what is a quantum?
it is a small discrete unit/packet of energy (plural quanta)
what is a photon?
a photon is a quantum associated with electromagnetic radiation
what was Newton’s theory of light (1672)?
he believed that light was composed of a stream of tiny corpuscles (particles), moving in straight lines from a light source, he supported this theory with his own laws of motion and showed that both reflection and refraction could be explained in terms of particles either bouncing off a surface or travelling more quickly as they move from a less dense to a more dense medium
what did Max Planck discover in 1901?
- he investigated black body radiation and found a link between energy and frequency of the radiation and that energy is emitted in discrete packets of energy (quanta)
- E is directly proportional to f
what is the photon model?
Einstein opened up the idea that light behaves like a particle in 1905 and that light exists in discrete packets of energy. It was de Broglie who proposed that light behaves both like a wave and a particle and as a result quanta of light or other electromagnetic energy became known as photons.
what is the equation used to work out the energy of a single photon?
E = hf
or E = hc/λ
what is a key property of photons?
as they exchange particles and have no mass they travel at c, the speed of light, they also have NO CHARGE
define the electronvolt
the electronvolt is defined as the kinetic energy gained by an electron when it is accelerated through a potential difference of 1 volt, very small unit of energy
what is 1eV equal to in joules?
E = QV
E = 1.6 x 10^-19 C x 1V
1 eV = 1.6 x10^-19J
outline and explain an experiment using LEDs to determine a value for the Planck constant
- the light emitted by LEDs comes in a range of different colours, because the colour or wavelength of the light is related to photon energy we can use different LEDs to a electrical circuit with a 6V power supply, a 1kΩ variable resistor, an ammeter and a voltmeter across the LED
- start off with no current flowing in the circuit, then adjust the variable resistor to increase the p.d until the LED just starts to light up (use a tube block light pollution and increase accuracy)
- record the activation voltage (Vo) across the LED and the wavelength of light the LED emits
- repeat this with at least 5 other LEDs (use different colours to get different wavelengths)
- plot a graph of v against (1/λ to obtain a straight line graph, the gradient will be equal to hc/e because of E = eV and E = hc/λ
- calculate the gradient, times by e and divide by c
- planck’s constant = 6.63 x 10^-34
what happens if you shine electromagnetic radiation of a particular frequency on the surface of a metal?
electrons are emitted from its surface, this phenomenon is known as the photoelectric effect and the electrons that are released are called photoelectrons
how does the photoelectric effect happen?
- free electrons on the surface of the metal absorb energy from the light
- if an electron absorbs enough energy, the bonds holding it to the metal brake and its emitted from the surface
- this is called the photoelectric effect and the electrons emitted are called photoelectrons
what is Einstein’s photoelectric equation and what does it mean?
hf = Φ + KEmax
where hf = the energy transferred to an electron
Φ = work function
KEmax = maximum kinetic energy of the photoelectrons
for an electron to be emitted from the surface of the metal what is necessary?
the energy gained from a photon must be greater than the work function of the metal if it isn’t the metal will heat up but no electrons will be emitted
define work function
the work function of a metal is the minimum energy required to release an electron from its surface, overcoming the electrostatic attraction between the positive metal ions
define threshold frequency
the threshold frequency is the lowest frequency of radiation that will results in the emission of electrons from a particular metal surface, for most metals this frequency occurs in the ultraviolet region of the electromagnetic spectrum
what is the formula for threshold frequency and where does it come from?
since for electrons to be released hf = Φ, the threshold frequency must be f = Φ / h
(since KEmax becomes zero as this is the minimum freq. to provide the minimum amount of energy to just release he electrons from the surface, not to make them move)
does the kinetic energy of the photoelectrons depend on the intensity of the radiation?
no
the kinetic energy of the electrons is independent of the intensity of radiation, because they can only absorb one photon at a time (there is a one-to-one interaction between a photon and a surface electron)
does the kinetic energy depend on the frequency of radiation?
yes
the frequency determines the amount of energy supplied by each photon to the photoelectrons
what does increasing the intensity do at the same frequency (above the threshold frequency)?
- this increases the rate of emission of photoelectrons, more photoelectrons are released but their KE does not increase
- rate of emission of photoelectrons above the threshold freq. is directly proportional to the intensity of the incident radiation
what is Φ dependent on?
the type of metal
outline an experiment to demonstrate the photoelectric effect with a gold-leaf electroscope
- set up a gold-leaf electroscope with a metal plate cap attached at the top of the stem
- the zinc plate is initially negatively charged
- when this is done the metal stem and the gold leaf will repeal each other
- the metal plate is then exposed to ultraviolet light and the photoelectric effect causes free electrons to be ejected
- this causes the electroscope to lose its negative charge (becoming slightly less negative or slightly more positive) meaning the gold leaf is no longer repelled and falls down
- when using visible light nothing happens as the frequency of visible light isn’t above the threshold frequency, no matter how bright or intense the beam of light is the photoelectric effect doesn’t occur
do all the photoelectrons that are emitted have the same kinetic energy?
no, the photoelectrons are emitted with a variety of kinetic energies ranging from zero to some maximum value, this value increases with the frequency of the radiation, and is unaffected by the intensity of the radiation
what occurrences show wave properties of light?
interference and diffraction, this can only be explained using waves interfering constructively and destructively
