Sporting Optics: Light

First light

WHOEVER BUILT THE first fire was underpaid. Not only did fire produce heat to keep people warm (and alive!), it enabled them to make tools for work and war.

Fire could cook meat and heat water. Fire cleared trees and brush to make room for crops. And fire gave light. Light to make the night friendly and to signal other people at a distance. Light to follow a trail, to define a perimeter, to read, to brighten huts, then villages, then cities. Light to give people more time.

Legend has it that Zeus didn’t trust humans with fire. Prometheus stole some from the mountain home of the gods and delivered it down to earth. We’ve since proven Zeus was right to have reservations. But on balance, fire has been a good thing.

“Bringing the fire” to the Olympic games commemorates the Greek myth and reminds us of the central and elemental role of fire (and its light) in our lives.

Fire is chemical energy released as heat and light. Electrical and atomic action can also cause heat and light. We’ve harnessed both only recently. Oil lamps fueled by animal fat, on the other hand, date back more than 15,000 years. The first recorded lighthouse, the Pharos of Alexandria, was a 260-foot tower completed in 280 B.C. It was wood- fired.

Gas lighting replaced oil in the 19 century. The lamps were brightened by use of a mantle, a chemically treated net placed over the gas flame.

The biggest fire, our strongest source of light, is the sun. An orb of flaming gases, it has burned non stop for as long as anyone can remember and is likely to continue burning even after you finish reading this guide. It is 93 million miles from earth. That’s just far enough to keep the earth from browning like a marshmallow held too close to a campfire. But close enough to keep us from freezing when our planet’s rotation, on its axis and around the sun, puts us on earth’s cold side.

Compared to many bright spots in the heavens, the sun is bumper- to-bumper close to earth. Stars that twinkle on a clear night can be so far off that measurement in miles become ludicrous. Instead, we gauge their distance in light-years. Light travels about 186,000 miles per second, so a light-year — the miles light travels in one year — amounts to quite some distance. That’s 186,000 miles x 60 seconds/minute x 60 minutes/hour x 24 hours/day x 365 days/year. That’s one light-year.

Danish astronomer Ole Roemer was probably the first to calculate the speed of light. He recorded the time it took Jupiter’s moons to circle the planet and noticed that some times the moons were behind or ahead of schedule. Over a year, he figured the lag was about 22 minutes and decided it was because of changes in the distance the light traveled from Jupiter to earth. Using simple math, he figured the speed of light to be 137,000 miles per second — a little low, as it turned out, but an impressive display of reasoning for the day. Roemer was born in 1644. Not until 1849 was a land-based measure taken of light speed. Armand Fizeau’s timer, a clever apparatus, had a toothed wheel that spun rapidly. Fizeau shot a light beam through the wheel at a mirror 5 1/2 miles away and registered the returning beam through another tooth. By knowing how fast the wheel turned, he was able to calculate the speed of the light. Leon Foucault, who worked with Fizeau, got roughly the same reading (185,000 miles per second) with a series of mirrors, one of which was spinning at a fixed rate.

Fig so-1-10

Incidentally, 186,000 is the accepted figure for light in a vacuum, air refractive index of 1. Dense air slows light. So does water, which throttles it to 140,000 miles per second. In optical glass, light travels at about 124,000 miles per second, and in a diamond, 77,000.

Consider that many prominent stars are hundreds of light-years away — that tonight we see light reflected from them before we were born — and you get a healthy dose of perspective. We’re tiny creatures on a speck of dirt so far from the hub of Creation that our ordinary use of words like “great,” “awesome,” “incredible,” “blockbuster,” and “world class” is shamefully self- serving at best.

On the other hand, we can’t be blamed for a limited frame of reference if the perimeters of our pasture extend only as far as we can see. Indeed, for most of us, sight defines the boundaries and qualities of our world. We humans have a poor sense of smell and mediocre hearing. We take the measure of space in units we can visualize (a pace, a football field) or in arbitrary units that can be replicated by anyone who can read a map or odometer. Visual images matter more than auditory messages. Advertisers pay more for a color page in Time magazine than for a minute spot on the radio. We keep putting up billboards, and advertising on ball caps. Given their druthers, most people want an eyeful of entertainment, not an earful.

They’d rather watch television or a video than treat only their ears to a radio program or audio tape. They pay to see a movie, whether or not it is as good as the book whose words must be imagined into pictures.

Fig so-1-11-0 White light includes all the colors; each registers individually when reflected as a certain wavelength.

Fig so-1-11-1 Swarovski’s Swarobright is a superb lens coating that improves images over a wide spectral.

We’re now so accustomed to the electric light that we keep some bulbs burning until we have to change them. Actually, the bulb doesn’t burn. The incandescent light owes its glow to electric current that passes through a filament, heating it. Electric lights dating to the early 1800s were called arc lamps. The current jumped a gap between two carbon rods, producing a very bright light. But the lamps were hard to install and proved a fire hazard. Thomas Edison tried many, many filaments in his search for a low- voltage lamp. When he first demonstrated a useful light in October 1879, he was actually a few months behind British inventor Joseph Swan, whose similar carbon-filament lamp also endured under partial vacuum.

After commercial production of electric lights began in the United States in 1880, hotel visitors had to be reminded by desk clerks that the new bulbs did not require a match for lighting. Modern incandescent lamps have a filament of coiled tungsten in argon and other inert gases. Though only about 8 percent of the electrical energy is converted to light. That light is very close in color to the sun’s, so colors of objects seen under incandescent light are about the same as what you’d see in daylight.

Incandescent lighting actually followed the forerunner of fluorescent lighting. In the mid 1850s, Johann Heinrich Wilhelm Geissler fashioned tubes for low-pressure gases, then passed electricity through the gases, which glowed as a result. Today, fluorescent lamps illuminate workbenches, street corners and farmyards. Unlike the incandescent bulb, these gaslights do not mimic the sun; colors under illumination depend on the gas used. Mercury vapor lamps have no red component, so there’s a blue look to its light. Low-pressure sodium street lights deliver a yellow hue. High-pressure sodium offers greater efficiency and truer colors, though its signature hue is bluish-pink.

Properties of natural light

Light seems to behave as if it were composed of gazillions of invisible particles. Isaac Newton had championed the particle theory, pointing out that “Light is never known to follow crooked Passages nor to bend into the Shadow.” Later studies would show, however, that light does indeed “bend” around corners. Newton also had trouble explaining refraction, the different speeds and angles of light traveling in distinct mediums. Particle acceleration and deceleration were easier to account for than were changes in light path.

Fig so-1-12-0 Shadow, backlighting and silhouettes are functions of light source position and the blockage of light rays.

In 1665, Francesco Grimaldi noticed that light behaved queerly when passed through a slit. It seemed to bend and spread. Grimaldi called this phenomenon diffraction. In 1690, the Dutch physicist Christian Huygens came to a thoughtful conclusion: Light was not a stream of particles, as nearly everyone thought. It traveled so fast, it must be a series of waves. Huygens proposed that in his book, Traite de la Lumiere. The waves, he said, were carried in a weightless, invisible “ether” that permeated space. The light waves could be bro ken into wavelets, which could combine to form a wave front. He surmised, correctly, that a wave theory would account for the refractive properties of light. No mean scholar, Huygens also constructed the first pendulum clock and discovered the rings around the planet Saturn. But it would be many years before wave theory met with public acceptance.

In 1801, English physicist Thomas Young placed a screen with two slits behind a screen with one. The effect was to scatter the light screen behind the two-slit screen showed evidence of wave action. That is, there were convergence bands, where light waves emerged from the two slits in step with each other; and there were interference bands, where light waves were out of step and canceled each other. Young likened his results to the wave action set up by two objects dimpling the surface of water. He postulated that the colors he saw on the rear screen each had specific wave characteristics. Though his conclusions were sound, reinforcing Grimaldi’s work, scientists of the day remained skeptical, convinced that light comprised particles.

Around the turn of the last century, William Crookes came up with a radiometer to measure the pres sure of light on a surface. Light alone did turn the instrument’s finely-balanced vanes. But when the radiometer’s glass bulb was evacuated, the vanes stopped. Since then, both particle and wave theories have been established — and vigorously defended — to explain the behavior of light.

Some of the first recorded studies of light focused on shadow. The great Italian artist and scientist Leonardo da Vinci sketched light rays from two spaced candles casting shadows on both sides of an object between them. That was around the beginning of the 16th century. Roughly a hundred years later, German mathematician Johannes Kepler published Astronomiae pars Optica, a discourse that explained how light traveled in straight lines, cast shadows and bent when moving from one substrate to another. Perhaps best remembered for his discovery of the elliptical paths of our planets, Kepler also figured out why some people cannot see clearly up close, and why others see fuzzy images far away.

Shadow served Christopher Columbus when he landed on Jamaica in 1504. Short of supplies, he was unable to get help from natives — until he remembered that an eclipse of the moon was due. He called the Indians together at the appropriate time and “commanded” the moon to vanish. The awe-struck people gave him what he wanted. Incidentally, an eclipse of the moon occurs when the earth gets between the sun and the moon. An eclipse of the sun follows movement of the moon between earth and sun. If much larger area 01 partial shadow, delivers a partial eclipse.

A small source of light blocked by an object close to an image-producing screen will form a sharp, clearly defined shadow. Move the screen or enlarge the light source, and a fuzzy image results. In the 18th century, a French government minister, Etienne de Silhouette, made “shadow portraits” as cheap substitutes for paintings. “Silhouette” has since come to mean any black shape seen against light.

Light can be reflected, as even cave dwellers must have realized when they looked into still water. In Greek mythology, the youth Narcissus grew too fond of his own reflection in a pool. He tried to touch it, fell in and drowned. In Egypt as early as 1300 B.C., polished bronze disks were used as mirrors — 1,000 years before the Greek mathematician Euclid figured out how light is reflected. About A.D. 1100, the Arab scientist Alhazen came up with a law that quite accurately described how light behaves when it bounces off a reflective surface. Clear glass backed by metal first appeared in Venice around A.D. 1300.

Reflection comprises two light rays: an incoming or incident ray and the outgoing or reflected ray. Looking in a mirror, you know you’re getting a reflected image. But your brain perceives an object on or surfaces yield reflected images that are true to form but reversed right and left (police cruisers, fire trucks and ambulances sometimes have hood lettering reversed so that when seen in a rearview mirror it will be a quick read). Convex surfaces bounce light rays outward, so the virtual image, a miniature, forms behind the mirror. In contrast, concave surfaces form the image in front of the bowl, where reflected rays cross.

Fig so-1-13-0 Early experiment showing light diffraction.

Refraction is the bending of light as it passes from one substance into another. In the second century A.D., Egyptian geographer Ptolemy devised the first law of refraction. It proved unreliable. Arab Alhazen had no better luck. In 1621, Willebrord Snell, a Dutch mathematician, determined that light bends in a precise and predictable manner, and that there was a specific ratio of the “angle of incidence” (light entering) to the “angle of refraction” (light path in the new substrate). He showed that every substance has a characteristic bending power or refractive index. Snell also pioneered the idea of triangulation to measure distance by using angles instead of point locations.

Bow-fishing, or even reeling an underwater plug with a visible leader, you see refracted light. The image of an arrow entering the water breaks at an angle, though the arrow goes straight (that’s why you must aim low; the image of the fish is higher than the fish!). The leader also bends at the water’s surface. Air in different temperature layers con ducts light along different paths, placing a distant image where it is not. A mirage occurs when a layer of warm air next to the ground is trapped by cooler air above. Light bent in an upside-down arc toward the horizontal line of vision eventually forms an upside-down virtual image on the ground. This is the mirage popularized in movies made in the desert. Mirage as a target- shooting or long-distance sighting problem is the flow of heat waves across the warm surface of the earth.

Here, too, lower air is warmer than upper air. You get the same effect if you look across the top of a hot charcoal grill in the back yard, or over a rifle or shotgun barrel after a few shots. Heat waves can make the target move. Refracted light tells you the target is where it isn’t. This mirage can help you determine wind direction and speed, because the heat waves boil vertically in still conditions, and run ahead of a breeze. They flatten out and vanish when the wind becomes strong.

When a layer of cool air is trapped by warm air on top, a far away object can appear to “loom” or hang above its real location. Light passes to you in an arc like a trajectory that bends toward the horizontal line of vision (an inverted version of the light path that delivers a mirage).

Refracted light was put to work by lacemakers in the early century. Water-filled spheres bent the incident light in such a way as to direct it onto a small part of the lace. The concentrated beam, focused on the fine embroidery, became a miniature spotlight.

Lenses use refracted light to correct and magnify images. The word “lens” comes from the Latin name for lentils. A lentil seed is small and round, with convex sides: a bulging disk. The earliest lenses were convex, making light bend inward. The bent rays exiting a convex lens meet at a place behind the lens called the principal focus. The distance from this point to the center of the lens is the focal distance. The shorter the distance, the more powerful the lens. Concave lenses bend light rays away from each other. In either case, your eye traces the light rays back in a straight line, not along the actual path traveled by the ray. Result: A bigger or smaller image is formed beyond or in front of the actual image.

Roger Bacon, an English friar whose work with chemicals gave gunpowder a European home, used a glass lens to magnify printing in a book. German and Italian glassmakers soon found a market for lenses and began grinding them. 1n1629, English King Charles I formed a spectacle-makers’ guild.

Bifocal glasses typically have a convex lens (for distant viewing) atop a concave lens (for reading close up). Aspheric lenses are neither simply convex nor simply con cave; their surfaces may feature more than one radius — for example, a convex periphery surrounding a concave center. Aspheric lenses in riflescopes and rangefinders can improve brightness and sharpness at field edges while making the entire field appear flat.

The effect of lens curvature stops at the surface — a fact used by Georges de Buffon in 1748 to trim the weight of ponderous lighthouse lenses. He cut out the inside glass, leaving the central lens and ribs of reflecting surfaces on the face. Emanating light traveled in a straight line from the light source to the face. Later, Augus tin Fresnel modified these lenses.

Fig so-1-14-0 Refraction occurs when light passes from one medium to another that slows or accelerates its travel.

Fig so-1-14-1 How the human eye sees.

Color

Sunlight comprises many colors, and they don’t all behave the same in glass. The brilliant Isaac Newton used a prism to split white light into a spectrum of colors. A screen showed the spectrum. More importantly, Newton demonstrated that the spectrum was not contributed by the prism. A small slit in the screen caught only one color that passed through to another prism. The beam exiting that prism was just one color. Newton published his findings in his second major scientific book, Opticks, in 1704. Like the French philosopher Rene Descartes before him, Newton correctly concluded that rainbows result from the refraction of light in raindrops. He included an explanation in his book.

The various colors combined in sunlight represent an almost continuous spectrum of wavelengths, from 220 to 2300 nanometers (a nanometer, or nm, is a billionth of a meter). Ultraviolet light is at the low end of the spectrum, from 220 to 400nm. We can’t see it. Infrared light, also invisible, extends from 700 to 2300 nm. Traveling through the atmosphere, “far” infrared waves are absorbed by carbon dioxide, water vapor and ozone. Ozone also filters out the “hard” ultraviolet light. Visible light, violet to red, occurs from 400 to 700 nm on the electromagnetic spectrum. In 1801, Thomas Young postulated that the human eye has color receptors sensitive to specific wavelengths. He was right. Color-specific nerve endings, or cones, put a label on the incident light. Equal proportions of red, green and blue (the “additive primary colors”) come through as white. Violet gives only one cone a signal, and the brain sees violet.

When light passes through a lens, colors separate by refracting at different angles. The result is chromatic aberration, or color fringing that detracts from the clarity of the image you’re viewing through the lens. Chromatic aberration is most noticeable near the edge of the lens. In 1733, English mathematician Chester Moor Hall came up with an achromatic lens comprising two lenses of different types of glass. This “doublet” delivered a color-true image because the rear glass brought together colors that had separated in the front glass.

Color appears as a reflection or as endemic to the light source. An incandescent filament produces its own color; a leaf is green because it absorbs all the component colors of sunlight except green, which it reflects. The red tint on the lens of a red-dot sight reflects red to increase the contrast of the dot against the target. Spectroscopes are now used to determine if colors are pure or formed by “subtraction” — absorption of colors that don’t appear. White cannot be made by subtraction, which is why you can’t add colored inks or paints to get white.

Photographers prefer the light of dawn and dusk to the white light of midday, because a red glow adds warmth and deepens the color of any landscape, and because the presence of shadows at day’s edge adds contrast or “snap” to the photo. At dawn and dusk the sun is just as intense as at midday, but Earth’s rotation has put the sun at an oblique angle to your position, and as the light must pass through more of the atmosphere, it loses more and more of its blue color component. By sunset, a clear sky may be very red.

Secondary colors result when the three primary colors are added in pairs. Red and green make yellow, green and blue make cyan, and red and blue make magenta. On their own, each secondary color is one primary color shy of white light. Overlap two secondary colors, and you get a primary color, because you’ve effectively subtracted two primary colors. Overlap three primary colors and you get black, because you’ve taken out all three primary colors.

Fig so-1-15-0 Color separation in a single convex lens can be corrected by use of a doublet.

Bringing things closer

Magnifying an object can be done with one convex lens, but bringing distant objects closer is a job for multiple lenses in series. Legend has it that Dutch spectacle-maker Hans Lippershay accidentally lined up two lenses on a far-away weathercock in 1608. The iron chicken suddenly looked as big as a cow! But in 1590, Zacharias Johnson claimed to have built a successful compound microscope. Whether it was Hans or Zack or, as some have suggested, Hans’s assistant, the alignment of lenses in tandem added versatility to optics. Now they could be used at a great distance to view things that were hard to reach.

Like stars.

In 1609, the Italian astronomer Galileo Galilei built his first telescope. With this and subsequent instruments magnifying up to 30 times, he discovered four of the moons that orbit Jupiter. He also found that the Milky Way comprised countless stars invisible to the naked eye.

Galileo’s first telescopes had a con vex front (objective) lens and a con cave rear (ocular) lens. Light rays bent by the front lens passed through the rear lens before converging, so the focal point of the front lens lay behind the rear of the telescope. In modern scopes, the focal point lies within the scope, where an upside- down image is formed. The image is righted by an erector lens. Johannes Kepler changed Galileo’s telescope to shift the image inside. He used a con vex ocular lens, but didn’t bother with an erector because distant stars had no top or bottom. Like his colleagues, Kepler had to put up with spherical aberration: a fuzzy image caused by the failure of light rays to meet at a common convergence point on the scope’s axis. The glass refracted each light ray a little differently than the next, and different parts of a lens would direct light to cross the lens axis at various distances behind it. Changing the lens curvature might have helped if the engineers had had our technology. But they didn’t. The solution: long focal length. In some of the first tube less instruments, the lenses were several hundred feet apart! Spherical aberration would later be corrected by gluing together two lenses with different refractive properties to form a single lens. The doublet would also eliminate chromatic aberration.

Fig so-1-16-0 One of many optical test machines used in Bushnell’s laboratory.

Fig so-1-16-1 Bushnell Light Transmission Machine: About 4% of all light passing through an air-to-glass surface is lost, mainly to reflection and refraction. Total loss: 8% per lens or doublet. Lens coatings reduce that loss dramatically.

Most of the first telescopes were refractors, with a big convex front lens that formed an image in front of the little concave rear lens, which bent the light rays again so they became parallel. But plate-size front glass caused unacceptable color dispersion; besides, these lenses were frightfully expensive and difficult to make. In 1668, Isaac Newton came up with a reflecting telescope that used mirrors instead of lenses to magnify. A large concave mirror at the rear of the scope collected light, reflecting it into a small central mirror, which delivered the light and image through a rear (“Cassegrain”) or side-mounted (“Newtonian”) eyepiece. Mirrors proved so successful that they’re still used in observatory telescopes.

As images in telescopes improved, four problems became apparent. Coma is a spherical aberration that looks like a teardrop-shaped blur. Lenses without coma are called aplanatic. Field curvature is another bugaboo, distinguished by a flat image in the middle of the lens, but lines that wander at its edge. You can’t eliminate this, but most good optics now have so little that you have to look for it to see it. Astigmatism, as in the human eye, occurs when lines at various angles can’t be brought into focus together. Distortion is a term that can be used to describe the image skewed by various aberrations. But a singular form of distortion occurs when magnification varies from one part of the field of view to the other. High magnification in the middle of a lens will make a square object seem to bulge; low magnification will cause the sides to suck in. An orthoscopic lens is one that’s been corrected for distortion.

Most of the instruments used to improve our vision in the outdoors are designed to bring things up close, to make them easier to identify and study. The binoculars, riflescopes and spotting scopes available now give us remarkably bright, sharp, flat images. But the light they transmit has the same properties that Newton, Galileo and Kepler worked so hard to fathom. Computer-ground lenses and tolerances controlled to the third decimal place by CNC machines — plus several hundred years of improvements in engineering — have made modern field optics far more effective. For most outdoors enthusiasts, they’re indispensable.

Still, you must have light.

Fig so-1-17-0 Big objective lenses give you a bigger bundle of light, a brighter picture at dawn and dusk.

Fig so-1-18-0 A spotting scope needs a tripod to make its high magnification useful.

Next: What is Good Glass?

Prev: Introduction (part 1, part 2)

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