electromagnetic waves edited

8/3/2019 Electromagnetic Waves Edited

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EXPERIMENT NO.4

To Study the Variation of Photoelectric Effect with Intensity ofLight .

APPARATUS

The apparatus, which I have used to conduct this experiment, includePhotocell, Electric Lamps, Battery, and Scale.

THEORY

Electromagnetic Waves;

At this point in the course we'll move into optics. This might seem like a

separate topic from electricity and magnetism, but optics is really a sub-topicof electricity and magnetism.

This is because optics deals with the behavior of light, and light is oneexample of an electromagnetic wave.

Photon

The quantum of electromagnetic energy, regarded as a discrete particlehaving zero mass, no electric charge, and an indefinitely long lifetime.

Phonon


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A quantum of acoustic energy, the level of which is a function ofthe frequencyof the acoustic wave. Note: Phonons in acoustics areanalogous to photons in electromagnetic. The energy of a phonon is usuallyless than 0.1 eV (electron-volt) and thus is one or two orders of magnitudeless than that of aphoton. When photons and phonons interact insemiconductors used in communications systems,

undesirable system behavior can occur.

Ionization of Atoms;

Ionization is the gain or loss of electrons. The loss of electrons, which is themore common process in astrophysical environments, converts an atom intoa positively charged ion, while the gain of electrons converts an atom into anegativelycharged ion.In the subsequent discussion, we will use the terms ionization and ionize inthe sense of losing electrons to form positive ions.

Threshold Frequency;

Threshold Frequency is the minimum frequency of radiant energy required tocompletely remove an electron from a metal. This applies particularly to

photoelectric components and cathode ray tubes.
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The Work

Function;

Electrons in the metal plate are held to the atoms in the plate with certainenergy. For the purposes of describing the photoelectric effect, this energy is

called the "work function." It is defined more rigorously as the energy thatwould be needed to push the electrons out aninfinite distance from the atomic nucleus. Wecan calculate this fairly easily for simple atoms by utilizing the equation forelectric potential energy, for anything beyond a hydrogen atom, the workfunction becomes too complicated to calculate, so we look it up. The energydepends on the charge of the nucleus, the charge of the electron, and thedistance between the nucleus and the electron.

Stopping Potential;

Voltage required stopping the outward movement of electrons emitted byphotoelectric or thermionic action.

EINSTINE THEORY OF PHOTOELECTRIC EFFECT;


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Einstein's theory predicts that the maximum kinetic energy is completelyindependent of the intensity of the light (because it doesn't show up in theequation anywhere). Shining twice as much light results in twice as many

photons, and more electrons releasing, but the maximum kinetic energy ofthose individual electrons won't change unless the energy, not the intensity,of the light changes.

The maximum kinetic energy results when the least-tightly-bound electronsbreak free, but what about the most-tightly-bound ones; the ones in whichthere is just enough energy in the photon to knock it loose, but the kineticenergy that results in zero. Setting Kmaxequal to zero for this cutofffrequency (nuc), we get:

nuc = phi / h

Or

The cutoff wavelength:

lambdac = hc / phi

These equations indicate why a low-frequency light source would be unableto free electrons from the metal, and thus would produce no photoelectrons.

PLANCKs constant;

Planck's constant, symbolized h, relates the energy in one quantum (photon)

of electromagnetic radiation to the frequencyof that radiation. In the

International System of units (SI ), the constant is equal to approximately

6.626176 x 10-34 joule-seconds. In the centimeter-gram-second (cgs ) or

small-unit metric system, it is equal to approximately 6.626176 x 10 -27 erg-

seconds.The energyE contained in a photon, which represents the smallest possible'packet' of energy in an electromagnetic wave, is directly proportional to thefrequency f according to the following equation:

E = hf

IfE is given injoules andfis given in hertz(the unit measure of frequency).

RELATIONSHIP b/w CURRENT & INTENSITY;

The photoelectric current is known to be directly proportional to theintensity of incident light with fixed frequency.
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The photoelectric effect is dependent upon various factors includingfrequency of light, intensity of light, nature of material, energy of light and

potential difference. However, even if the photoelectric effect is caused, thephotoelectric current which is produced as a result of it may vary if intensityof light is changed, provided that frequency of illumination is greater than

threshold frequency. To determine the impact of changing light intensity onphotoelectric effect while keeping the other factors constant, an experimentwas performed. The impact was seen by changing the distance of lightsource from photocell and the recording the readings on Micrometer.

KINETIC ENERGY OF PHOTOELECTRONS:

The kinetic energy of photoelectrons increases when light of high energy fallson the surface of matter. When energy of light is equal to threshold energythen electrons are emitted from the surface whereas when energy is greaterthan threshold energy then photoelectric current is produced. The thresholdfrequency is not same for all kinds of matter and it varies from matter tomatter.

MAX PLANCKs QUANTUM THEORY:-

The history of quantum mechanics dates back to the 1838 discovery ofcathode rays byMichaelFaraday. This was followed by the 1859 statementof the black body radiation problem by Gustav Kirchhoff, the 1877 suggestionby Ludwig Boltzmann that the energy states of a physical system can bediscrete, and the 1900 quantum hypothesis of Max Planck.

Planck's hypothesis that energy is radiated and absorbed in discrete"quanta", or"energy elements", precisely matched the observed patternsof black body radiation. According to Planck, each energy element E is

proportional to its frequency:

Where h is Planck's constant. Planck cautiously insisted that this wassimply an aspect of the processes of absorption and emission of radiationand had nothing to do with the physical reality of the radiation itself.

However, in 1905 Albert Einstein interpreted Planck's quantum hypothesisrealistically and used it to explain the photoelectric effect, in which shininglight on certain materials can eject electrons from the material.

ALBERT EINSTINEs EXPLANATION

The other exemplar that led to quantum mechanics was the study ofelectromagnetic waves such as light. When it was found in 1900 by Max


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Planck that the energy of waves could be described as consisting of smallpackets or quanta, Albert Einstein further developed this idea to show that anelectromagnetic wave such as light could be described as a particle - latercalled the photon - with a discrete quanta of energy that was dependent onits frequency.[5] This led to a theory of unity between subatomic particles andelectromagnetic waves called waveparticle duality in which particles and

waves were neither one nor the other, but had certain properties of both.

Procedure;

After arranging all the equipment, I put the diagram in front of me andI started making neat and tight connections by following thediagram. The negative terminal of the battery was connected tocathode K of the photo cell C through a rheostat andgalvanometerG. The anode A of the photocell C was connectedto the positive terminal of the battery through a key.

After arranging the apparatus, I put the light lamp at some distancefrom the photocell.

The distance was measured by using a scale. The lamp was positioned in such a way that it was facing cathode of thephotocell. When I switched on the lamp, the light fell on cathodeand it emitted electrons. Since cathode was negatively chargedtherefore, it repelled the electrons and emitted them towardsanode, the positive terminal of the photocell. The movement ofelectrons from cathode to anode produced photoelectric currentin the circuit. I would like to mention here, that the photocellwhich I had used had a concave cathode to give a convergingbeam of electrons to anode.

When I ensured that the apparatus was well arranged andphotoelectric current was being produced, I changed the distanceof the lamp from cathode of photocell and noted down the newdistance. While changing the distance I checked the deflection inthe micro ammeter. I noted down the reading of deflection ofMicro ammeter.

The relation between intensity of illumination and distance is asfollows:

I = constant/d

It means that intensity of light is inversely proportional to thesquare of distance. Since I was taking the readings of distanceand galvanometer therefore, this formula could be used todetermine the intensity of light. One thing which I consideredwhile changing the distance was that whenever I changed thedistance of the lamp from the cathode, I moved the lamp linearly,to keep the angle of incident rays same from the cathode.


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I took few readings of distance of lamp from cathode of photocell anddeflections in Micro ammeter. Since I did not change the angle atwhich light was falling on the cathode, therefore, I ensured thatthe relation between intensity of light and square of distance waslinear.

I = constant/d

I = 1/d

APPLICATIONS OF PHOTOELECTRIC EFFECT;

The photo-cell is the most ranging of applications of the photoelectriceffect. It is most commonly found in solar panels. It works on thebasic principle of light striking the cathode which causes theemission of electrons, which in turn produces a current.

The related field of astronomy, the photo-electric effect is used in theform of photo-multiplier tubes and charge coupled devices(CCDs).CCDs are made of a thin wafer of silicon which is sensitive tolight, on top or bottom of which is placed a tight array of pixels. Theentire detector is usually a few square centimeters in size, about thesize of a nickel, and it can be as thin as a micron, or a tenth of thediameter of a human hair. Pixels are laid out on the silicon wafer in arectangular grid pattern, usually several hundred to a few thousandon a side. In order to fit so many pixels on such a small surface, the

pixels themselves must also be very small; each measuresapproximately 10 to 20 microns across.

The main

application of the photoelectric effect is the process where light istransformed (via photovoltaiccell) to electric current. Thegiven figure is a diagram ofan apparatus in which the

photoelectric effect canoccur. An evacuated glass orquartz tube contains ametallic plate E connected tothe negative terminal of thebattery and another metallic

plate C that is connected tothe positive terminal of thebattery.When the tube is kept in thedark, the ammeter reads

zero, indicating no current inthe circuit. However, when


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the plate E is illuminated by light having a wavelength shorter than someparticular wavelength that depends on the metal used to make plate E, acurrent is detected by the ammeter, indicating a flow of charges acrossthe gab between plates E andC. This current arises from photoelectronsemitted from the negative plate (the emitter) and collected at the positive

plate (the collector).

REFERENCEs;

University physics

wikipedi.com

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