Module 4 Quantum Physics Flashcards
What is a photon?
A quantum of energy of EM radiation.
State the equation relating Energy and the frequency of light.
E=hf
What is h, what is it’s value?
It is the planck’s constant.
h=6.63x10^-34
Can a photon transfer some of it’s energy while interacting?
No, it can only transfer all or none of it’s energy.
Define what is threshold voltage and what is the efficiency of transfer of electrical energy to light energy at threshold voltage?
It is the minimum voltage required by an LED to emit light.
100% efficiency.
Describe an experiment in which you can use LEDs to figure out value for h.
Apparatus: A number of LEDs of known wavelength, A source of emf, wires, variable resistor, voltmeter.
Attach the source of emf, LED and variable resistor in series. Attach the voltmeter parallel to the LED.
Make the resistance of the circuit such that no current passes through it and LED doesn’t light up, by increasing the resistance of the variable resistor.
Then decrease the resistance of the variable resistor by small amounts until the LED just lights up. Measure the voltage across the LED at this point.
Take an average of three readings of this threshold voltage and use other LEDs to find their threshold voltage as well.
Plot a graph of V against 1/Lambda by
V=E/Q, V=hc/Qxlambda,
V=hc/Q x 1/lambda +0.. y=mx+c
Find gradient and times it by Q/c to find h.
Why isn’t Joules used to measure Energy of photon?
What other unit is used? What is it’s value in joules?
Joules is too big for use in stating photon energy, hence electron volts are used instead as 1eV =1.6x10^-19J
Define an electron-volt and write the definition in equation form.
It is the kinetic energy gained by an electron as it is accelerated through a pd of 1 volt.
eV=1/2mv^2
What is the photoelectric effect?
It is the emittance of electrons from the surface of a metal when an em radiation of high enough frequency/energy is shone on the metal.
Describe the gold leaf experiment.
An electroscope with a zinc cap, outside evacuated chamber, connected to a brass stem and the brass stem connected to a gold leaf, in an evacuated chamber are negatively charged. This causes the stem to repel the gold leaf making it float away.
When em radiation of high enough energy (or threshold frequency) is shone on the zinc cap, the electrons on the surface of the metal are emitted, causing the negative charge on the electroscope to be lost.
This causes the gold leaf to fall towards the stem.
How does the gold leaf electroscope prove the particulate nature of Em radiation?
The electrons are emitted when incident radiation is of a high enough frequency called the threshold frequency. According to wave theory the electrons should’ve been emitted eventually.
The emitted electrons had kinetic energies ranging from zero to a maximum value, showing that the max ke was directly proportional to the frequency of the incident photons. According to wave theory the ke should be proportional to intensity but increasing intensity does not increase max ke.
What is directly proportional to intensity of EM radiation in gold leaf electroscope?
The rate of photoelectrons emitted per second, if the frequency of em radiation is above or equal to threshold frequency
State what the wave theory expects from the gold leaf experiment.
The wave theory states that energy of wave is proportional to intensity and that energy can be added up on surface electrons causing them to be emitted eventually regardless of the frequency of the em radiation.
An increase in intensity is expected to increase ke of emitted photoelectrons.
What is the energy gained by an electron equal to when, em radiation is shone on a metal.
What should the energy be equal to, to allow electron emission?
Energy gained =hf.
Energy = work function of metal
What is the name of electrons emitted by photons?
Photoelectrons.
