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Dual Nature of Light: Energy Quantization and Wave-Particle Duality

This chapter explores the concepts of energy quantization and the dual nature of light. Topics covered include electromagnetic radiation, waves, wavelength and frequency, the photoelectric effect, and evidence of the particle nature of light.

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Dual Nature of Light: Energy Quantization and Wave-Particle Duality

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  1. Chapter 7 Atomic Structure and Periodicity g orbital dz2 orbital

  2. Electromagnetic Radiation

  3. Electromagnetic Radiation Electric Field and Magnetic Field Traveling at Right Angle

  4. Waves • Waves have 3 primary characteristics: • 1. Wavelength : distance between two peaks in a wave. • 2. Frequency : number of waves per second that pass a given point in space. • 3. Speed c: speed of light is 2.9979  108 m/s.

  5. Relation between wavelength and Frequency: Inverse Proportionality • (m)   (s-1) = c (m/s) 1Hz = cycles/s = s-1 1 nm = 10-9 m; 1 Ǻ = 10-10 m; 1 pm = 10-12 m 1nm = 10 Ǻ

  6. As the amplitude increases, the intensity (the brightness) increases Intensity is proportional to A2: I  A2

  7. Nature of Matter? End of 19 th Century: It was believed that MATTER and ENERGY are different No Intermingling between the two forms Particles: Mass; momentum; Localized (position specified) Waves: Massless; delocalized BUT WAS THIS REALLY TRUE? The Answer turns out to be No

  8. UV-Catastrophe: significant historical importance heat blackbody ?!! UV-Catastrophe Planck: E = nh  Planck constant h = 6.626 10-34 j.s n = 1, 2, 3, 4…  frequency s-1 Meaning: Energy is quantized

  9. Planck ContributionQuantization of Energy-- Max Planck: (summer of 1900) E = n h  Planck constant h = 6.626 10-34 j.s n = 1, 2, 3, 4…  frequency s-1 Energy transfer in matter is quantized Energy is absorbed and emitted in discrete quanta Each has an energy = h  = hc 

  10. The Photoelectric Effect Explained by Einstein  Increasing the intensity of light increased the number of photoelectrons, but not their maximum kinetic energy.  Red light will not cause the ejection of electrons from Na, however much the intensity is increased.  A weak violet light will eject only a few electrons, but their maximum kinetic energy is greater than those ejected by intense light of longer wavelengths.

  11. Summary of experimental results from the photoelectric effect • Electrons are not ejected when light shined is below a minimal frequency, called the threshold frequency o • The kinetic energy increased as the frequency of light increased above o • The magnitude of current was independent of  but increased as the intensity of light increases • The kinetic energy of the e was independent of the light intensity

  12. DUAL NATURE of LIGHT Wave-Particle Duality Albert Einstein (Nobel Prize 1921): Light long thought of as waves is now theorized to consist of a stream of particles called photons: Each photon has an energyE = h  Therefore light has a wave nature and a particle nature In the special theory of relativity (1905):E = mc2 mc2 = h c / mc = h/ m.v = linear momentum, a property of particles

  13. (620 nm) E = hν=Eo+K.E.= h νo + ½ mv2 Threshold frequency Kinetic energy of the electron

  14. Evidence of the particle nature of light: Compton’s experiment (1922) Collisions between X-rays and electrons showed that photons actually have momentum as calculated by formula: mc = h/ 

  15. It was thought before that scattering takes place as follows: Emits a spherical wave of the same frequency Beam of radiation Oscillating electron: vibrates with the frequency as the beam of the waves The particle nature of the photon (therefore particle-particle collision) explained why the scattered X-rays had a different frequency than the incident X-rays.

  16. Summary: what do we know so far? 1. Energy quantization: energy can be absorbed or emitted by matter in discrete units called quanta (quantum) 2. Light has particulate and wave nature, known as the dual nature of light

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