50 Fun Facts About Light

Sunlight crosses some 150 million kilometers to reach Earth, a trip that takes roughly how long?

About 8 minutes pass between light leaving the Sun and landing on Earth, so the Sun in the sky is always the Sun as it was that long ago.

Albert Einstein won his 1921 Nobel Prize for work on which topic, rather than for relativity?

Photoelectric effect research earned the prize: light striking a metal knocks electrons loose, which Einstein explained in 1905 by treating light as tiny packets of energy.

On a phone or TV screen, which color appears where red and green light overlap at full strength?

Yellow appears because colored lights add together instead of muddying like paint. Screens use the same trick to build every shade from only red, green and blue dots.

Which illusion makes a hot road ahead look wet, as if a pool of water were lying on it?

Mirage comes from a Latin word meaning to look at or wonder at. The shimmering water is really an image of the sky, bent toward the eye by hot air near the ground.

Stars twinkle when viewed from the ground because their light is disturbed on its way through what?

Earth's atmosphere is turbulent, and its shifting pockets of air nudge a star's pinpoint of light around. Planets are near enough not to be such tiny points, so they shine more steadily.

Which invisible light, found in 1800 with a thermometer, carries signals from TV remote controls?

Infrared sits just beyond the red end of the visible spectrum. Astronomer William Herschel detected it through the warmth it gave a thermometer.

Which light, invisible to people, lets honeybees see hidden landing patterns on flower petals?

Ultraviolet markings called nectar guides steer pollinators toward a flower's nectar. A meadow buttercup looks plain to people but shows a darker center in that light.

What is the production of light by living things such as fireflies and deep-sea fish called?

Bioluminescence is so common in the ocean that about three-quarters of the animals counted in video surveys off California could make their own light.

Despite sounding like a measure of time, a light-year is a unit used by astronomers to measure what?

Distance is what it measures: one light-year is how far light travels in a year, about 9.46 trillion kilometers.

Which element was detected in sunlight in 1868, decades before anyone isolated it on Earth?

Helium is named after helios, the Greek word for the Sun. Astronomers spotted its unfamiliar line in sunlight during an eclipse, and it was not isolated on Earth until 1895.

Which English scientist used a prism in the 1660s to show that white light is a blend of colors?

Isaac Newton went a step further, using a lens and a second prism to merge the colors back into white. That proved the prism had not simply stained the light.

What is the bending of light called as it enters water, making a half-submerged pen look broken?

Refraction happens because light slows down in water. Entering at an angle, one side of the beam slows before the other, and the whole beam swings onto a new path.

The Moon makes no light of its own, so moonlight is sunlight that has undergone what?

Reflection off the Moon is surprisingly weak: its surface is about as dark as asphalt, and it outshines everything else at night mainly because it looms so large in the sky.

A black T-shirt warms up faster in sunshine than a white one mainly because of which process?

Absorption turns light into heat, and black dye soaks up nearly every visible wavelength. White cloth bounces most of that energy away again.

Which word describes frosted glass, which lets light through but blurs the view behind it?

Translucent materials pass light but jumble its direction. Frosted glass is ordinary clear glass with one side sandblasted or acid-etched into tiny pits.

A magnifying glass, which can focus sunlight to start a fire, uses a lens of which shape?

Convex lenses bulge outward, so they pull parallel rays together to a single point. That hot spot is why the same lens can enlarge a stamp or set paper smoldering.

Which technology carries internet data between continents along the ocean floor?

Optical fiber is a flexible strand of glass that traps light in its core. The beam keeps bouncing off the inner wall, so it follows the cable even around bends.

Which device, first built in 1960 around a ruby crystal, makes the beam used in barcode scanners?

Laser began as an acronym whose first two words are 'light amplification'. Theodore Maiman built the first one, which shone red.

Which type of sunglasses lens cuts the glare bouncing off water and wet roads?

Polarized lenses grew out of Edwin Land's Polaroid filter, which lets through only light waves vibrating in one direction. His company later became famous for instant cameras.

Which pigment makes leaves look green by soaking up red and blue light far more than green?

Chlorophyll takes its name from Greek words meaning pale green and leaf. The green light it mostly ignores bounces around inside the leaf and back out to the eye.

Which color of visible light has the shortest wavelength, at the far end of the spectrum from red?

Violet is a true spectral color, made by a single wavelength of light. Purple looks similar but is a mixture the eye builds from red and blue.

Which property of a light wave, measured in hertz, counts the crests passing a point each second?

Frequency decides the color the eye sees. Visible light vibrates hundreds of trillions of times each second, with blue faster than red.

Which unit, printed on lamp packaging, measures how much visible light a bulb gives off?

Lumen labels are required by law in many places, because a watt figure only says how much electricity a lamp uses, not how bright it is.

Which part of the eye, the dark spot at its center, widens in dim light and narrows in sunshine?

Pupil size is set by the iris, the ring of muscle around it. Human ones are round, but cats and foxes have vertical slits and goats have horizontal ones.

Sharp shadows and pinhole cameras both depend on the fact that light in clear air travels in what?

Straight lines are why a pinhole camera needs no lens: each ray passes through the hole without swerving, so the picture lands flipped on the back wall.

Light in empty space covers 186,000 miles every second, which is roughly how many kilometers?

300,000 kilometers a second is nature's speed limit: no matter, energy or information can travel through space any faster.

What happens to sunlight, far more strongly for blue than red, that makes the daytime sky blue?

Scattering by air sends blue light bouncing across the whole sky, while red and orange mostly carry on through. That leftover light is why the setting Sun looks red.

Seen from an airplane high above the ground, a rainbow can sometimes appear as which shape?

A full circle is every rainbow's true shape. On the ground the horizon hides the lower half, which is why people usually see only an arc.

What name do physicists give to a single particle of light, a packet of energy that has no mass?

Photon became the standard name after the chemist Gilbert Lewis popularized it in 1926, some twenty years after Einstein argued that light arrives in packets.

Which lamp technology earned a 2014 Nobel Prize after scientists made it shine blue in the 1990s?

Light-emitting diode lamps at first glowed only red, and white lighting and full-color screens had to wait until Shuji Nakamura, Hiroshi Amano and Isamu Akasaki added blue.

Which layer at the back of the eye turns light into nerve signals using its rods and cones?

Retina cells called cones handle color in bright light, while the far more numerous rods work in dim light, which is why colors fade at dusk.

What is the darkest inner part of a shadow called, where the light source is completely blocked?

Umbra is Latin for shadow and the root of the word umbrella. Anyone standing inside the Moon's during an eclipse sees the Sun completely covered.

In which of these clear materials does light travel slowest, at under half its speed in space?

Diamond cuts light to roughly 41 percent of its top speed, compared with about 75 percent in water. That extreme slowing helps give a cut gem its brilliance.

Which of the seven rainbow colors is named after a plant dye once shipped to Europe from Asia?

Indigo comes from the Latin indicum, meaning Indian, after the dye's source. The same dye, now mostly made synthetically, is what turns denim jeans blue.

Light is which kind of wave, vibrating at right angles to its direction of travel, unlike sound?

Transverse waves wiggle sideways as they move forward, like a ripple flicked along a string. Sound is the opposite kind, squeezing and stretching the air along its path.

Which substance was thought to fill all of space and carry light, until an 1887 test found none?

Aether was called luminiferous, meaning light-bearing. Albert Michelson and Edward Morley looked for Earth's motion through it and found nothing, a result that helped clear the way for relativity.

Which effect splits white light into colors in a prism and causes color fringes in cheap lenses?

Dispersion arises because each color travels at a slightly different speed in glass, so each one bends by a different amount.

Which wave effect colors soap bubbles as light bounces off both sides of the thin film?

Interference happens when the two bounced waves overlap: some colors reinforce each other and others cancel, and the mix shifts as the film changes thickness.

Which process lets glow-in-the-dark toys keep shining long after the lamp is switched off?

Phosphorescence is a slow-motion cousin of fluorescence: the material soaks up light but hands it back gradually instead of at once.

Which unit of length has been defined since 1983 by how far light travels in a split second?

Meter length once depended on a metal bar kept as the official prototype. Now it is how far light goes in 1/299,792,458 of a second, which fixes light's speed as an exact number.

Which Briton, who helped decode the Rosetta Stone, showed with two slits that light is a wave?

Thomas Young let light from two narrow openings overlap and saw bright and dark stripes, a pattern only waves can make. He judged it the most important of his many achievements.

Which Scottish physicist showed in 1865 that light is a wave of electric and magnetic fields?

James Clerk Maxwell found that his electric and magnetic waves should move at almost exactly the measured speed of light. He also presented the first durable color photograph, in 1861.

Ole Rømer showed in 1676 that light has a finite speed by timing eclipses of a moon of which planet?

Jupiter's moon Io kept emerging from eclipse later than expected whenever Earth was moving away. Rømer blamed the delay on light needing extra time to catch up.

Which term do astronomers use for the share of sunlight a planet sends back into space?

Albedo comes from the Latin for whiteness. Fresh snow sends back about nine-tenths of the sunlight that hits it, while charcoal returns only a few percent.

What do astronomers call the stretching of light from receding galaxies toward longer wavelengths?

Redshift measurements let Edwin Hubble report in 1929 that the farther off a galaxy lies, the faster it appears to retreat, a cornerstone of the case for an expanding universe.

Which law, named for a Dutch astronomer, predicts how sharply light bends entering a new material?

Snell's law is named for Willebrord Snellius, who derived it in 1621, although the Persian scientist Ibn Sahl had found the same rule in Baghdad in 984.

Which Basra-born scholar argued in his Book of Optics that sight works by light entering the eye?

Ibn al-Haytham, known in Europe as Alhazen, overturned the ancient belief that eyes see by sending out rays of their own. He also insisted that ideas be tested by experiment.

Which blue glow fills nuclear reactor pools when particles outrun light's slowed speed in water?

Cherenkov radiation is light's answer to a sonic boom. Nothing outpaces light in empty space, but water slows it enough for fast electrons to get ahead.

Which Dutch scientist, discoverer of Saturn's moon Titan, described light as a wave in 1690?

Christiaan Huygens also invented the pendulum clock, the most accurate timekeeper for almost 300 years. His wave idea waited more than a century for experiments to back it.

Which part of the electromagnetic spectrum has the longest wavelengths, some wider than Earth?

Radio waves were first generated in a laboratory by Heinrich Hertz in 1887, and for years afterward they were known as Hertzian waves.