There's More to Light Than Meets the Eye
Light is a form of energy. We see it in many different forms all around us. It’s the sunlight that pours through your bedroom window and wakes you up. It’s the glow of your computer screen and the flash of lightning during a thunderstorm. It’s the crackling flames in a fireplace. All are forms of light.
Fire is a source of both light and heat. Fire gives off different amounts of light and appears to us as different colors. Take a closer look at a candle flame, camp fire, or fire in a fireplace and see what colors you notice.©Peter Haworth
These are all forms of visible light – the light we can see. But visible light is only a narrow sliver of the universe’s light energy. Most light energy is invisible. Scientists refer to all forms of light energy — the visible and the invisible — as electromagnetic radiation.
Incandescent Light Is Hot Stuff
The Sun is a giant ball of burning gas that gives off a lot of light. This light is what makes life on Earth possible. Plants need the Sun’s light to grow and make oxygen. Without the Sun, Earth would not be a very green planet.©SOHO (ESA & NASA)
Visible light can be produced in many ways. Most commonly light is produced when something becomes very hot, or incandescent. The Sunis our biggest and most important source of incandescent light. With a surface temperature of about 5500° C, the Sun produces vast amounts of visible and invisible light.
An electric light bulb gives off light when electricity heats up a filamentto about 2200° C, causing it to glow. If you’ve accidentally touched a glowing light bulb, then you know it’s a source of hot, incandescent light. Ouch!
A light bulb is a source of incandescent light. This kind of light is produced with heat which is why most light bulbs have protective covers or shades around them. This is also why we have to be careful when decorating with strands of light bulbs.©Richárd Dancsi
Luminescent Light: It Glows
A firefly might look like it is carrying a tiny light bulb but it is actually making its own light. Some animals make light by changing chemicals in their bodies. This makes them glow, just like a bulb or tiny flame.©M.Imamori/Foto Natura/Minden Pictures
Light made without heat isluminescent light. Running an electric current through a gascauses the gas to temporarily absorb energy and then give off light without also producing heat. This is how fluorescent and neon lights work.
Some plants and animals give off light by changing chemical energy in their bodies into light energy. This kind of light is calledbioluminescence. Fireflies that flicker on and off on summer nights are bioluminescent.
What Do Waves Have to Do with Light?
Light can be seen in many colors but what we normally think of as light, is usually white light. White light is all the colors mixed together and is what a lamp or flashlight might give off.©Maxime Caissy
Light energy behaves like a wave. Like all waves, light waves havewavelengths and frequencies. Light from the Sun or a table lamp is white light, which is actually a blend of light of different colors known as thevisible spectrum.
How Can Light Make Electricity?
People have created technology that uses large panels, called solar panels, that can collect and convert energy from sunlight into energy that can be used to power appliances or heat a home.©Roger Taylor/DOE/NREL
But wait! Light can also behave like a stream of particles. These particles are tiny units of light energy called photons. When photons from a light beam strike certain materials, such as silicon, they can make electricity. It’s called the photoelectric effect. The photoelectric effect is what makes solar-powered calculators work and what makes large solar panels heat a home.
Stars are very bright, they have what scientists call a high luminous intensity, but because they are so far away, they often look dim or twinkley to us. A star that looks very bright must be putting off a lot of light!©M.Durham/GLOBIO.org
The amount of light a source produces is its luminous intensity. Scientists measure luminous intensity in units called candelas. Luminous intensity is not quite the same thing as brightness. Light from sources that are far away provides less illumination, or surface light, than light from a nearby source of the same luminous intensity. Distant stars, for example, pump out tremendous numbers of candelas. But they are so far away they may not provide any illumination on Earth.
Reflection: The Bouncer
Light rays travel outward in all directions along a straight path – until they hit something. Then they can bounce, bend, be absorbed, or bepolarized. Reflection occurs when light bounces off a surface and back to our eyes. This is how we see objects. Some objects reflect more light rays than others. When you look in a mirror, the mirror reflects almost all the light that hits it. The light reflects back at the same angle and you see a clear image of yourself.
Rainbows are just light tricks. All the tiny droplets of rain refract the sunlight peaking through the clouds. This creates a rainbow.©G.Ellis/GLOBIO.org
Refraction: The Bends
When light travels from air to a denser form of transparent matter such as glass or water, it slows down and bends. This is refraction. If you’ve ever noticed that a straw in a glass of water looks “broken,” then you’ve seen refraction in action. Raindrops refract sunlight and disperse it to form a rainbow.
Absorption: The World in Color
The amount of light reflected and absorbed by objects helps explain why we see different colors. Most objects absorb some wavelengths of light and reflect others. For example, a lime looks green. This is because the lime reflects light in the green wavelength but absorbs light in the other visible wavelengths.
Objects look a certain color to us because of light. This lime looks green because it reflects only green light. A red car looks red because it absorbs the light of all the spectrums except red.©Adriana Poveda
How about objects that look black or white? Light hitting an object such as a glass of milk reflects all the visible color wavelengths. When the colors of the visible spectrum are blended, they look white. But when an object looks black, it’s because all the colors are being absorbed andnone are reflected.
Polarization: Sun Block
White light is made of light waves that are vibrating in different directions, at right angles to the path of the light ray. Passing light through a filter called a polarizing filter lets only waves that are vibrating in one direction get through. The light that gets through the filter is polarized, because all the waves are vibrating in one direction. There are a number of practical uses for polarization. Polarizing sunglasses, for example, block light waves to reduce glare.
Bright sunlight isn’t very good for your eyes and can make it hard to see when it is very bright. Sunglasses can help block some of the light and if they are polarized can also reduce glare, making it much more fun to play in the sun.©Silvia Cosimini
Early Reflections on Light
Humans have always been curious about light. The ancient Greeks thought that light traveled out from our eyes to let us see objects. In about A.D. 1000, an Arab thinker named Alhazen became the first to recognize that we see objects because light reflects off them.
More Enlightening Discoveries
The scientist Isaac Newton made important discoveries about light that helped future scientists in their studies.©Godfrey Kneller
Several important breakthroughs came in the 1600s. English scientist Isaac Newton (1642–1727) experimented with prisms and discovered that white light is a blend of all the colors: red, orange, yellow, green, blue, and violet. Newton also believed that light was made up of particles. The Dutch scientist Christiaan Huygens (1629–1695) was the first to suggest that light behaves like a wave.
Waves or Particles: The Great Debate
Scientists could not agree on whether light behaved like particles or waves. But in the 1800s, Scottish scientist James Clerk Maxwell (1831–1879) developed the theory of electromagnetism. His theory identified light as an electromagnetic wave, that is, a wave produced by electric and magnetic fields. Finally. That was settled. Light was a wave.
Or was it? In the early 1900s, the particle theory surfaced again. German scientists Max Planck (1858–1947) and Albert Einstein (1879–1955) did experiments that suggested that light was given off in tiny packets, or quanta. This led to the development of quantum theory; a revolutionary concept that helped explain how light could behave both like a wave and a stream of particles.
Two Who Turned on the Lights
In the 1870s, American inventor Thomas Edison and British inventor Joseph Swan found that electricity could be used to heat up a filament so that it gave off heat and light energy. Placing the filament inside an empty glass ball allowed it to burn brightly without catching fire. The light bulb was born. By the end of the 1800s, electric lighting was becoming common in homes, businesses, and factories, changing life forever. Never again would daily activities have to end when the sun went down.
Light rays that are bent by a lens are what make it possible to see this tiny chicken embryo. This is the same technology used in telescopes to see planets and stars in our solar system.©Frederic Carmel
Increased knowledge about how light works did more than light up our homes. For instance, the understanding of refraction brought about the invention of the lens. Lenses bend light rays so that people can clearly see things that are very small or very distant. Eyeglass lenses correct people’s vision by bringing fuzzy images into sharp focus. Lenses in microscopeshelp scientists see tiny life-forms. Telescope lenses help scientists get a better look at faraway stars and planets.
Lasers can be used for lots of things. They can be used in special surgery, to cut metal, and for fun things like this laser show. Without lasers, you wouldn’t be able to listen to your favorite CD!©Martin Simonis
Einstein’s theories about light helped bring about the invention of lasers. A laser produces a narrow, high-energy beam of light that is all the same wavelength. Laser light has an incredible range of uses. The concentrated energy in laser beams can be used to slice effortlessly through steel. It also can be aimed very precisely to perform delicate surgery on eyes. Laser beams are also used to etch information into CDs and DVDs. Lasers also help decode the information on CDs and DVDs.
Another milestone in light technology was the discovery that print, voices, music, and images could be converted into pulses of laser light and carried great distances through fiber optic cables. Most long-distance phone calls on land lines travel over fiber optic networks. So do your favorite TV shows. Chances are a fiber optic network is connecting you to the Internet so you can read this article!