Electromagnetic radiation (waves) is simply another term for light. Light waves are fluctuations of electric and magnetic fields in space. Radiation is energy emitted in the form of waves (light) or particles (photons).
Historically, scientists who subscribed to the wave theory centered their arguments on the discoveries of Dutchman Christiaan Huygens. Wave proponents envisions light as wave-like in nature, producing energy that traverses through space in a manner similar to the ripples spreading across the surface of a still pond after being disturbed by a dropped rock.
Those who subscribe to particle theory cite Sir Isaac Newton's prism experiments as proof that light travels as a shower of particles, each proceeding in a straight line until it is refracted, absorbed, reflected, diffracted or disturbed. Particle proponents hold that light is composed of a steady stream of particles, like droplets of water sprayed from a garden hose nozzle.
Alfred Einstein, Max Planck, Neils Bohr and others attempted to explain how electromagnetic radiation can display what is now called "wave-particle duality." For instance, low frequency electromagnetic radiation tends to act more like a wave than a particle; high frequency electromagnetic radiation tends to act more like a particle than a wave.
Visible light is electromagnetic radiation at wavelengths which the human eye can see. We perceive this radiation as colors. Light broken up into its component colors is called the light spectrum. The rainbow (or a light passing through a prism) reflects this spectrum, consisting of red, orange, yellow, green, blue, indigo, and violet. The different colors of light correspond to the different energies of the light waves.
Visible light is based on a simple model of propagating rays and wave fronts, a concept first proposed in the late 1600s by Dutch physicist Christiaan Huygens. The way visible light is emitted or absorbed by substances, and how it predictably reacts under varying conditions as it travels through space and the atmosphere, forms the basis of color. Isaac Newton discovered white light is made up of all the colors of the visible spectrum.
The electromagnetic (EM) spectrum is a name that scientists give to varying types of radiation as a group. Radiation is energy that travels and spreads out as it goes, such as visible light that comes from a lamp or radio waves that come from a radio station. The electromagnetic spectrum is the full range of electromagnetic radiation, consisting of gamma rays, X-rays, ultraviolet rays, visible light (optical), infrared, microwaves, and radio waves.
Many sources emit electromagnetic radiation, and are generally categorized according to the specific spectrum of wavelengths generated by the source. Long radio waves are produced by electrical current flowing through huge broadcast antennas, while shorter visible light waves are produced by the energy state fluctuations of negatively charged electrons within atoms. The shortest form of electromagnetic radiation, gamma waves, results from decay of nuclear components at the center of the atom.
Hotter, more energetic objects and events create higher energy radiation than cool objects. Only extremely hot objects or particles moving at very high velocities can create high-energy radiation like X-rays and gamma-rays.
Electromagnetic radiation can be described in terms of a stream of photons, which are massless particles traveling in a wave-like pattern and moving at the speed of light. A photon is the smallest (quantum) unit of light/electromagnetic energy. Photons are generally regarded as particles with zero mass and no electric charge.
After more than 300 years of measuring the speed of light, the Seventeenth General Congress on Weights and Measures defined the speed of light at 299,792.458 kilometers per second. Consequently, the meter is defined as the distance light travels through a vacuum in 1/299,792,458 seconds. The speed of light is frequently rounded to 300,000 kilometers (or 186,000 miles) per second.
Light traveling in a uniform substance, or medium, propagates in a straight line at a relatively constant speed, unless it is refracted, reflected, diffracted, or disturbed in some manner. This was understood and described as far back as 350 BC by the ancient Greek scholar, Euclid, in his landmark treatise Optica.
Light waves come in many frequencies. The frequency is the number of waves that pass a point in space during any time interval, usually one second. It is measured in units of cycles (waves) per second, or Hertz (Hz). The frequency of visible light is referred to as color, and ranges from 430 trillion Hz, seen as red, to 750 trillion Hz, seen as violet. The full range of frequencies extends beyond the visible spectrum, from less than one billion Hz, as in radio waves, to greater than 3 billion billion Hz, as in gamma rays.
Light not only vibrates at different frequencies, it also travels at different speeds. Light waves move through a vacuum at their maximum speed, 300,000 kilometers per second or 186,000 miles per second, which makes light the fastest phenomenon in the universe. Light waves slow down when they travel inside substances, such as air, water, glass or a diamond. The way different substances affect the speed at which light travels is key to understanding the bending of light, or refraction.
The amount of energy in a light wave is proportionally related to its frequency: High frequency light has high energy; low frequency light has low energy. Gamma rays have the most energy, and radio waves have the least. Of visible light, violet has the most energy and red the least.
By the late 1960s, lasers were becoming stable research tools with highly defined frequencies and wavelengths. It quickly became obvious that a simultaneous measurement of frequency and wavelength would yield a very accurate value for the speed of light, similar to an experimental approach carried out by Keith Davy Froome using microwaves in 1958.
Several research groups in the United States and in other countries measured the frequency of the 633-nanometer line from an iodine-stabilized helium-neon laser and obtained highly accurate results. In 1972, the National Institute of Standards and Technology employed the laser technology to measure the speed at 299,792,458 meters per second (186,282 miles per second), which ultimately resulted in the redefinition of the meter through a highly accurate estimate for the speed of light.
This was confirmed later in 1983 by the Seventeenth General Congress on Weights and Measures. Thus, the meter is defined as the distance light travels through a vacuum during a time interval of 1/299,792,458 seconds. In general, however, (even in many scientific calculations) the speed of light is rounded to 300,000 kilometers (or 186,000 miles) per second.
Arriving at a standard value for the speed of light was important for establishing an international system of units that would enable scientists from around the world to compare their data and calculations.
Einstein's Theory of Relativity implies that nothing can go faster than the speed of light.
All light-natural and artificial-is made up of a collection of one or more photons propagating through space as electromagnetic waves. For example, a light source in a room produces photons and objects in the room reflect those photons. The eyes absorb the photons and that is how we see.
The mechanism involved in producing photons is the energizing of electrons orbiting each atom's nucleus. Electrons circle the nucleus in fixed orbits, the way satellites orbit the Earth. An electron has a natural orbit that it occupies. When an atom is energized, its electrons move to higher orbits.
A photon of light is produced whenever an electron in a high orbit falls back to its normal orbit. During the fall from high energy to normal energy, the electron emits a photon (a packet of energy) with very specific characteristics. The photon has a frequency, or color, that exactly matches the distance the electron falls.
As an example, sodium vapor lights, the kind seen in parking lots, are yellow. A sodium vapor light energizes sodium atoms to generate photons. The energy packets generated by the falling sodium electrons fall at a wavelength that corresponds to yellow light.
The most common way to energize atoms is with heat, the basis of incandescence. A normal 75-watt incandescent bulb (or any wattage) is generating light by using electricity to create heat.
Halogen lamps use electricity to generate heat, but contain a filament that runs hotter than incandescent bulbs. Gas lanterns use natural gas or kerosene as the source of heat. Fluorescent lights use electricity to directly energize atoms rather than requiring heat. In Indiglo watches, voltage energizes phosphor atoms. Fireflies use a chemical reaction to energize atoms.
Each photon contains a certain amount (or bundle) of energy, and all electromagnetic radiation consists of these photons. The only difference between the various types of electromagnetic radiation is the amount of energy found in the photons. Radio waves have photons with low energies, microwaves have a little more energy than radio waves, infrared has still more, then visible, ultraviolet, X-rays, and the most energetic of all are gamma-rays.
Whether it's a signal transmitted to a radio from a broadcast station, heat radiating from a fireplace, X-rays producing images of teeth, or the visible and ultraviolet light emanating from the sun, the various categories of electromagnetic radiation all share identical and fundamental wave-like properties.
What light is and the properties it contains will continue to be one of the most fascinating subjects of scientific inquiry in the future.
Light as Energy
The amount of energy falling on the Earth's surface from the sun is approximately 5.6 billion billion (quintillion) megajoules per year. Averaged over the entire Earth's surface, this translates into about 5 kilowatt-hours per square meter every day. The energy input from the sun in a single day could supply the needs for all of the Earth's inhabitants for a period of about 3 decades.
Only in the last few decades has mankind begun to search for mechanisms to harness the tremendous potential of solar energy. This intense concern has resulted from a continuing increase in energy consumption, growing environmental problems from the fuels that are now consumed, and an ever-present awareness about the inevitable depletion of fossil fuel.
Related topics include photosynthesis, the photoelectric effect, solar cells, charge-coupled devices, fuel cells, and nuclear fusion.
Green plants absorb water and carbon dioxide from the environment, and utilizing energy from the sun, turn these simple substances into glucose and oxygen. With glucose as a basic building block, plants synthesize a number of complex carbon-based biochemicals used to grow and sustain life. This process is termed photosynthesis, and is the cornerstone of life on Earth.
Solar cells convert light energy into electrical energy either indirectly by first converting it into heat, or through a direct process known as the photovoltaic effect. The most common types of solar cells are based on the photovoltaic effect, which occurs when light falling on a two-layer semiconductor material produces a potential difference, or voltage, between the two layers.
The voltage produced in the cell is capable of driving a current through an external electrical circuit that can be utilized to power electrical devices.
Fuel cells (hydrogen) are designed to utilize a catalyst, such as platinum, to convert a mixture of hydrogen and oxygen into water. An important byproduct of this chemical reaction is the electricity generated when hydrogen molecules interact (through oxidation) with the anode to produce protons and electrons.
Power over optical fiber will replace electrical copper wires, such as those that connect sensors to monitor fuel tanks on airplanes, eliminating the fear of short circuits and sparks. Fiber optic systems are being designed to use a laser for injecting power in the form of light into a fiber-optic cable and a photovoltaic (PV) array to convert the light back into electricity for powering devices. Photonic power devices are scheduled to replace electrical transformers now currently used in power grids.
Current transformers are large, expensive to maintain, and heat up. To prevent temperatures from rising to dangerous levels and to reduce power leaks, oil and gas are used as insulators. But oil is flammable and can make transformers explode at high temperatures. Photonic Power offers the option of measuring high currents by placing a transducer directly on the line, eliminating the use of transformers to overcome voltage differences. The power-over-fiber system converts electricity directly to light.
Photonics began in the 60s with the invention of the laser followed in the 70s with optical fiber as a medium for transmitting information using light beams. A tremendous amount of information can be transmitted using optical fiber, so much so, it serves as the infrastructure for the Internet. So, we use light not only to see but also to communicate.