G482 Electrons , Photons and Waves - Quantum Physics

Quantum Physics Energy of a photon The photoelectric effect Wave–particle duality Energy levels in atoms

Energy of a Photon What is a Photon ? a quantum/packet/of electromagnetic energy

Energy of a Photon How do you calculate the energy of a photon ?

The Photoelectric Effect Describe the Photoelectric Effect A photon is absorbed by an electron in a metal surface causing an electron to be emitted. Energy of photon = hf Energy is conserved. Only electrons with energy above the work function energy will be emitted. Only light with a frequency greater than the threshold frequency will cause emission Energy=work function + Max KE of electron. The work function is the minimum energy required to release an electron from the surface. Number of electrons emitted depends on light intensity. ( Energy of Electron does not)

The Photoelectric Effect Describe the Photoelectric Effect Experiment A clean zinc plate is mounted on the cap of a gold leaf electroscope where the plate is initially charged negatively . Shine a UV light on the plate and watch the gold leaf collapse as charge leaves the plate, indicating the emission of electrons

The Photoelectric Effect Photoelectric Effect Graphical Analysis

The Photoelectric Effect Photoelectric Effect Graphical Analysis As intensity goes up more electrons emitted As frequency goes up it does not affect the number of electrons emitted As intensity goes up it does not affect the energy of the electrons.

The Photoelectric Effect Photoelectric Effect Numerical Analysis To convert from eV to J multiply by 1.6 x 10 -19

The Photoelectric Effect Describe the LED Experiment to find h Adjust the potential divider to low/zero voltage Connect flying lead to one LED Increase voltage until LED just lights/strikes Repeat several times and average to find Vmin Repeat for each LED Shield LED inside opaque tube to judge light more accurately

Wave Particle Duality When do waves behave like particles ? In the photoelectric effect – the energy of the light does not depend on the amplitude of the wave but on its frequency.

Wave Particle Duality When do particles behave like waves ? Travelling electrons are diffracted by graphite producing a series of diffraction rings. Maximum diffraction occurs when the de Broglie wavelength of the electrons is similar to the gap between layers of carbon atoms.

Wave Particle Duality What is the de Broglie wavelength for an electron ? Travelling electrons are observed to behave as waves/show wavelike properties Where the electron wavelength depends on its speed/momentum

Energy Levels in Atoms What is an Emission Line Spectrum? light emitted from excited isolated atoms produces a line spectrum a series of sharp/bright/coloured lines against a dark background

Energy Levels in Atoms What is an Absorption Line Spectrum? in an absorption spectrum a series of dark lines appears against a bright background/within a continuous spectrum

Energy Levels in Atoms Why are spectra important? They reveal that electrons within atoms can only occupy certain energy levels.

Explaining Line Spectra

Energy Levels in Atoms Numerical and Graphical Analysis An energy-level diagram for the hydrogen atom Numerical and Graphical Analysis Electrons have discrete energy levels within atoms - they cannot have any energy . hf or hc/λ = E1 – E2