Electromagnetic Radiation

AGEN-689 – Advances in Food Engineering

Electromagnetic wavesLight, microwaves, X-rays, and TV and radio transmissionsThey are all the same kind of wavy disturbances that repeats itself over a distance called wavelengthThe different names refer to different wavelengths

Same kind of waves with different λAll forms of electromagnetic radiation can travel in vacuumOther waves, sound waves need some kind of material (air or water) in which to move

Electromagnetic waves and vacuumVisible light can travel in vacuum (we see it)So all those kind of waves can travel tooThey all have the same speed = the speed of light

Young’s experiment19th century – the nature of lightHow could energy be transmitted through space (even through vacuum)Mechanical model for light: Young’sInterference patterns is formed in a double slit experiment

Young’s Mechanical ModelWave propagation through spaceProperties like frequency, ν, wavelength, λRelationship:

Interference fringes

cf =λ

The ether conceptLight propagates in wave motionAn unknown substance existed that supported the propagation of wave motionEther – certain elastic ppt to support the propagation of the wave motion

Electromagnetic waveMaxwell, 1864Prediction of the velocity of an electromagnetic wave from first principlesThe speed of an electromagnetic wave in a vacuum: Permeability constant

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Permittivity constant

Maxwell findsElectromagnetic waves are known to be energy propagated through spaceOne of the simplest model for this generation – an oscillating electric chargeAn electron oscillating in a linear path will produce time varying electric and magnetic fields that are propagated through space and that constitute an electromagnetic wave front

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Maxwell’s classical theory of electromagnetism

Energy is emitted in form of electromagnetic waves when an electric charge is accelerated; A charge undergoing simple harmonic motion will generate electromagnetic waves with same frequency as the moving charge

Electromagnetic spectrumElectromagnetic waves exist in a continuous spectrum that extends over many decades of frequencyIt includes radiations from the lowest frequencies (radio waves) to the highest (X-rays and gamma rays)

X-rays and Gamma raysHave sufficient energy to ionize atoms with which they interactCalled ionizing radiation

λfc =Speed of light in vacuum(and air) = 2.998x10^8 m/s)

X-ray

Ionization

Photoionization

Speed of light Τλ

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Relationship between f or λ and energy of radiation

Not as obvious as the simple relationship between frequency and wavelengthTheoretical explanation for cavity radiation remained unsolved for many years

Blackbody radiation" or "cavity radiation" refers to an object or system which absorbs all radiation incident upon it and re-radiates energy which is characteristic of this radiating system only, not dependent upon the type of radiation which is incident upon it. The radiated energy can be considered to be produced by standing wave or resonant modes of the cavity which is radiating.

Plank’s theory (1901)Atoms constituting the radiating surface of the cavity were electromagnetic oscillators with characteristics frequenciesThese oscillators would absorb and emit electromagnetic radiation from and to the cavityProposed the ‘quantum theory’ of electron radiation

Plank’s theoryAn oscillator can have only energies given by

nhfE =Quantum number Plank’s constant

The oscillator does not emit radiation continuously, by in ‘packets’ or quanta. The energy of the quanta will vary as the irradiator moves from one quantum level to another

Quantum theory of electromagnetic radiation – Planks’

The energy emitted by an oscillator is determined by its frequency, f, constant, h, and integer, nThe oscillator can only have a number ‘quantized’ energy steps determined by the value of nThe oscillators do not radiate energy in a continuum, but in quantized steps determined by n

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Plank’s constant = h = 6.63 x 10^-34 J s = 4.14 x 10^-15 eV s

The Photoelectric EffectIt's been determined experimentally that when light shines on a metal surface, the surface emits electrons. For example, you can start a current in a circuit just by shining a light on a metal plat

PhotonsBased on Planck's work, Einstein proposed that light also delivers its energy in chunks; light would then consist of little particles, or quanta, called photons, each with an energy of Planck's constant times its frequency. In that case, the frequency of the light would make a difference in the photoelectric effect

Einstein's photoelectric discoversHigher-frequency photons have more energy, so they make the electrons come flying out faster; thus, switching to light with the same intensity but a higher frequency should increase the maximum kinetic energy of the emitted electronsIf you leave the frequency the same but crank up the intensity, more electrons should come out (because there are more photons to hit them), but they won't come out any faster, because each individual photon still has the same energy. If the frequency is low enough, then none of the photons will have enough energy to knock an electron out of an atom. With a few simple measurements, the photoelectric effect would seem to be able to tell us whether light is in fact made up of particles or waves

The Photoelectric effect – Einstein (1906)

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If the potential across the electrodes is reversed at the time of electron emission, a potential, Vo, can be found that is just adequate to stop the current flowThis is the stopping potentialThe maximum kinetic energy of the emitted photoelectrons is:

The contradiction with classical physics

1. The electrons were emitted immediately - no time lag!2. Increasing the intensity of the light

increased the number of photoelectrons, but not their maximum kinetic energy!3. Red light will not cause the ejection of

electrons, no matter what the intensity!4. A weak violet light will eject only a few

electrons, but their maximum kinetic energies are greater than those for intense light of longer wavelengths!

The quantum explanationAnalysis of data from the photoelectric experimentshowed that the energy of the ejected electrons was proportional to the frequency of the illuminating light. This showed that whatever was knocking the electrons out had an energy proportional to light frequency. The remarkable fact that the ejection energy was independent of the total energy of illumination showed that the interaction must be like that of a particle which gave all of its energy to the electron! This fit in well with Planck's hypothesis that light in the blackbody radiation experiment could exist only in discrete bundles with energy

Einstein's theoryLight travels through space in packets, named photonsThe energy of each photon is the product of its frequency and Plank’s constant

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Minimum energy required for the electron to escape fromits position in the metal surface = work function for a particularsurface and it is a characteristic of the material

Photoelectric effectMost commonly observed phenomena with light can be explained by wavesBut the photoelectric effect suggested a particle nature for light

Mass-Energy EquivalenceIn relativistic terms (i.e. rapidly moving objects)Mass and energy relationship

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Rest energy