Light: Wave or Particle?

Eric Saltsman - Physics Class - 12th Grade

Light is a form of energy visible to the human eye that is radiated by moving charged particles. There are many unanswered questions about light. Light is very hard to study due to its high speed. One major question about light is the uncertainty of whether light is a wave or whether it is a particle. The is some evidence pointing in both directions but no conclusive proof confirms that it can be classified as either one. Scientists have learned through experimentation that light behaves like a particle at times, and like a wave at other times. Whenever light acts like a particle, we called the particles that make up the light either a photon or quantum. In 1900, Max Planck proposed the existence of a light quantum, a finite packet of energy which is a photon.

Photons may be described as tiny packets of light energy. Photons are unlike conventional particles, such as specks of dust, because photons do not have to have a specific volume in space. Photons are always arranged in an electromagnetic wave of a definite frequency which indicates that photons are part of a wave. The frequency of the photon forms a single spectrum line that represents a particular wavelength or color. In 1900, a German physicist named Max Planck discovered that light energy is carried by photons. He found that the energy of a photon is equal to the frequency of its electromagnetic wave multiplied by a constant called "h", which stands for Planckís constant. However, this constant is very small because each photon carries little energy. Using the Joule as the unit of energy, Planck's constant is 6.626 x 10^(-20) joules per second in scientific notation. Photons are different from particles of matter in that they have no mass and always move at the constant speed of light which is 300,000 km/sec (186,000 mi/sec). Sir Isaac Newton was for the idea that light is a particle(photon), although, Newton had a hard time explaining the effect of interference on the moving photons.

Another way light is thought of as traveling is in the form of a wave. It is thought this because light also shows wavelike characteristics. When light diffracts, or bends slightly as it passes around a corner, it shows wavelike behavior. If it were a particle, it would not bend around a corner. The waves are called "matter waves". Matter waves have a specific wavelength and the wavelength is inversely proportional to the particleís momentum. Matter waves also explain the arrangement of electrons in separate orbits. The waves associated with light are called electromagnetic waves because they consist of changing electric and magnetic fields. A wave is a continuous phenomenon, which means that when it travels, its electromagnetic field must move at each of the infinite number of points in every small part of space. When we add heat to any system to raise its temperature, the energy is shared equally among all the parts of the system that can move. When this idea is applied to light as a wave, with an infinite number of moving parts, it would require an infinite amount of heat to give all the parts equal energy. But thermal radiation, the process in which heated objects emit electromagnetic waves, occurs in nature without having to add an infinite amount of heat. The wave theory of atomic particles has given scientists a greater understanding of the structure of atoms and their nuclei.

A debate arises: "Is light a wave or a particle?" It canít be both because the models of waves and particles are very different. The way they behave when traveling and when passing through mediums is very different. It seems that your conclusion depends on what type of experiment you are doing. In 1905, Einstein said that a ray of light travels in the path of the photon. The traveling photons are in great number and travel in straight lines. To understand the nature of light, and how it is normally created, it is necessary to study matter at its atomic level. The motion of electrons, leads to the emission of light in most sources. The first successful theory of light wave motion in three dimensions was proposed by the Dutch scientist Christiaan Huygens in 1678. Huygens suggested that light wave peaks form surfaces like the layers of an onion. In a vacuum, or a uniform material, the surfaces are spherical. These wave surfaces advance, or spread out, through space at the speed of light. Huygens also suggested that each point on a wave surface can act like a new source of smaller spherical waves, which may be called wavelets, that are in step with the wave at that point. The envelope of all the wavelets is a wave surface. An envelope is a curve or surface that touches a whole family of other curves or surfaces like the wavelets. This construction explains how light seems to spread away from a pinhole rather than going in one straight line through the hole. The same effect blurs the edges of shadows. Huygens's principle, with minor modifications, accurately describes all forms of wave motion.

There is also a theory which light is said to travel in both a wavelike manner and a particle-like manner. This is called the wave-particle duality and was proposed in 1924 by de Broglie, who also developed the idea of the "matter wave" which is a basic part of the wave-particle duality theory. The wave-particle duality theory says that photons do have a mass and travel along on a wave. This was a combination of the wave and particle theories of light and now is the generally accepted explanation of the way light travels. Heisenberg showed that the wave-particle duality leads to the uncertainty principle. The uncertainty principle states that the position and velocity of a particle cannot simultaneously be measured with exactness. This principle is valid because a particle has certain wave properties. The wave-particle duality theory made since after Einstein connected matter and energy.

Einstein taught us that matter is just another form of very, very condensed energy. Albert Einstein's theory into the equivalence of matter and energy, expressed as the famous equation: E=mc^2, has been confirmed countless times. At the time, it was much easier to demonstrate light as a wave. The process also occurs naturally when a star shines because the atoms in its core fuse, transforming a very small sliver of matter into light. And when particles of matter and antimatter meet, they annihilate each other in a burst of energy. But like any equation, E=mc^2 works both ways. Meaning that it should be possible to convert massive amounts of energy into small amounts of matter. A team of physicists has now transformed light into matter. "We're able to turn optical photons into matter," says Princeton physicist Kirk McDonald, leader of the team. And physicists who smash atoms together have witnessed the conversion of energy into matter in virtual photons that flit in and out of existence just long enough to spawn the particles of exotic matter which can be observed in particle accelerators. But the photons formed like this are not under the direct control of physicists. The photons arise as part of a complex chain of events starting with just a collision of two particles of matter. Until now, no one had directly created matter from light. The key piece was a laser capable of packing a tremendous amount of energy into a small space. By focusing this pulse on an area of just 16-millionths of a square inch, the physicists bathe a spot with an incredibly intense electromagnetic field. But even with this crowd of high-power photons squeezed together, the energy is still only about a millionth of what's needed to make matter. The problem is that the laser's greenlight photons donít provide enough force. Photons, which are massless, can sometimes siphon off part of the energy of a high-speed particle with mass. This occurs because the total energy of the particle, which includes its mass, may exceed that of the photon. The point of trying to make matter by combining energy in the form of photons is to prove that the light traveling as photons have a "mass". Even though the mass is very small, it was made by just light.


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