Thursday, July 30, 2015

What are the limits of human vision?

From observations of distant galaxies light years from us to the perception of invisible colors, Adam Hedheyzi on BBC explains why your eyes can do incredible things. Take a look around. What do you see? All these colors, the walls, the windows, everything seems obvious, as it should be here. The idea that we all see it because the particles of light - photons - that bounce off of objects and fall into human vision, it seems incredible.

This bombardment of photon absorbed, about 126 million light-sensitive cells. Different direction and energy of the photons are transmitted to our brains in different shapes, colors, and brightness, filled with images of our multi-colored world.

Human vision is remarkable, obviously, have a number of limitations. We cannot see radio waves emanating from our electronic devices, cannot see the bacteria under your nose. But with advances in physics and biology, we can determine the fundamental limitations of the natural view. "Everything that you can distinguish between thresholds, the lowest level, above and below which you cannot see," - says Michael Landy, a professor of neurology at New York University.


We will begin to consider these visual thresholds in the light of - pardon the pun - that many associate with the vision first: color.

Why do we see purple, brown and not dependent on the energy or wavelengths of the photons incident on the retina at the rear of our eyeballs. There are two types of photoreceptors, rods and cones. Cones are responsible for color and sticks allow us to see the shades of gray in low light conditions, such as at night. Opsin, or pigment molecules in the cells of the retina absorb electromagnetic energy of incident photons, generating an electrical impulse. This signal goes through the optic nerve to the brain, where it is born conscious perception of colors and images.

We have three types of cones and related opsins, each of which is sensitive to photons of a certain wavelength. These cones are denoted by the letters S, M and L (short, medium and long wave respectively). Short wave we perceive blue, long - red. Wavelengths between them, and combinations thereof are converted into full rainbow. "All the light that we see, in addition to artificially created by a prism or smart devices like lasers, is a mixture of different wavelengths, - says Landy."

Of all the possible photon wavelength cones show our small band of 380 to 720 nanometers - what we call the visible spectrum. Beyond our range of perception is the infrared and radio spectrum, the latter wavelength range is from a millimeter to a kilometer long.

Over our visible spectrum to higher energies and shorter wavelengths, we find the ultraviolet spectrum, x-rays, and then on top - a gamma-ray spectrum, the wavelengths of which reach one trillion meter.

While most of us are limited to the visible spectrum, people with aphakia (absence of the lens) can see in the ultraviolet spectrum. Afak is usually created as a result of surgical removal of cataracts or congenital defects. Typically, the lens blocks UV light, so without it people can see beyond the visible spectrum, and to perceive wavelengths up to 300 nm in a bluish tinge.

A study in 2014 showed that, relatively speaking, we all can see infrared photons. If two infrared photon accidentally fall into the retinal cells simultaneously, almost their combined energy, converting them from an invisible wavelength (eg 1000 nm) in the visible 500-nanometer (cold green for most of the eye).

How many colors can we see?
A healthy human eye has three types of cones, each of which can distinguish about 100 different colors, so most researchers agree that our eyes can generally distinguish about a million shades. However, color perception - a rather subjective capacity that varies from person to person, so to determine the exact figures is difficult.

"It is rather difficult to pass it on figures - says Kimberly Jamieson, Researcher, University of California at Irvine. - What one person sees, it can only be part of the colors seen by the other person. "

Jamison knows what he's talking, because it works with "tetrahromatami" - people who have "superhuman" vision. These rare individuals, mostly women, have a genetic mutation that gave them additional fourth cones. Roughly speaking, thanks to a fourth set of cones, tetrahromaty can see 100 million colors. (People with color blindness, dichromates are only two kinds of cones and can see about 10 000 colors).

How many photons at least we need to see?
In order to color vision worked, cones, usually need a lot more light than their counterparts’ sticks. Therefore, in low light color "goes out", because the fore monochromatic sticks.
In ideal laboratory conditions and locations of the retina, where sticks are largely absent, the cones can be activated by only a handful of photons. Yet stick better job in terms of the scattered lights. Experiments have shown 40s, one quantum of light is enough to get our attention. "People can respond to a single photon, - said Brian Wandell, a professor of psychology and electrical engineering at Stanford. - There's no point in even greater sensitivity. "

In 1941, researchers at Columbia University have set people in a dark room and gave their eyes adjust. Sticks took several minutes to reach full sensitivity - that's why we have problems with vision, when suddenly the lights go out.

The researchers then lit the blue-green light in front of the test persons. At a level exceeding the statistical coincidence, the participants were able to capture the light, when the first 54 photons reach their eyes.

After compensation for loss of photon absorption by other components of the eye, the researchers found that for five of the photons activate five individual sticks that give participants a sense of light.

What is the limit of small and distant that we can see?
This fact may surprise you: there are no internal restrictions smallest or the most distant thing we can see. As long as the objects of any size, at any distance transmit photons cells of the retina, we can see them.

"Anything that excites the eye, is the amount of light that reaches the eyes - says Landy. - The total number of photons. You can make the light source is ridiculously small and remote, but if it emits photons powerful, you see him. "

For example, conventional wisdom has it that a dark clear night we can see the candle flame from a distance of 48 kilometers. In practice, of course, our eyes would just swim in photons, so the stray light rays from a distance just lost in this mess. "When you increase the intensity of the background, the amount of light that you need to see something, increased", - said Landy.

Night sky with a dark background dotted with stars is a striking example of the range of our vision. Stars are huge; many of those that we see in the night sky make millions of kilometers in diameter. But even the nearest stars are at least 24 trillion kilometers away from us, but because so small for our eyes, that they cannot tell. And yet we see them as a strong point of emitting light as photons cross the cosmic distances and fall into our eyes.

All the individual stars that we see in the night sky, are in our galaxy - the Milky Way. The most distant object we can see with the naked eye, is beyond our galaxy: a galaxy of Andromeda, located 2.5 million light-years from us. (Although this is controversial, some individuals claim that they can see the galaxy of the Triangle in an extremely dark night sky, and it is three million light years from us, just have to take their word for it).

Trillion stars in the Andromeda Galaxy, given its distance, in a vague blur glowing piece of sky. Yet its dimensions are enormous. From the point of view of the apparent size of even being quintillion kilometers away, the galaxy is six times wider than the full Moon. However, our eyes up so few photons that this celestial monster is almost imperceptible.

How serious can be the vision?
Why cannot distinguish individual stars in the Andromeda Galaxy? The limits of our visual resolution, or visual acuity, impose limitations. Visual acuity - is the ability to distinguish details such as points or lines, separated from each other so that they are not merged. Thus, we can assume the limits of the number of "points", which we can discern.

Boundaries acuity establishes several factors such as the distance between the rods and cones, retinal packed. Also important is the optics of the eyeball, which, as we have said, it prevents all possible photons in the light-sensitive cells.

In theory, studies have shown that the best that we can see, it's about 120 pixels per degree arc, unit of angular measurement. You can imagine it as a black and white chess board 60 by 60 cells, which fit on the nail arm. "This is a clear pattern that you can see," - said Landy.

Visual inspection, such as tables with small letters that guided by the same principle. These limits sharpness explain why we cannot discern and focus on a single dull biological cell width of a few micrometers.

But do not write off the accounts themselves. Million colors, single photons, galactic worlds beyond kvantilliony kilometers away - not bad for a bubble of jelly in our sockets connected to the 1.4-pound sponge in our skulls.

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