wave-particle duality of light

WAVE-PARTICLE DUALITY OF LIGHTWAVE-PARTICLE DUALITY OF LIGHTIn 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 interferenceEvidence 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)

PHOTONPHOTON

We have seen that light comes in discrete units (photons) with particle properties (energy and momentum) that are related to thewave-like properties of frequency and wavelength.

MATTER WAVESMATTER WAVES

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In 1923 Prince Louis de Broglie postulated that ordinary matter can havewave-like properties, with the wavelength λ related to momentum p in the same way as for light

de Broglie wavelength

de Broglie relation346.63 10 Jsh

Planck’s constant

Prediction: We should see diffraction and interference of matter waves

De Broglie

NB wavelength depends on momentum, not on the physical size of the particle

ELECTROMAGNETIC ELECTROMAGNETIC WAVESWAVES

JAMES MAXWELL - 1864JAMES MAXWELL - 1864 DISTRIBUTION OF DISTRIBUTION OF

ELECTRIC CHARGES IN ELECTRIC CHARGES IN SPACE PRODUCE SPACE PRODUCE MAGNETIC FIELDS AND MAGNETIC FIELDS AND ELECTRIC FIELDS.ELECTRIC FIELDS.

MAGNETIC FIELD LINES MAGNETIC FIELD LINES ARE CONTINUOUS LOOPSARE CONTINUOUS LOOPS

ELECTRIC FIELD LINES ELECTRIC FIELD LINES BEGIN AND END ON BEGIN AND END ON ELECTRIC CHARGESELECTRIC CHARGES

CHANGING ELECTRIC FIELDS CHANGING ELECTRIC FIELDS PRODUCE MAGNETIC FIELDSPRODUCE MAGNETIC FIELDS

CHANGING MAGNETIC FIELDS CHANGING MAGNETIC FIELDS PRODUCE ELECTRIC FIELDSPRODUCE ELECTRIC FIELDS

ELECTRIC FIELDSELECTRIC FIELDS

MAGNETIC FIELDSMAGNETIC FIELDS

IF TWO CHARGED PARTICLES IF TWO CHARGED PARTICLES OSCILLATE THEN YOU CREATE AN OSCILLATE THEN YOU CREATE AN OSCILLATING ELECTRIC FIELDOSCILLATING ELECTRIC FIELD

AN OSCILLATING ELECTRIC FIELD AN OSCILLATING ELECTRIC FIELD WILL CREATE AN OSCILLATING WILL CREATE AN OSCILLATING MAGNETIC FIELD MAGNETIC FIELD

ELECTROMAGNETIC WAVEELECTROMAGNETIC WAVE THE ELECTRIC FIELD AND MAGNETIC FIELD THE ELECTRIC FIELD AND MAGNETIC FIELD

ARE AT RIGHT ANGLES TO EACH OTHER ARE AT RIGHT ANGLES TO EACH OTHER AND THE DIRECTION OF PROPAGATIONAND THE DIRECTION OF PROPAGATION

6 CHARACTERISTICS OF 6 CHARACTERISTICS OF E-M WAVESE-M WAVES

ALWAYS PRODUCED WHEN CHARGES ALWAYS PRODUCED WHEN CHARGES ARE ACCELERATED. THIS CAUSES A ARE ACCELERATED. THIS CAUSES A LOSS OF ENERGY. THAT ENERGY IS LOSS OF ENERGY. THAT ENERGY IS TRANSMITTED THROUGH THE FIELD.TRANSMITTED THROUGH THE FIELD.

THE FREQUENCY OF THE E-M WAVE THE FREQUENCY OF THE E-M WAVE IS DETERMINED BY THE VIBRATING IS DETERMINED BY THE VIBRATING SOURCESOURCE

THE SPEED OF ALL E-M WAVE IN A THE SPEED OF ALL E-M WAVE IN A VACCUM IS 3.00 X 10VACCUM IS 3.00 X 1088 M/S. M/S. ALL E-M WAVES ALL E-M WAVES OBEY THE UNIVERSAL WAVE EQUATION OBEY THE UNIVERSAL WAVE EQUATION

(V = f x (V = f x λλ AND FOR E-M WAVES C = f x AND FOR E-M WAVES C = f x λλ))

ELECTRIC AND MAGNETIC FIELDS ELECTRIC AND MAGNETIC FIELDS OSCILLATE AT RIGHT ANGLES TO EACH OSCILLATE AT RIGHT ANGLES TO EACH OTHER AND TO THE DIRECTION OF OTHER AND TO THE DIRECTION OF PROPAGATIONPROPAGATION

EXHIBIT MANY WAVE PROPERTIES EXHIBIT MANY WAVE PROPERTIES LIKE – INTERFERENCE, DIFFRACTION, LIKE – INTERFERENCE, DIFFRACTION, REFRACTION, POLARIZATIONREFRACTION, POLARIZATION

CARRY ANGULAR AND LINEAR CARRY ANGULAR AND LINEAR MOMENTUMMOMENTUM

1887 - E-M WAVES 1887 - E-M WAVES WERE FIRST WERE FIRST DETECTED BY DETECTED BY HEINRICK HERTZ HEINRICK HERTZ (RADIO WAVES)(RADIO WAVES)

1889 – MARCONI 1889 – MARCONI TRANSMITTED TRANSMITTED RADIOWAVES OVER RADIOWAVES OVER THE OCEAN WITH NO THE OCEAN WITH NO PHYSICAL PHYSICAL CONNECTIONCONNECTION

The Electromagnetic SpectrumThe Electromagnetic SpectrumMore than meets the eye!More than meets the eye!

WavelengthWavelength The distance from one wave crest to the nextThe distance from one wave crest to the next

Radio waves have longest wavelength and Radio waves have longest wavelength and Gamma rays have shortest!Gamma rays have shortest!

Examples from Space!Examples from Space!

Wavelength UnitsWavelength Units

Meters (like on last slide)Meters (like on last slide)• More commonly in nanometers (1 nm = More commonly in nanometers (1 nm =

1010-9-9 meters)meters) Angstroms still usedAngstroms still used

• Named for Swedish Astronomer who Named for Swedish Astronomer who first named these wavelengthsfirst named these wavelengths

• 1 nanometer = 10 A1 nanometer = 10 Aoo