Untitled Quiz

The Nature of Light

• Leonardo da Vinci first proposed a wave model of light in the late 1400s.
• Francesco Grimaldi described diffraction experiments in a paper in the 1660s.
• Christiaan Huygens set out wave theory in Traité de la lumière in 1690.
• Robert Hooke and Robert Boyle supported wave theory in the late 1600s.
• Isaac Newton rejected wave theory, arguing light properties are explained by particle model.
• The particle theory was generally accepted for the next century.
• Thomas Young gave an account of double-slit interference in 1802.
• Leon Foucault showed light traveled more slowly in water than air in 1853.
• Albert Einstein showed photoelectric effect is consistent with particle theory in 1905.
• Louis de Broglie produced theory of wave-particle duality in the 1920s.
• Davisson and Thomson independently showed electrons can be diffracted by crystals in 1927.

History of Quantum Mechanics

• Scientists were fascinated by how hot objects glowed differently with increasing temperature in the early 1900s.
• Higher temperature results in increased intensity and frequency of emitted light.
• All objects above absolute zero (0K or -273°C) radiate electromagnetic waves.
• Warm objects, around 30-40°C, emit infrared radiation, which is lower in frequency.
• A theoretical blackbody absorbs all electromagnetic waves at temperatures lower than the environment.
• A blackbody emits all electromagnetic waves at temperatures higher than the environment.
• Intensity of electromagnetic waves emitted by a blackbody is dependent on temperature.
• The higher the temperature, the lower the wavelength (higher frequency) of electromagnetic waves emitted by a blackbody.

The Ultraviolet Catastrophe

• Classical physics assumed light was continuous, explaining the visible spectrum as a range of colors.
• Theory created a problem where a blackbody would release infinite energy in the ultraviolet range if light was continuous.
• This ultraviolet catastrophe was uncovered by Rayleigh and Jeans.
• The ultraviolet catastrophe violates the conservation of energy law.

Planck's Quantum Theory

• Maxwell Planck proposed a new theory to solve the ultraviolet catastrophe.
• Energy emitted and absorbed by an object must be in discrete quantities called quanta.
• Planck's Contribution: Observed and analyzed radiation data from blackbodies.
• Theorized the energy of radiation and its frequency are related by Planck's constant (h = 6.63x10-34).

Planck's Law vs. Rayleigh-Jeans Law

• Both laws show similarities at higher wavelengths.
• At lower wavelengths, the Rayleigh-Jeans law tends to infinity.

Einstein's Light Quantum Theory

• Light travels as packets of energy called photons.
• Each photon's energy is proportional to its frequency.
• Described by the formula E = hf, where h equals 6.63 x 10^-34 Js
• A photon is defined as a discrete packet of energy.

Photon Relationships

• If the frequency of a photon increases, its energy increases.
• Increasing the energy of a photon will decrease it's wave length

Calculating Frequency and Energy of a photon

• The frequency of an electromagnetic wave with a wavelength of 600nm is 5x10^14 Hz.
• The energy of a photon of the same wave is 3.3x10^-19 J.

Conversions Using Electron Volts

• Electron volt (eV) is a unit for energy.
• It is suitable when working with small energy values such as within an atom.
• To convert from electron volts to joules, multiply by 1.6 x 10^-19.
• To convert from joules to electron volts, divide by 1.6 x 10^-19.
• 1 eV is equal to 1.6 x 10^-19 C .

Energy Level Diagrams

• Free electrons on the surface have zero energy.
• Electrons gain energy to move up energy levels.
• Electrons must gain the exact amount of energy, equal to the difference in energy levels.

Excitation

• Electrons exist in discrete energy levels.
• Excitation is when electrons gain the exact energy to move up an energy level.
• The energy gained is equal to the difference between the two energy levels (E2 - E1).
• Electrons can gain energy by absorbing the exact amount of energy from one photon.
• Electrons can gain energy during a particle collision

De-excitation

• De-excitation occurs when electrons lose the EXACT energy to move down an energy level.
• The energy lost is equal to the difference between the two energy levels (E2 - E1).
• Electrons can only lose energy by emitting a photon of electromagnetic radiation.
• The ground state is the lowest energy level that can be occupied by an electron.
• The ground state is often labeled with a quantum number of "n = 1".

Relationship between light and colours

• Wavelengths for the visible light spectrum range from 390nm to 700nm.

Hydrogen Atom Transitions

• An electron in a hydrogen atom can undergo 10 different transitions to lead to either excitation or de-excitation.

Line Absorption Spectra

• White light passes through a cold gas where the atoms become excited.
• Missing Wavelengths: Electrons exist in discrete energy levels.
• When visible light photons pass through an elemental gas.
• Absorption occurs when electrons excite to a higher energy level
• Absorption only occurs when photons of energy equal the difference between two energy levels are absorbed.
• A photon’s energy equal its frequency (E = hf).
• Specific frequencies of photons are absorbed.
• Absorbed frequencies appear as black lines on the absorption line spectra.

Line Emission Spectra

• A hot gas has electrons already in an excited state.
• Specific wavelengths of light on a line emission's spectrum originate from discrete energy levels.
• The electrons releases photons when they de-excite.
• Wavelengths of light appear as coloured lines on the line emission spectra.

Intensity

• Radiation Flux measures the amount of power per given area, it is the the term intensity.
• SI Units for intensity are W/m^2
• The surface are can be calculated by A = πr^2
• If the Power is known, then the energy can be calculated.