Electromagnetic Radiation. AGEN-689 Advances in Food Engineering

Transcription

1 Electromagnetic Radiation AGEN-689 Advances in Food Engineering

2 Electromagnetic waves Light, microwaves, X- rays, and TV and radio transmissions They are all the same kind of wavy disturbances that repeats itself over a distance called wavelength The different names refer to different wavelengths

3 Same kind of waves with different λ All forms of electromagnetic radiation can travel in vacuum Other waves, sound waves need some kind of material (air or water) in which to move

4 Electromagnetic waves and vacuum Visible light can travel in vacuum (we see it) So all those kind of waves can travel too They all have the same speed = the speed of light

5 Young s experiment 19 th century the nature of light How could energy be transmitted through space (even through vacuum) Mechanical model for light: Young s Interference patterns is formed in a double slit experiment

6 Young s Mechanical Model Wave propagation through space Properties like frequency, ν, wavelength, λ Relationship: Interference fringes f λ = c

7 The ether concept Light propagates in wave motion An unknown substance existed that supported the propagation of wave motion Ether certain elastic ppt to support the propagation of the wave motion

8 Electromagnetic wave Maxwell, 1864 Prediction of the velocity of an electromagnetic wave from first principles c = The speed of an electromagnetic wave in a vacuum: Permeability constant 1 µ ε o o Permittivity constant

9 Maxwell finds Electromagnetic waves are known to be energy propagated through space One of the simplest model for this generation an oscillating electric charge An 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 +

10 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

11 Electromagnetic spectrum Electromagnetic waves exist in a continuous spectrum that extends over many decades of frequency It includes radiations from the lowest frequencies (radio waves) to the highest (Xrays and gamma rays)

12 X-rays and Gamma rays Photoionization X-ray Ionization Have sufficient energy to ionize atoms with which they interact Called ionizing radiation c = fλ Speed of light in vacuum (and air) = 2.998x10^8 m/s)

13 Speed of light Τ λ f T f c c 1 = T = period(time it takes for the charge to oscillate once) = frequency = the number of completed periods in 1sec λ = T = fλ

14 Relationship between f or λ and energy of radiation Not as obvious as the simple relationship between frequency and wavelength Theoretical 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.

15 Plank s theory (1901) Atoms constituting the radiating surface of the cavity were electromagnetic oscillators with characteristics frequencies These oscillators would absorb and emit electromagnetic radiation from and to the cavity Proposed the quantum theory of electron radiation

16 Plank s theory An oscillator can have only energies given by E = nhf 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

17 Quantum theory of electromagnetic radiation Planks The energy emitted by an oscillator is determined by its frequency, f, constant, h, and integer, n The oscillator can only have a number quantized energy steps determined by the value of n The oscillators do not radiate energy in a continuum, but in quantized steps determined by n E = n hf Plank s constant = h = 6.63 x 10^-34 J s = 4.14 x 10^-15 ev s

18 The Photoelectric Effect It'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

19 Photons Based 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

20 Einstein's photoelectric discovers Higher-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 electrons If 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

21 The Photoelectric effect Einstein (1906) 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 flow This is the stopping potential The maximum kinetic energy of the emitted photoelectrons is: K = ev max o

22 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!

23 The quantum explanation Analysis of data from the photoelectric experiment showed 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

24 Einstein's theory Light travels through space in packets, named photons The energy of each photon is the product of its frequency and Plank s constant hf = Eo + K max Minimum energy required for the electron to escape from its position in the metal surface = work function for a particular surface and it is a characteristic of the material

25 Photoelectric effect Most commonly observed phenomena with light can be explained by waves But the photoelectric effect suggested a particle nature for light

26 Mass-Energy Equivalence In relativistic terms (i.e. rapidly moving objects) Mass and energy relationship E for v E 0 = = = 0 mc mc v c 2 2 Rest energy