5. Electron deflection So, how did Thompson realised that the “cathode rays” are streams of negative particles? The cathode rays could be deflected by electric, or magnetic fields. Draw the path of the electrons that go through the hole in the anode and between the positive and negative electrodes. Cathode Anode Electrodes
6. Electron-volt We can measure the E k (Kinetic Energy) of a charge which is accelerated across a potential difference using this formula: Kinetic Energy (J) = Charge (C) x Voltage (V) The charge of the electron (elementary charge) is e = 1.60 x 10 -19 C So, we can define a new unit of energy, the ELECTRONVOLT: One electronvolt is the Kinetic Energy gained by an electron when it is accelerated through a potential difference of one volt .
7. Neon Lamps But, why do fluorescent tubes emit light of different… The gas that fills the tubes is different, so it emits different colour light when an electric current flows through it. C O L O U R S And why do different gases emit different colours? To answer this question we must understand the nature of light and electromagnetic radiation, and the structure of the atom.
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9. The Hydrogen Spectrum All elements have their own line spectrum emitted when an electric charge is passed through their vapour. For an hydrogen discharge tube this is the line spectrum we would obtain: The lines on the spectrum are the wavelengths of the light produced by the discharge through a hydrogen gas. What is this light made of? 400 nm 500 nm 600 nm 700 nm 656 nm 486 nm 434 nm 410 nm
10. The Photon: a massless particle We’ve always thought of light as a wave, because it behaves like a wave in many cases (e.g. refraction, reflection, diffraction…). However, Einstein discovered that in some instances light behaves like a particle. He called these “particles” PHOTONS. His observations extend to all electromagnetic waves. What are they made of? What are they made of? What do they carry? What do they carry? How is the energy carried affected? On what does their energy depend?
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12. Energy of a photon We can measure the energy of a photon using Einstein’s equation: h = 6.63 x 10 -34 Js Planck constant f = frequency of photon/electromagnetic radiation c = 3 x 10 8 m/s speed of light in a vacuum = wavelength of photon/electromagnetic radiation
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14. Energy Levels and Photon Emission Calculate the energy of all the photons that can be emitted by this atom. E is the energy gap between two energy levels. Which of them is not part of the visible spectrum? E = 0.66 eV E = 1.90 eV Ground State 1 st Exited State 2 nd Exited State 1 st to ground state 2 nd to 1 st state 2 nd to ground state
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