50 Fun Facts About Optics

Which vehicle's name is often painted in reverse so that drivers ahead can read it in a mirror?

Ambulance is spelled out in mirror writing so the word reads the right way round in a rear-view mirror. Leonardo da Vinci wrote most of his personal notes in the same reversed script.

Which legume gave the lens its Latin name, because a lens bulging on both sides has the same shape?

Lentil in Latin is simply lens. English borrowed the word in the 1690s for glass that regulates light rays, by analogy with the shape of the seed.

How long does sunlight take to reach Earth, when light from the next nearest star takes over four years?

8 minutes is the trip through space, so we see the Sun as it was a little while ago. The energy first spends 10,000 to 170,000 years working its way out from the Sun's centre.

Which colour appears on a screen when red and green light are combined at equal intensity?

Yellow is a secondary colour of light, made on screens whose pixels glow only red, green and blue. Cyan and magenta are the other two secondaries.

Which Founding Father is generally credited with inventing bifocal glasses for near and far vision?

Benjamin Franklin was one of the first people known to wear split lenses. One account says he cut his lenses in half so he could lip-read French speakers at court.

Which space telescope launched in 1990 with a misshapen main mirror that astronauts fixed in 1993?

Hubble's main mirror was smooth to about 10 billionths of a metre, yet its outer edge was too flat by roughly two thousandths of a millimetre, enough to blur its view.

In a doubtful legend, which Greek inventor burned Roman ships at Syracuse using mirrors and sunlight?

Archimedes gets the credit only in later tellings: none of the three earliest accounts of the siege, by Polybius, Livy and Plutarch, mentions mirrors.

Which colour, named after a plant dye, sits between blue and violet in the seven-colour spectrum?

Indigo joined the list in the 1600s, when the spectrum's colours were matched to the seven notes of a musical scale. Some scientists think that indigo was simply what we now call blue.

Which effect, caused by light bending through hot air, makes a distant road look covered in water?

Mirage images are real enough to photograph: the water is light from the sky, bent by the hot air just above the road. Only the mind's reading of it as a puddle is wrong.

Which toy, invented by David Brewster, uses tilted mirrors to turn beads into symmetrical patterns?

Kaleidoscope was Brewster's own coinage, from Greek words meaning the observation of beautiful forms. He hit on the idea in 1814 while experimenting with light bouncing between glass plates.

Which sky sight always appears opposite the Sun and looks like a full circle from an aircraft?

Rainbow light is bent on entering a water droplet, bounced off the back of it and bent again on the way out. From the ground only an arc of the circle is usually visible.

Which word describes the bending of light that makes a pencil in water look broken at the surface?

Refraction happens because light changes speed as it passes from one material into another. The same bending makes a pool look shallower than it really is.

Which English scientist split white light with one prism and recombined the colours with another?

Isaac Newton gave the band of colours its name, spectrum. In 1668 he also completed the earliest known working reflecting telescope, which gathers light with a mirror instead of a lens.

Which astronomer made his own telescope in 1609 and soon discovered Jupiter's four largest moons?

Galileo Galilei at first described the moons as three fixed stars lying in a line through Jupiter. Their shifting positions over the next nights showed him they were orbiting the planet.

Which light-sensitive layer at the back of the eye plays the role of film or a sensor in a camera?

Retina tissue grows out of the developing brain in the embryo, so it counts as part of the central nervous system. It is, in effect, brain tissue at the back of the eye.

Which French mathematician proposed in 1662 that light follows the path that takes the least time?

Pierre de Fermat proposed it to explain how light bends between two materials. The idea was controversial at first because it seemed to credit nature with knowledge and intent.

Which instrument uses two parallel mirrors set at 45 degrees to let a hidden viewer see over walls?

Periscope tubes with two added lenses let soldiers keep watch from the trenches of World War I. Submarine versions swap the mirrors for prisms and add magnification.

Which gemstone's high dispersion splits white light into flashes of colour that jewellers call fire?

Diamond shows little of this as a rough stone, so it has to be cut and polished. In 1919 Marcel Tolkowsky calculated a cut that balances white brilliance against coloured fire.

Which physicist won a Nobel Prize for explaining the photoelectric effect rather than for relativity?

Albert Einstein proposed in 1905 that light itself consists of localized particles. Nearly all physicists rejected the idea at first, including Max Planck and Niels Bohr.

Which word describes frosted glass, which lets light through but scatters it so that images blur?

Translucent comes from Latin words meaning to shine through. Glass is frosted by sandblasting or acid etching, which pits one side so that it scatters light instead of passing a clear image.

Which mirror shape is used for shop security mirrors because it shows a wider view than a flat one?

Convex mirrors make everything look smaller, which fits more of the scene in. That is why some car wing mirrors carry a warning that objects are closer than they appear.

Which eye condition blurs distant objects but not near ones, and is corrected with concave lenses?

Myopia was named by the physician Galen, from Greek words meaning to close the eye. Johannes Kepler later showed that lenses curving inward could correct it.

Which name, popularised by chemist Gilbert Lewis in 1926, is given to a single particle of light?

Photon particles are massless, so they can only move at one speed, the speed of light in a vacuum. The idea of light as units of energy grew out of work on the photoelectric effect.

Which precious metal coats the glass in the mirror design credited to Justus von Liebig in 1835?

Silver had long been polished into mirrors on its own, and such mirrors were in wide use in the Roman Empire. Many modern mirrors use a thin coating of aluminium instead.

Which handheld instrument has prisms inside each barrel to turn the image the right way up?

Binoculars built with prisms also became shorter, since the light path is folded inside. Until the 1890s the same job was done by long twin telescopes with extra lenses.

In which country were the first eyeglasses made around 1290, according to a friar's sermon of 1306?

Italy has the evidence in that sermon, in which Giordano da Pisa said the art of making glasses was not yet twenty years old. Venice had rules on selling them by 1301.

Which type of sunglasses lens, first sold in 1936 with Edwin Land's filter, cuts glare off water?

Polarized lenses block glare reflected at some angles off shiny non-metallic surfaces. Edwin Land gave his patented filter a brand name that became famous: Polaroid.

Which type of three-dimensional image, recorded with laser light, won Dennis Gabor a Nobel Prize?

Hologram research began as a side effect in 1948, when Gabor was looking for a way to improve image resolution in electron microscopes. The field took off only after the laser arrived in 1960.

Which structures were the stepped lenses of physicist Augustin-Jean Fresnel designed to be used in?

Lighthouses gave the design its nickname, the invention that saved a million ships. The first one was lit in July 1823, and single-piece versions later went into car headlamps.

Which law of light bending bears a Dutch astronomer's name, though Ibn Sahl found it in Baghdad in 984?

Snell's law went unpublished in the lifetime of Willebrord Snellius, who derived it in 1621. René Descartes reached it independently and printed it in 1637.

Which British physicist gives his name to the scattering of sunlight that makes the daytime sky blue?

Lord Rayleigh's effect is much stronger for blue light than for red as sunlight crosses the air. At twilight the blue has been scattered out of the low Sun's light, leaving it reddish.

Which wave effect, named by Francesco Grimaldi, makes the fine tracks of a CD shimmer with colour?

Diffraction comes from a Latin verb meaning to break into pieces, because Grimaldi saw light breaking up into different directions. His observations were published only after his death.

Which wave effect in thin films produces the swirling colours seen on soap bubbles and oil on water?

Interference occurs between light reflected from the top and the bottom of the film. As the film's thickness varies from place to place, different colours are strengthened or cancelled.

Which light detectors in the human eye typically come in three types and make colour vision possible?

Cone cells are active in daylight, while rod cells take over in dim light. Comparing the signals from the three types is what lets the brain tell colours apart.

Which invisible radiation did William Herschel discover in 1800 through its effect on a thermometer?

Infrared turned out to carry slightly more than half of the energy that reaches Earth from the Sun. Herschel presented his results to the Royal Society of London the same year.

Roughly how far does light travel in one second, enough to lap Earth's equator more than seven times?

300,000 km is the rounded figure; the accepted value is 299,792 km per second. It is exact by agreement, because the metre is now defined by how far light travels in a set time.

Which organisms did Antonie van Leeuwenhoek first document, using single-lens microscopes he made?

Bacteria were among the moving specks he decided were little animals. A draper by trade and largely self-taught, he is now known as the father of microbiology.

Which gemstone did Theodore Maiman use to build the first working laser in 1960?

Ruby in that first laser was a synthetic rod, and such rods are usually pumped by a flashtube to give pulses of deep red light. Maiman fired his for the first time on May 16.

Which device projects an upside-down image of the scene outside through one small hole?

Camera obscura boxes fitted with a lens became popular aids for drawing and painting, since an artist could trace the projected image. They later evolved into the photographic camera.

Which kind of image, like the one behind a flat bathroom mirror, cannot be projected onto a screen?

Virtual images are only an appearance, because the light rays never actually meet at the spot behind the glass. A real image, formed where rays do converge, can be caught on a screen.

From which imaginary line are a light ray's angles of incidence and reflection at a mirror measured?

Normal lines stand perpendicular to the mirror at the point where the ray lands. The name traces back to the Latin norma, a carpenter's square.

Which part of a shadow is the darkest, innermost region, where the light source is completely blocked?

Umbra is Latin for shade or shadow, and the same root gave English the word umbrella. An observer standing inside it sees the light source totally hidden.

Which camera setting, given as an f-number, also controls how much of a photo appears in sharp focus?

Aperture is the opening that limits how much light passes through the lens. A smaller opening means a larger f-number, less light and more of the scene in focus.

Which British polymath argued that light is a wave and also helped decipher the Rosetta Stone?

Thomas Young first demonstrated the idea with water waves in a ripple tank. He judged establishing the wave theory of light to be the most important of his many achievements.

Which scholar from Iraq, called the father of modern optics, wrote the Book of Optics between 1011 and 1021?

Ibn al-Haytham was the first to correctly show that we see because light enters the eye, not because the eye sends rays out. He also argued that vision takes place in the brain.

Which Dutch scientist set out a wave theory of light in 1690, long after finding Saturn's moon Titan?

Christiaan Huygens also invented the pendulum clock, the most accurate timekeeper for almost 300 years. He was the first to explain Saturn's odd appearance as a thin, flat ring.

Which Danish astronomer first showed light has a finite speed, by timing Jupiter's moon Io in 1676?

Ole Rømer made the discovery while working at the Royal Observatory in Paris. The speed his timings implied was well below the modern value, but it showed that light is not instant.

Which lens flaw, reduced by pairing crown and flint glass, arises because glass bends each wavelength differently?

Chromatic aberration shows up as fringes of colour along the boundaries between dark and bright parts of an image. A two-glass lens built to cancel most of it is called an achromat.

Which unit do eye care professionals use to state the optical power of a corrective lens?

Dioptre values are one divided by the distance, in metres, at which a lens brings light to a focus. Albrecht Nagel first suggested numbering lenses this way in 1866.

Which colour does an object appear when its surface absorbs all the wavelengths of visible light?

Black is taken to an extreme by Vantablack, a coating of carbon nanotubes claimed to absorb up to 99.965% of the visible light that strikes it.