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3.12 Properties of Photons

Chapter Summary/Important Vocab

  • Waves and Light

  • Quantum Theory

  • Electromagnetic Spectrum

  • Atomic Emission Spectrums

Components of a Wave

Low & High Frequency Waves:

The formula

c = hv

  • c = Speed of Light (3 × 10^8 m/s)

  • h = Wavelength (m)

  • v = Frequency ( s^-1 or Hz)

Quantum Theory and Planck

Max Planck 1900

  • Hypothesized that the energy radiated from a heated object, such as a stove element or a lightbulb filament, is emitted in discrete units quanta.

  • As energy increases, so does intensity.

Planck

E = hv

  • E = Energy per Quantum (J)

  • h = Planck’s Constant (6.63 × 10^-34 Js)

  • v = Frequency (s^-1)

The Photoelectric Effect

  • The science world knew that certain clean metal surfaces would shed electrons when certain frequencies of light were shined on them.

  • 1st Fact: Highly intense low frequency light does not eject any electrons, even if it shines on the surface for several days.

  • 2nd Fact: When the threshold frequency is reached, electrons are ejected immediately.

  • 3rd Fact: increasing the Intensity of the Light at a frequency that will cause electrons to eject results at a higher ejection rate. However, all ejected electrons share the same velocity.

  • 4th: Increasing the frequency of the light increases the velocity of the ejected electrons. However

  • , all ejected electrons share the same velocity.

Einstein’s Theory (1905)

  • A Beam of light is a stream of particles called photons.

  • The energy of a photon is related to its frequency according to E = hv.

  • The quantum of Planck is a particle - a photon.

  • If the frequency of a photon is below a certain threshold, no electrons are ejected.

  • If the frequency of the photon is or above a certain threshold, its energy is transferred to the electron.

Planck and Einstein

E = hv

  • E = Energy per Photon (J)

  • h = Planck’s

  • Constant (6.63 × 10^-34 Js)

  • v = Frequency (s^-1)

The Electromagnetic Spectrum is a Continuous Spectrum of Light

Atomic Emission Spectrum

Why do we have different colors of Light?

  • As wavelength frequency changes, color changes.

  • But there is a duality to light:

    • It behaves like a wave, and

    • It behaves like a particle (A photon)

  • As wavelength/frequency changes, the energy per photon changes.

    • E = hv

Absorbing Photons

  • When a photon is absorbed by an atom or molecules, an electron moves up one or more energy levels.

  • The increase in energy is equal to the energy of the photon that was absorbed.

  • The increase in energy is also equal to the difference in energy between the two energy levels.

Emitting Photons

  • When a photon is emitted from an atom or molecule, an electron moves down one or more energy levels..

  • The decreases in energy is equal to the energy of the photon that was released.

  • The decrease in energy is also equal to the difference between the two energy levels.