WAVE THEORY Group 2 Buensuceso Elagio Emman Gines

WAVE THEORY Group 2: Buensuceso, Elagio, Emman, Gines, Diokno

Contents Huygens’ Wave Theory Thomas Young’s Double Slit Experiment EM Wave Thoery Hertz’s Experiment on EM Waves

Huygens’ Wave Theory

Huygens’ Wave Theory l l In the late 1600 s, many people began asking if light is made up of particles, or waves ? Sir Isaac Newton, a famed scientist, supported theory that light was made up of tiny particles, maintaining the stance of previous scientists.

Light l l However, we know that light is part of the electromagnetic spectrum, the spectrum is the collection of all waves, which include visible light, Microwaves, radio waves ( AM, FM, SW ), X-Rays, and Gamma Rays. This was proven eventually in 1678, by a Dutch Scientist named Christiaan Huygens.

Christiaan Huygen l l Christiaan Huygens, believed that light was made up of waves vibrating up and down perpendicular to the direction of the light travels (transverse waves), and therefore formulated a way of visualizing wave propagation. This became known as 'Huygens' Principle'.

Huygens’ Wave Theory l l Huygens’ theory was the successful theory of light wave motion in three dimensions. It also suggested that light wave peaks form surfaces like the layers of an onion.

Wave Theory l l In a vacuum, or other mediums, the light waves are spherical, and these wave surfaces advance or spread out as they travel at the speed of light. This theory explains why light shining through a pin hole or slit will spread out rather than going in a straight line.

Newton vs Huygens l l Some of the experiments l This was due to the conducted on light theory, unexplained both the wave and particle, phenomenon of light had some unexplained results: as Newton could not Polarization - scientists explain the phenomenon of were familiar with the light interference, this forced fact that wave motion Newton's particle theory in favor of the wave theory. was parallel to the However, it was eventually direction of wave travel, realized that matter and NOT perpendicular to waves exhibited properties the to the direction of of the other. wave travel, as light does.

Huygens’ Principle l l He also found that a surface containing many separate wave sources appeared as a single wave front with the shape of the surface. This wave front is termed a 'Huygens combination' of the separate waves. This explains how matter's spherical In-waves are formed. The Out-waves of others combine to form a Huygens 'combination wave front' which forms the spherical In-wave of our wave-centers.

l Christian Huygens proposed a hypothesis for the geometrical construction of the position of a common wavefront at any instant during the propagations of waves in a medium. The postulates are: l Every point on the given wavefront called primary wavefront* acts as a fresh source of new disturbance, called secondary wavelets** that travel in all directions with the velocity of light in the medium. l A surface touching these secondary wavelets tangentially in the forward direction at any instant gives a new wavefront at that instant. This is the secondary wave front. * - The envelope of all wavelets in the same phase - having received light from sources in the same phase at the same time is called a wave front. ** - All points lying on small curved surfaces, that receive light at the same time from the same source (primary or secondary) are called wavelets.

Thomas Young’s Double Slit Experiment

l The double-slit experiment in quantum mechanics is an experiment that demonstrates the inseparability of the wave and particle natures of light and other quantum particles. A coherent light source illuminates a thin plate with two parallel slits cut in it, and the light passing through the slits strikes a screen behind them. The wave nature of light causes the light waves passing through both slits to interfere, creating an interference pattern of bright and dark bands on the screen. However, at the screen, the light is always found to be absorbed as discrete particles, called photons. l If the light travels from the source to the screen as particles, then the number that strikes any particular point on the screen should be equal to the sum of those that go through the left slit and those that go through the right slit. In other words, the brightness at any point should be the sum of the brightness when the right slit is blocked and the brightness when the left slit is blocked. However, it is found that blocking one slit makes some points on the screen brighter and other points darker. This can only be explained by the alternately additive and subtractive interference of waves, not the exclusively additive nature of particles.

l Although the double-slit experiment is now often referred to in the context of quantum mechanics, it is generally thought to have been first performed by the English scientist Thomas Young in the year 1801 in an attempt to resolve the question of whether light was composed of particles (Newton's "corpuscular" theory), or rather consisted of waves traveling through some ether, just as sound waves travel in air. The interference patterns observed in the experiment seemed to discredit the corpuscular theory, and the wave theory of light remained well accepted until the early 20 th century, when evidence began to accumulate which seemed instead to confirm the particle theory of light. l It was shown experimentally in 1972 that in a Young slit system where only one slit was open at any time, interference was nonetheless observed provided the path difference was such that the detected photon could have come from either slit. The experimental conditions were such that the photon density in the system was much less than unity.

l A Young double slit experiment was not performed with anything other than light until 1961, when Claus Jönsson of the University of Tubingen performed it with electrons, and not until 1974 in the form of "one electron at a time", in a laboratory at the University of Milan, by researchers led by Pier Giorgio Merli, of LAMEL-CNR Bologna. l The results of the 1974 experiment were published and even made into a short film, but did not receive wide attention. The experiment was repeated in 1989 by Tonomura et al at Hitachi in Japan. Their equipment was better, reflecting 15 years of advances in electronics and a dedicated development effort by the Hitachi team. Their methodology was more precise and elegant, and their results agreed with the results of Merli's team. Although Tonomura asserted that the Italian experiment had not detected electrons one at a time —a key to demonstrating the wave-particle paradox—single electron detection is clearly visible in the photos and film taken by Merli and his group.

EM Wave Theory

Electromagnetic waves It consists of electric and magnetic field components which oscillate in phase perpendicular to each other and perpendicular to the direction of energy propagation.

EM wave theory The theory of electromagnetic waves was first postulated by James Clerk Maxwell and was confirmed by Heinrich Hertz. Maxwell derived a wave form of the electric and magnetic equations, revealing the wave-like nature of electric and magnetic fields, and their symmetry. Maxwell proved that light therefore is an electromagnetic wave through his equations.

EM wave theory l A spatially-varying electric field generates a time-varying magnetic field and vice versa. Neither can exist by themselves. l Therefore, as an oscillating electric field generates an oscillating magnetic field, the magnetic field in turn generates an oscillating electric field, and so on. l These oscillating fields together form an electromagnetic wave.

Electromagnetism l Electromagnetic waves are a form of traveling electric and magnetic transverse waves l A charged (positive/negative) particle can create an electric field around it. The force of an electric field acts to electric charges just like how a gravitational field would act to masses. l When the charge start to oscillate, back and forth, the oscillation of the electric field will create a magnetic field that is at right angles to the electric field l The oscillation of the magnetic field would create another electric field and continue to create each other in the process. Unlike a STATIC field, a wave cannot exist unless it is moving. Once created, an electromagnetic wave will continue on forever unless it is absorbed by matter.

Hertz’s Experiment on EM Waves

l In 1887, Hertz designed a brilliant set of experiments that tested Maxwell's hypothesis. He used an oscillator made of polished brass knobs, each connected to an induction coil and separated by a tiny gap over which sparks could leap. Hertz reasoned that, if Maxwell's predictions were correct, electromagnetic waves would be transmitted during each series of sparks. To confirm this, Hertz made a simple receiver of looped wire. At the ends of the loop were small knobs separated by a tiny gap. The receiver was placed several yards from the oscillator. l According to theory, if electromagnetic waves were spreading from the oscillator sparks, they would induce a current in the loop that would send sparks across the gap. This occurred when Hertz turned on the oscillator, producing the first transmission and reception of electromagnetic waves. Hertz also noted that electrical conductors reflect the waves and that they can be focused by concave reflectors. He found that nonconductors allow most of the waves to pass through. Another of his discoveries was the photoelectric effect.

l Earlier in 1886, Hertz developed the Hertz antenna receiver. This is a set of terminals that is not electrically grounded for its operation. He also developed a transmitting type of dipole antenna, which was a center-fed driven element for transmitting UHF radio waves. These antennas are the simplest practical antennas from a theoretical point of view. l Hertz made observations of the photoelectric effect and of the production and reception of electromagnetic (EM) waves using an apparatus. His receiver consisted of a coil with a spark gap, whereupon a spark would be seen upon detection of EM waves. He placed it in a darkened box to see the spark better. He observed that the maximum spark length was reduced when in the box. A glass panel placed between the source of EM waves and the receiver absorbed ultraviolet radiation that assisted the electrons in jumping across the gap. l When removed, the spark length would increase. He observed no decrease in spark length when he substituted quartz for glass, as quartz does not absorb UV radiation.

l Through experimentation, he proved that transverse free space electromagnetic waves can travel over some distance. This had been predicted by Maxwell and Faraday. With his apparatus configuration, the electric and magnetic fields would radiate away from the wires as transverse waves. Hertz had positioned the oscillator about 12 meters from a zinc reflecting plate to produce standing waves. Each wave was about 4 meters. Using the ring detector, he recorded how the magnitude and wave's component direction vary. Hertz measured Maxwell's waves and demonstrated that the velocity of radio waves was equal to the velocity of light. The electric field intensity and polarity was also measured by Hertz. l His discoveries would later be more fully understood by others and be part of the new "wireless age". In bulk, Hertz' experiments explain reflection, refraction, polarization, interference, and velocity of electric waves.

Sources http: //en. wikipedia. org/wiki/Waveparticle_duality#Huygens_and_Newton http: //www. nightlase. com. au/education/optics/light. htm http: //www. juliantrubin. com/bigten/youngdoubleslit. html http: //en. wikipedia. org/wiki/Electromagnetic_radiation http: //science. hq. nasa. gov/kids/images/ems/consider. html http: //physics. tamuk. edu/~suson/html/4323/emtheory. html http: //www. juliantrubin. com/bigten/hertzexperiment. html http: //en. wikipedia. org/wiki/Heinrich_Hertz#Electromagnetic_research http: //en. wikipedia. org/wiki/Heinrich_Hertz http: //people. seas. harvard. edu/~jones/cscie 129/nu_lectures/lecture 6/hertz/Hertz _exp. html