taking photographs of animals.

The tapetum lucidum seen in a dissected calf’s eye.

“Eyeshine” is very obvious in this photograph of a raccoon.

* Interestingly, the splitting of rhodopsin into photorhodopsin and retinal seems to be the fastest chemical reaction that has been directly studied.

Triboelectric envelope

The triboelectric effect (the prefix tribo- comes from the Greek τρίβω for “rubbing” or “friction”) results in the creation of a charge difference between two surfaces: one becomes positive and the other negative. The difference in charge is neutralised when a spark jumps between the two surfaces.

Opening “self-stick” envelopes quite often results in a noticeable triboelectric effect, as I discovered when opening this week’s copy of The Economist.

Arbitrage at the speed of light

arb·it·rage n /ˈɑrbɨtrɑːʒ/
the practice of taking advantage of a price difference between two or more markets.

The image most people have of stock markets is of men (and it is always men) in suits using hand signals and shouted verbal commands to buy and sell stocks and shares; this system is called “open outcry” and in reality is used only very rarely.

The vast majority of trading now takes place via computer, and this has altered the way in which markets operate. Not only are traders using computers, but now the traders are computers, operating at very high speeds to execute pre-programmed trading strategies.

As computer hardware and software have improved it is no longer the speed at which computers operate that is most important, but rather the time taken for light to travel down the optical fibre between trading locations. Typical trading latencies are now below five hundred microseconds, enabling traders to make more than two thousand trades per second.*

Because the speed of light has become the limiting factor the physical location of trading offices is becoming more and more important. Well-positioned traders (if you’ll excuse the pun) can take advantage of the difference in price between two markets – buying low in one market and selling high in another – for a profit.

For example: imagine three traders buying and selling aluminium on the London Metals Exchange through the LMEselect electronic trading system. One trader is located in London, one in Dubai (5500km from London) and the other in Singapore (10800km from London). The speed of light in an optical fibre is about 200 million metres per second so any change in price reaches the London trader almost immediately but takes 28 milliseconds to reach Dubai and 54 milliseconds to reach Singapore. The trader in Dubai has an extra 26 milliseconds to act – enough time for more than fifty 500 microsecond trades – before the information reaches Singapore. If the trader in Dubai is trading metals in both London and Singapore then it becomes possible to buy low in London and sell high in Singapore before price information can pass between the two.

In a recent paper†, academics Alexander Wissner-Gross and Cameron Freer show that “there exist optimal locations from which to coordinate the statistical arbitrage of spacelike separated securities” and plot these locations on a map.

The red dots represent exchanges, the blue dots the optimal location of trading nodes between each pair of exchanges.

As the authors point out:

“[W]hile some nodes are in regions with dense fibre-optic networks, many others are in the ocean or other sparsely connected regions, perhaps ultimately motivating the deployment of low-latency trading infrastructure at such remote but well-positioned locations.”

This suggests that the location of exchanges and the speed of light may become the deciding factors as to where trading offices are sited.

* See, for example, Tara Bhupathi. 2010. “Technology’s Latest Market Manipulator? High Frequency Trading: the Strategies, Tools, Risks, and Responses”, North Carolina Journal of Law & Technology 11(2): 377-400.

† Alexander Wissner-Gross and Cameron Freer. 2010. “Relativistic Statistical Arbitrage”. Physical Review E 82(5): 056104-056110. doi:10.1103/PhysRevE.82.056104

Snow-covered UK seen from space

In January I posted an image from NASA’s MODIS satellite showing the UK covered in snow. I’m doing the same for the recent snow; these images were taken today (1st December) between 1110 and 1430.

The problem with this image is that snow and clouds are both white so it’s difficult to tell the difference between the two. Luckily NASA also provides some false colour imagery at long wavelengths (670 nanometres, 876nm and 2155nm) that makes this job easier.

Ice is very absorbent in the 2155nm band (shortwave infrared) that is assigned to the red channel of the image, but reflects in the visible red (670nm) and near infrared (876nm) bands that are assigned to the green and blue channels respectively, causing ice to appear cyan. Vegetation is absorbent in both the near and shortwave infrared which leaves it looking green.

Higher resolution imagery is available from the NASA MODIS Rapid Response System.

Optical phenomena

Environmental Graffiti has a great post about optical (light) phenomena; I’ve picked out a couple of my favourites:

Crepuscular Rays

Crepuscular rays are caused by the scattering of beams of light; beams created by an object such as a tree or a cloud.

Star Trails

Star trails are created when the shutter of a camera is left open for a long time (a matter of hours) and the Earth’s rotation causes the star to move through the sky, leaving a trail behind on the image. The trails are formed in circles around the North Star Polaris, (or the South Star Sigma Octanis in the southern hemisphere), the only star that appears stationary from Earth because it is directly above the axis of rotation.

Circumhorizon Arc

A circumhorizon arc is caused by the refraction of light through ice crystals in cirrus clouds; it’s a bit like a rainbow stuck in the sky.