What Is Light
Light and eyes go together. They are in that category of useful mysteries. It is not alone. We use the warmth of the campfire without knowing much about thermodynamics . We drink water without knowing much about hydration. And for many of us, we dress ourselves in clothes without knowing much about style.
We may not understand magnets but we know how to use them
Magnetism has something to do with planet cores, poles and other stuff.
And they can pick up paper clips.
Electricity is what happen when a wall switch is turned or, a toaster needs power or the television is turned on. It has something to do with burning coal, solar panels and rushing water. It can have diesel or alternating current. And it needed to turn on the lights.
Our knowledge of light is practical. It may be a mathematical constant and the fastest thing in the universe. Or it might not be. It is useful for starting the day, taking pictures and finding my socks.
It turns out that magnetism, electricity and light are independent of each other. Think of a giant wall. The height is magnetism..it can vary from short to tall, meaning it can vary in intensity from small to large, but it only goes up the down. The width of our imaginary wall is electricity. It varies in intensity too (narrow to wide).
The two walls are independent of each other. They are orthogonal; at 90-degrees to each other. When that wall of magnetism and electricity comes toward you, that’s light.
Magnetism and electricity are independent but they can be synchronized.
Light is the result of that synchronization.
Light travels. It moves. It is in motion.
Primary Properties
Polarization
Light travels in waves. It is not easily detected but light fluctuates, coming and going so rapidly we can’t see it. It looks continuous to us. But the waves which result from this polarization impact our vision. Waves give us color and brightness.
Frequency
Light waves, like ocean waves, vary in frequency. think of waves as vibrations. When you slowly shake the ear drum, we perceive low tones. When it is rapidly shaken, we hear high tones.
In vision, we are shaking photoreceptors. We interpret these shakings as colors. It all has to do with how frequent the skating is.
Cosmic radiation is almost constant, X-rays are 10,000 slower, and purple is 4000 times slower than X-rays. Red is twice as slow as purple.
We can quantify wave frequency by measuring the length of time between peaks. Think of ocean waves. A calm sea has slow moving waves. One wave reaches shore, and there is along pause before the next one arrives.
The length of time between peaks is the wavelength. Slower waves have longer wavelengths. Faster waves come more often and have shorter wavelengths.
Cosmic radiation has an effective wavelength of zero. It is too fast for us to see. At the other end, infrared, radio and electricity are too slow for us to see.
Amplitude
The height of the wave is interpreted by us as intensity. Small waves are dim. Tall waves are bright. Big waves are perceived as bright.
A slow, large wave is sensed as bright and red. Medium sized waves which are large are interpreted as green, of medium intensity. Small rapid waves (short wavelength) are din and purple.
Both frequency and amplitude result in perceptions. There is nothing in the waves, just variations in size and speed.
Direction
Left on its own, light travels in a straight line. Assuming unchanging optical density, light continues in straight lines from its source. That’s how we get shadows. If light flowed around objects, there would be no shadows. Everything would be evenly lit.
‘Because light traveling straight lines, it has directionality. Light radiates from its source. It is emitted from its source and continues going until it runs into something.
Most natural sources of light radiate light indiscriminately. They radiate in every direction.
Radiance is the amount of energy from a light source.Low radiance means low energy output. It is measured in lumens.
Illumination is the amount of light falling on a surface. The surface facing a light source will have more illumination than the surface in shadow.
Light travels in straight lines, hits objects and creates shadows. When it hits an object, lights bounces off it into our eyes.
The strength of reflection is a function of the object’s color, smoothness and relative position. Whit surfaces bounce well. Darker colors make less successful reflectors. Smoothness is also important. Objects with lots of soft nook and crannies don’t reflect well. For reflections, smooth is better.
But the primary factor to consider is the relative position of light source, object and viewer. A light source far away won’t reflect as much as a closer one. A distant star won’t produce reflections as straws from our local star. Distance matters. Similarly, reflections are easier to perceive when the distance between object and viewer is optimal.
Although relative positions are important, the source can be direct, diffused or reflected. We don’t care if the light comes from the sun or the moon. We don’t care if the sky is cloudy or clear. We don’t can if the light is bounced off the water or sand. All of those are fine with us. As long as there is enough light to see, we’re happy.
What is less clear is that color occurs in the brain.
When light hits an object, the object can absorb some of that radiation. If it absorbs all of it, we see the object as black, because there is nothing passed on to our eyes. If the object absorbs nothing, everything bounces off the object, hits our eyes and we call that white.
Notice that the object has no color; all it has is absorption. As light bounces off (reflects), we see what is left over. A red table absorbs all of the energy from slow and medium wavelength waves. Something green absorbs fast and slow waves but reflects medium wavelengths.
Another feature of light is that it refracts when traveling into a new medium. When the density changes, light changes directions.
Traveling through the air, light flies pretty straight. But when it goes from air to water, it’s angle is changed. You notice this when looking at someone in a swimming pool. Their legs look distorted because light is being tilted by the change of medium. We don’t actually lose our legs when they get wet. The light changes before it hits the person, and changes again when it is reflected back. But the two refractions don’t balance each other out.
Seeing
The selective gathering of light, the physical recording of the pattern of light energy received from the outside world, the transduction of that information into neural signals, and the interpretation of that data.
Light gathering. The simplest organisms don’t do much to select light. They just sit and wait. If light hits them, they move toward it. But most sophisticated creatures are more active in aiming their sensors, and in selecting what to watch.
Physical recording. Light must be physically received. There have to be receptors which respond to light. In complex visual systems, these receptors have proteins which change shape or process when activated by photons. There has to be some kind of reception system.
Transduction. Seeing also includes transforming light patterns into patterns of neural activity. The eye is a transducer. Its whole job is to translate from one medium to another. Colors, shades, movement, and depth must be represented in patterns of neural activity.
Interpretation. Seeing is completed by the brain. The mind actively processes information as it comes in. If we think the data is language, we process it differently than music. Seeing is not a passive process. We are actively engaged in perception.
Eyes
Eyes come in a variety of styles. At its basic level, an eye has a photoreceptor that responds to photons. Upon activation, the receptor transforms light into nerve impulses. Directional sensitivity is also important. If a photoreceptor responds to light from any direction, it cannot determine the direction of light. This is fine for simple creatures but doesn’t give you many response options. You can only determine the overall amount of light.
If you set out to design the best eye, you a number of options.
Eye Cups
One solution’ to the problem of directional sensitivity is to place a receptor inside a depression or cup-like structure. Only light from specific angels is detected. Light from other directions bounces off the cup. Depending on the depth of the cup, these receptors can be very directionally sensitive. Only light coming from a particular direction will be seen.
A nice accessory for your eye cup is to have pigmented cells behind eye cups:, preventing light from bouncing back.
Mermis nigrescens is a parasitic worm which attaches to grasshoppers.It comes in male (~4 cm) and female (20 cm) versions. Basically, it is a worm. Smooth, long, thin and tapered in front. If it detects light, it moves toward it.
Compound Eyes
This was all the rage 300 million years ago. It is still seen in insects, and a few other animals. Although visual acuity is generally bad, it varies greatly. Usually you get very near sighted vision but a wide field of microscopic vision. The larger the array, the better the vision. To approach average human vision; it would require an array 1m in diameter.
Pinhole Eye
You may have built a pinhole camera and marveled at the sharpness of the photos it can take. You take a cardboard box, soda can, or anything you have lying around. Make it light tight except for one small pin-sized whole. Loading photosensitive paper in it (when in the dark), and seal it up.
Cover the pin hole with your finger, and take your camera out into bright light. Place it on something steady, aim it at something that doesn’t move. Take your finger off the hole, guess at how long to leave it uncovered, cover it when you think it has been long enough, and take it back into the dark. Remove the film and process.
This is how they did it in those pictures of the Old West where the people looked frozen. Not a friendly process but the pictures were externally clear.
But pin holes typically don’t make good eyes. One pin hole is very li,I ting. You can improve your design my making an arrays of pinholes, each with a receptor. An image can then be represented as a series of inputs from these receptors.
The Nautilus, sometimes called a squid twitch in a shell, uses pinhole technology. The advantage of a pinhole eye is that it is possible to get sharp images. The disadvantage is it requires a lot of light. Either you need high intensity of light or long exposures. Also, you can’t change focus.
A better design choice is to fill the pinhole with a lens. Without a lens, the hold in the head approach is limited to a single ray of light. A lens can give tou a broader field of view, and, perhaps, the ability to change focus.
Lens Eyes
We have worked our way up to vertebrates. And a few invertebrates, like the octopus .We are no longer limited to a single ray. The lens both selects and focuses light rays. The lens takes a beam of light and aims it at the photoreceptors.
The human eye is a good example of a lens eye. It produces good quality pictures. It doesn’t have the best lens, the best focus, or built in binocular vision. It is not the best visual system but it fits out needs well.
Chameleons have eyes which work independently, swivel essily, and provide 360-degree coverage. Falcons have front-facing eyes for improve binocular vision and fantastic distance vision. Owls have very large eyes, compared to their body size, giving great night vision. Horses have binocular vision, which is why they lift their head to see you better.
Out good-enough system has a cornea which provides ⅔ of our focus, and a flexible lens switch fine tunes the aim of light rays to hit the fovea.
The cornea is curved, and has air on the frontside and water behind it. When a beam of light hits this combination, the rays are refracted toward the middle of the eye.
The more focused beam passes through the iris. Everything that doesn’t pass through is blocked by the colored part of the iris (unless you’re an albino). Now that the beam has been refracted and filtered, it hits the lens. The lens can bend a bit, adjusting its flatness to improve focus. The focus is intended to insure that the light beam hits a tiny 1/16 inch wide spot called the fovea.
The fovea is a small spot in the macula (depression) on the retina. The retina is a net of neural cells sandwiched between two blood supplies. The net is composed of photoreceptors and support cells. The blood supply in front feeds the support cells. The blood supply in the back feeds the photoreceptors.
Once light has been transducer by the photoreceptors, it travels out of the eye and heads toward the back of the brain, making two stops. The first stop is the optic chiasm transfer station. The second stop is the LGN preprocessing center.
Optic Chiasm
Each eye has two neural streams One stream carries data from the left region of the eye. The other stream carries data from the right side of eye. Since the eyes are curved, the left side of the eye is aimed to the right; the right field of vision. The right side of the eye looks to the left; left field of vision.
At the optic chiasm, visual information is redirected.The two right field of view streams (left side of each each) are combined. The two streams from the right sides of the eyes (left field of vision) are also combined.
The outside streams go to their respective side. The left side of the left eye goes to the left hemisphere. The right side of the right eye goes to the right hemisphere.
The streams of data cross over to the opposite sides.
The result is the two data stream that look toward the right side of the body are both in the left hemisphere. And the two left field of view streams are directed to the right hemisphere. In consequence, each hemisphere has a copy of the data looking toward the other side of the body. The left hemisphere has the right looking data. The right hemisphere has the left looking data.
LGN
The second stop on our journey to the back of the head is at the lateral geniculate nucleus LGN). This six layered structure is part of the thalamus constellation. Pretty much every goes through some portion or another of this cluster.
The LGN swaps some information with the other thalamic structures, does some light pre-processing, and send the signals on. We then finally reach our goal: the striate cortex of the occipital lobe.
Occipital Lobe
Two lobes, one in each hemisphere, interconnected by fast neural connections. This is the primary projection site for vision. It is striped, hence the description of it as a striated cortex.
It processes the incoming visual images and then pssses on the results two the other lobes of the brain. One stream of information goes to the parietal lobe to identify where an object is.
The other main data stream goes to the temporal lobe to identify what the object is.
The frontal lobes takes all of this analyzed data from the other lobes and makes judgments about what to do.
Vision starts with light and eb=nds with thinking. The light is transducer in patterns of neural activity, pushed, pulled and synthesized. It is a highly effective system. Very useful.
Not the best in every category but a great stems for a highly mobile entity that has to solve problems in a wide range of settings.
Humans aren’t uniquely good at very many things. Aside from thinking and language, our only real skills are long distance running (other animals run faster, we go farther) and rock throwing. We can quite easily hit an object with a rock, up to 20 feet away. A large portion of the credit for that feat is our complex, binocular, integrated visual system
Our visual system has one great advantage. The optics may not be the best but we have attached our camera to a supercomputer.
Take an okay lens camera and attach it to a group of supercomputers. We take the raw data and use it to fill in the gaps, recall previous experiences, evaluate current conditions, predict future states and evaluate risk.
The light we see is part of a larger context. We can’t see (detect) much of the spectrum. It is a lot bigger than our small slice. We see green really well, red pretty well and purple okay.
We can’t see all of the electromagnetic spectrum a butterfly sees but we can use what we have to catch a butterfly.
Human Eyes
Want to jump ahead?
- What Is Perception?
- Perceptual Efficiency
- Vision
- Principles
- Depth
- Light & Eyes
- Eye
- Retina
- Color Vision
- LGN
- Occipital Lobe
- Pathways
- Taste
- Simple
- Tongue
- Throat
- Smell
- Basic
- Nose
- Olfactory bulb
- Flavor
- Touch
- Receptor
- Pressure
- Haptic Perception
- Temperature
- Pain
- Itch
- Hearing
- Ear
- Cochlea
- Pathway
- Temporal Lobe
- Vestibular
- Visceral
- Proprioception
- Time
