Electromagnetic Radiation and Quantum Phenomena Flashcards
What is the photoelectric effect?
If an free electron on the surface of a metal absorbs enough energy, the bonds holding it to the metal break and the electron is released as a photoelectron
For most metals, in what range is the necessary frequency to emit photoelectrons?
In the ultraviolet range
State the 4 main conclusions of the photoelectric effect experiment
- For a given metal, no photoelectron are emitted below a certain frequency, the threshold frequency
- The photoelectrons are emitted with a variety of kinetic energies, ranging from zero to a maximum value. This max value varies depending on the frequency of radiation
- Intensity of radiation is amount of energy per second hitting an area of the metal, max kinetic energy is unaffected by variations in intensity
- Number of photoelectrons emitted per second is proportional to intensity of radiation
Define Threshold frequency
Minimum energy needed to overcome the work function
Define Work function
Minimum energy required for electron to leave the surface of a metal
Describe an experiment to demonstrate the photoelectric effect
- A zinc plate is attached to the top of an electroscope( a box containing a piece of metal with a strip of gold leaf attached)
- The zinc plate is negatively charged which repels the gold leaf, causing it to rise up
- UV light is then shone on the zinc plate, the light’s energy then causes electrons to be lost from the zinc plate via the photoelectric effect
- As zinc plate and metal lose their charge, gold leaf no longer repels, so falls back down
What happens if the energy provided to an electron is less than the work function?
The electron will shake about a bit, then emit the energy as a photon
Define photon
A quanta of energy
Define stopping potential
The potential difference needed to stop the fastest moving electrons travelling with maximum kinetic energy. Work done by the p.d in stopping the fastest electrons is equal to the energy they were carrying
Define the electronvolt
A unit of energy. The energy gained when an electron is accelerated by a p.d of 1V
Describe discrete energy levels in atoms
- Electrons in an atom can only exist in certain energy levels
- Each level is numbered, with n = 1 being the ground state
- An electron is excited when it’s above ground state
How can electrons move down an energy level?
By emitting a photon
Define excitation
When an electron moves to a higher energy level by absorbing a photon with the exact energy difference between the two levels
What is ionisation?
When an electron is removed from the atom. The energy of each energy level within an atom shows the amount of energy needed to remove an electron from that level
Define ionisation energy
The amount of energy needed to remove an electron from the ground state
Describe how fluorescent tubes works(6 marks)
- Contain mercury vapour, across which a high voltage is applied
- This voltage accelerates fast moving free electrons, ionising some of the mercury atoms, producing more free electrons
- When the flow of free electrons collide with the electrons in the mercury atoms, the atomic mercury electrons are excited to a higher level
- The excited electrons emit high energy photons in the UV range
- Photons emitted has varying energies and wavelength corresponding to different transitions of electrons
- A phosphorus coating on the inside of the tube absorbs the photons, exciting its energy levels to a much higher energy levels
- These electrons cascade down the energy levels and lose energy by emitting many lower energy photons of visible light
What is an emission spectrum?
A spectrum of bright lines on a dark background corresponding to different wavelength caused by the de-excitation of electrons in the atoms of the gas
What is an absorption spectrum?
A light spectrum with dark lines corresponding to different wavelength of light that have been absorbed by the atoms in the gas
What happens when you compare the line absorption and emission spectra of a particular gas?
Black lines in the absorption spectrum match up to bright lines on the emission spectrum
Define Wave-particle duality
All particles have both particle and wave properties. Waves can also show particle properties
Give 2 examples of wave-particles duality
- Diffraction
- The photoelectric effect
Describe how diffraction patterns can be observed
- Electrons are accelerated in high velocities in a vacuum and then passed through a graphite crystal
- As the electrons pass through the spaces between the crystal atoms, they diffract like waves passing through a narrow slit and produce patterns of rings
According to wave theory, what factors increase the spread of the diffraction pattern?
- Increasing wavelength
- A smaller electron accelerating voltage
What are the disadvantages of light microscopes compared to electron microscopes?
- Diffraction effects blur detail on an image
- Light blurs out more detail than electron waves do as the wave length is shorter. allowing it to resolve finer detail
State the wavelength range of visible light
380nm-750nm
State the colours of visible light, from the longest wavelength to the shortest wavelength
- Red
- Orange
- Yellow
- Green
- Blue
- Violet
State the wavelength range of violet visible light
380nm-450nm
State the wavelength range of blue visible light
450nm-495nm
State the wavelength range of green visible light
495nm-570nm
State the wavelength range of yellow visible light
570nm-590nm
State the wavelength range of orange visible light
590nm-620nm
State the wavelength range of red visible light
620nm-750nm
