Lectures on light - The Nature of Light Before the...

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The Nature of Light Before the beginning of the nineteenth century, light was considered to be a stream of particles The particles were either emitted by the object being viewed or emanated from the eyes of the viewer Notes Newton was the chief architect of the particle theory of light – He believed the particles left the object and stimulated the sense of sight upon entering the eyes 1642-1727 Isaac Newton
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Nature of Light – Alternative View Christian Huygens argued that light might be some sort of a wave motion Thomas Young (1801) provided the first clear demonstration of the wave nature of light Notes – He showed that light rays show constructive and destructive interference with each other – Such behavior could not be explained by particles 1629 - 1695 Christian Huygens Thomas Young 1773 - 1829
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More Confirmation of Wave Nature During the nineteenth century, other developments led to the general acceptance of the wave theory of light Maxwell asserted that light was a form of high-frequency lectromagnetic ave ave of Notes electromagnetic wave (wave of electric and magnetic field) Hertz confirmed Maxwell’s predictions 1831-1909 James Clerk Maxwell 1857-1894 Heinrich Hertz
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• This is a pictorial representation, at one instant, of a sinusoidal, linearly polarized electromagnetic wave moving in the x direction Electromagnetic waves ( ) ( ) 2 2 2 2 2 , 1 , t t x E c x t x E = Wave equation for light wave Notes E and B vary sinusoidally with V isible light has the wavelength below 1 micrometer
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Particle Nature • Some experiments could not be explained by the wave nature of light • The photoelectric effect (also discovered by Hertz) was a major phenomenon not explained by waves hen light strikes a metal surface, Notes – When light strikes a metal surface, electrons are sometimes ejected from the surface – The kinetic energy of the ejected electron is independent of the intensity of the light – The wave theory of light predicted higher kinetic energy for higher intensity
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Particle Nature, cont. • Einstein (in 1905) proposed an explanation of the photoelectric effect that used the idea of quantization – The quantization model assumes that the energy of a light wave is present in articles called hotons Notes particles called photons – The energy of each particle of light is proportional to the frequency of the light wave E = hƒ h is Planck’s constant: h = 6.63 x 10 -34 J . s 1879 - 1955 Albert Einstein
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Dual Nature of Light • In view of these developments, light must be regarded as having a dual nature • Light exhibits the characteristics of a wave in some situations and the characteristics of a particle in other situations Notes – Nature prevents testing both qualities at the same time – The explanation of the particle-wave duality of light (and any other particle or wave) is given by quantum mechanics – We will only study situations when light behaves as a wave
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Lectures on light - The Nature of Light Before the...

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