WAVE-PARTICLE DUALITY OF LIGHT

1. WAVE-PARTICLE DUALITY OF LIGHT In 1924 Einstein wrote:- “ There are therefore now two theories of light, both indispensable, and … without any logical connection.” • Evidence for wave-nature of light • Diffraction and interference • Evidence for particle-nature of light • Photoelectric effect • Compton effect • Light exhibits diffraction and interference phenomena that are only explicable in terms of wave properties • Light is always detected as packets (photons); if we look, we never observe half a photon • Number of photons proportional to energy density (i.e. to square of electromagnetic field strength)

2. PHOTON

3. Planck’s constant De Broglie MATTER WAVES We have seen that light comes in discrete units (photons) with particle properties (energy and momentum) that are related to the wave-like properties of frequency and wavelength. In 1923 Prince Louis de Broglie postulated that ordinary matter can have wave-like properties, with the wavelength λ related to momentum pin the same way as for light de Broglie relation de Broglie wavelength NB wavelength depends on momentum, not on the physical size of the particle Prediction:We should see diffraction and interference of matter waves

4. ELECTROMAGNETIC WAVES

5. JAMES MAXWELL - 1864 • DISTRIBUTION OF ELECTRIC CHARGES IN SPACE PRODUCE MAGNETIC FIELDS AND ELECTRIC FIELDS. • MAGNETIC FIELD LINES ARE CONTINUOUS LOOPS • ELECTRIC FIELD LINES BEGIN AND END ON ELECTRIC CHARGES

6. CHANGING ELECTRIC FIELDS PRODUCE MAGNETIC FIELDS • CHANGING MAGNETIC FIELDS PRODUCE ELECTRIC FIELDS

7. ELECTRIC FIELDS

8. MAGNETIC FIELDS

9. IF TWO CHARGED PARTICLES OSCILLATE THEN YOU CREATE AN OSCILLATING ELECTRIC FIELD • AN OSCILLATING ELECTRIC FIELD WILL CREATE AN OSCILLATING MAGNETIC FIELD

10. ELECTROMAGNETIC WAVE • THE ELECTRIC FIELD AND MAGNETIC FIELD ARE AT RIGHT ANGLES TO EACH OTHER AND THE DIRECTION OF PROPAGATION

11. 6 CHARACTERISTICS OF E-M WAVES • ALWAYS PRODUCED WHEN CHARGES ARE ACCELERATED. THIS CAUSES A LOSS OF ENERGY. THAT ENERGY IS TRANSMITTED THROUGH THE FIELD. • THE FREQUENCY OF THE E-M WAVE IS DETERMINED BY THE VIBRATING SOURCE

12. THE SPEED OF ALL E-M WAVE IN A VACCUM IS 3.00 X 108 M/S. ALL E-M WAVES OBEY THE UNIVERSAL WAVE EQUATION (V = f x λ AND FOR E-M WAVES C = f x λ) • ELECTRIC AND MAGNETIC FIELDS OSCILLATE AT RIGHT ANGLES TO EACH OTHER AND TO THE DIRECTION OF PROPAGATION

13. EXHIBIT MANY WAVE PROPERTIES LIKE – INTERFERENCE, DIFFRACTION, REFRACTION, POLARIZATION • CARRY ANGULAR AND LINEAR MOMENTUM

14. 1887 - E-M WAVES WERE FIRST DETECTED BY HEINRICK HERTZ (RADIO WAVES) • 1889 – MARCONI TRANSMITTED RADIOWAVES OVER THE OCEAN WITH NO PHYSICAL CONNECTION

15. More than meets the eye! The Electromagnetic Spectrum

16. Wavelength • The distance from one wave crest to the next • Radio waves have longest wavelength and Gamma rays have shortest!

17. Examples from Space!

18. Wavelength Units • Meters (like on last slide) • More commonly in nanometers (1 nm = 10-9meters) • Angstroms still used • Named for Swedish Astronomer who first named these wavelengths • 1 nanometer = 10 Ao