Photon,Particleoflightenergy, Or Energy That Is Generated by Moving Electric Charges. Energy

Photon

I / INTRODUCTION

Photon,particleoflightenergy, or energy that is generated by moving electric charges. Energy generated by moving charges is called electromagnetic radiation. Visible light is one kind of electromagnetic radiation. Other kinds of radiation include radio waves, infrared waves, and X rays. All such radiation sometimes behaves like a wave and sometimes behaves like a particle. Scientists use the concept of a photon to describe the effects of radiation when it behaves like a particle. (See Wave Motion)

Mostphotonsareinvisible to humans. Humans only see photons with energy levels that fall within a certain range. We describe these visible photons as visible light. Invisible photons include radio and television signals, photons that heat food in microwave ovens, the ultraviolet light that causes sunburn, and the X rays doctors use to view a person’s bones.

Thephotonisanelementary particle, or a particle that cannot be split into anything smaller. It carries the electromagnetic force, one of the four fundamental forces of nature, between particles. The electromagnetic force occurs between charged particles or between magnetic materials and charged particles. Electrically charged particles attract or repel each other by exchanging photons back and forth.

II / CHARACTERISTICS

Photonsareparticles with no electrical charge and no mass, but they do have energy and momentum, a property that allows photons to affect other particles when they collide with them. Photons travel at the speed of light, which is about 300,000 km/sec (about 186,000 mi/sec). Only objects without mass can travel at the speed of light. Objects with mass must travel at slower speeds, and nothing can travel at speeds faster than the speed of light.

Theenergyofaphoton is equal to the product of a constant number called Planck’s constant multiplied by the frequency, or number of vibrations per second, of the photon. Scientists write the equation for a photon’s energy as E=hv, where h is Planck’s Constant and v is the frequency. Photons with high frequencies, such as X rays, carry more energy than do photons with low frequencies, such as radio waves. Photons that are visible to the human eye have energy levels around one electron volt (eV) and frequencies from 1014 to 1015 Hz (hertz or cycles per second). The number 1014 is a 1 followed by 14 zeros. The frequency of visible photons corresponds to the color of their light. Photons of violet light have the highest frequencies of visible light, while photons of red light have the lowest frequencies. Gamma rays, the highest-energy photons of all, have energies in the 1 GeV range (109 eV) and frequencies higher than 1018 Hz. Gamma rays are only produced in special experimental devices called particle accelerators and in outer space.

Althoughmomentumisusually considered a property of objects with mass, photons also have momentum. Momentum determines the amount of force, or pressure, that an object exerts when it hits a surface. In classical physics, or physics that deals with the behavior of objects we encounter in everyday life, momentum is equal to the product of the mass of an object multiplied by its velocity (the combination of its speed and direction). While photons do not have mass, scientists have found that they exert extremely small amounts of pressure when they strike surfaces. Scientists have redefined momentum to include the force exerted by photons, called light pressure or radiation pressure.

III / HISTORY

Philosophersfromasfar back in history as the Greeks of the 5th century bc have thought about the nature of light. In the 1600s, scientists began to argue over whether light is made of particles or waves. In the 1860s, British physicist James Clerk Maxwell discovered electromagnetic waves, waves of electromagnetic energy that travel at the speed of light. He determined that light is made of these waves, and his theory seemed to settle the wave versus particle issue. His conclusion that light is made of waves is still valid. However, in 1900 German physicist Max Planck renewed the argument that light could also act like particles, and these particles became known as photons. He developed the idea of photons to explain why substances, when heated to higher and higher temperatures, would glow with light of different colors. The wave theory could not explain why the colors changed with temperature changes.

Mostscientistsdidnot pay attention to Planck’s theory until 1905, when German-born American physicist Albert Einstein used the idea of photons to explain an interaction he had studied called the photoelectric effect. In this interaction, light shining on the surface of a metal causes the metal to emit electrons. Electrons escape the metal by absorbing energy from the light. Einstein showed that light behaves as particles in this situation. If the light behaved like waves, each electron could absorb many light waves and gain more and more energy. He found, however, that a more intense beam of light, with more light waves, did not give each electron more energy. Instead, more light caused the metal to release more electrons, each of which had the same amount of energy. Each electron had to be absorbing a small piece of the light beam, or a particle of light, and all these pieces had the same amount of energy. A beam of light with a higher frequency contained pieces of light with more energy, so when electrons absorbed these particles, they too had more energy. This could only be explained using the photon view of radiation, in which each electron absorbs a single photon and gains enough energy to escape the metal.

Todayscientistsbelieve that light behaves both as a wave and as a particle. Scientists detect photons as discrete particles, and photons interact with matter as particles. However, light travels in the form of waves. Some experiments reveal the wave properties of light; for example, in diffraction, light spreads out from a small opening in waves, much like waves of water would behave. Other experiments, such as Einstein’s study of the photoelectric effect, reveal light’s particle properties.

Contributed By:
Cindy Schwarz