Origins of Quantum Theory

Origins of Quantum Theory

• Chemists use the specific wavelengths of light emitted by stimulated atoms to identify elements.
• Light is a form of energy known as electromagnetic radiation.
• Visible light is only a small portion of the electromagnetic spectrum, which includes X-rays, ultraviolet radiation, microwaves, and radio waves.
• Stronger electromagnetic radiation has higher frequency, such as alpha and gamma rays.
• Weaker electromagnetic radiation has lower frequency and lower energy.

Two Important Experimental Observations

• These observations established the quantum theory of light.
• Blackbody radiation describes an ideal object that absorbs all light and emits electromagnetic radiation based on its temperature (Kirchoff 1859).
• Max Planck initiated quantum radiation research, noting that no light is reflected from an ideal blackbody.
• Planck's explanation (1900) stated that energy from a blackbody is quantized, meaning it is restricted to whole number multiples of certain energy
• The photoelectric effect involves the release of electrons from a substance when light strikes a metal (Hertz 1887).
• The electrons are released in specific amounts fixed by energy, which is quantized.

Light and Quantum Theory

• Maxwell (1900s) described light as an electromagnetic wave that exerts force on charged particles.
• In 3000 BC, Greeks considered light to be a spectrum of particles.
• Newton described light as particles.
• The photoelectric effect shows the effect of electromagnetic radiation/light on substances such as metals.
• For each path of light, there is a specific amount of fixed energy that is transferred.
• Classical theory suggests brightness (intensity) of light dictates the kinetic energy of liberated electrons, but this is false.
• The frequency (color/energy) of light is the most important factor in the photoelectric effect, not the intensity.
• Einstein (1905) explained that the size of a quantum of electromagnetic energy is directly related to frequency, with one photon of energy ejecting one electron.
• Quantum theory states electromagnetic energy cannot be infinitely subdivided.
• Energy exists as packets or quanta called photons, relating to a specific frequency of light.
• Intensity does not change the energy of electrons, only the number of electrons that are released.
• Lower frequency equals lower energy.
• The equation Ephoton = hv relates energy and frequency, where E is energy, h is a constant, and v is frequency.
• Einstein received the Nobel Prize for explaining the photoelectric effect, yet Max Planck is considered the father of quantum theory.
• Planck developed the original idea, and Einstein further explained it with the photoelectric effect.

Planck's Quantum Hypothesis

• When solids are heated, they emit light; white light is a combination of all colors, meaning hotter objects emit more wavelengths.
• The spectrum of emitted light forms a bell-shaped curve dependent on temperature.
• Planck hypothesized the energies of oscillating atoms in heated solids are multiples of small, discrete quantities of energy; energy is not continuous.
• Planck did not pursue this hypothesis, but Einstein did.
• Light emitted by a hot solid is quantized, arriving in bursts, packages, or bundles rather than a continuous stream.
• As temperature increases, the proportion of larger quanta becomes greater.
• The number and kind of quanta determine the color of the heated object.

The Photoelectric Effect

• Experimentally, light shining on a metal surface releases electrons.
• An ammeter records the electric current as the number of electrons in the circuit.
• Einstein proposed that light consists of a stream of energy packets or quanta called photons.
• A photon of red light has less energy than a photon of UV light.
• Ejection of electrons from a metal surface can be explained by photon-electron collision, where photon energy is transferred to an electron.
• This breaks the free electron to break free from the atom releasing extra energy.
• A certain minimum amount of energy (E) to be absorbed from a single photon to free an electron.
• One electron absorbs one photon, therefore, intensity/amount is not important.
• Quantum is a small, discrete, indivisible quantity.
• Photon is a packet or quantum of electromagnetic energy.
• The energy, E, of a photon depends on the wavelength or frequency.
• Electromagnetic radiation of long wavelength and low frequency, like radio waves, are composed of lower-energy photons.
• Short-wavelength radiation with high frequencies is composed of higher-energy photons.
• Photon energy is directly proportional to frequency and inversely proportional to wavelength.

Experiment Results Summary

• Current is linearly proportional to intensity.
• Current appears with no delay.
• Electrons are emitted only if light frequency exceeds a threshold.
• Maximum energy that electrons come off with increases linearly with frequency.