Module 4 - Quantum (Photons) Flashcards
What was Max’s Plancks Theory
That radiation was emitted in discrete packets of energy (quanta)
What is a photon
A quantum of energy emitted by electromagnetic radiation
Describe the relationship between the energy of the radiation and the frequency of the radiation
The greater the frequency of the radiation the greater the energy emitted:
E α f
Describe the relationship between the energy of the radiation and the wavlength of the radiation
The greater the wavelength of the radiation the less the energy emitted:
E α 1/λ
What is the rest mass of a photon and hence what is its speed
- 0kg
- Thus it travels at the speed of light
What is the equation for the energy of a photon, in two forms
E = hf
E = hc/λ
What is the value of Plancks Constant
6.63×10⁻³⁴
What is the unit for Planck’s constant
- Joule seconds [Js]
What is meant by one Electronvolt
- The energy transferred to or from an electron as it passes through 1V of p.d
- This is because E = QV, and Q = e, meaning that the energy transferred is equal to 1V x e
What is the value of one eV in Joules
1.6 x 10⁻¹⁹ J
What is the Electronvolt and why is it used
- It is an alternative unit of energy, appropriate for measuring energy at a quantum scale
What is the equation of J to eV
J = eV x (elementary Charge)
If an electron is accelerated through a pd of n Volts, what is the gain in Ek in eV
n (eV)
What is meant by the threshold voltage of an LED
- The value of potential difference below which no current will flow throgh a diode
What is the threshold voltage of an LED dependent on and what is the trend
- The colour of the LED
- Lowest to Highest:
- R-G-B
What does a high threshold voltage entail
A higher energy per unit of charge in order to emit photons
Why is it that at the threshold pd, the energy transferred by an electron in the LED is approximately equal to the energy of a single emitted photon
- One electron causes the emission of one photon, by the transfer of electric to light energy
- When the photon has enough energy to be emitted, the energy it has gained is approximately equal to the intial energy that the electron had
What is the setup of the Plancks Constant experiment
- Put an LED in parallel to a voltmeter and in series with an ammeter
What is the methodology for the Plancks Constant experiment
- Gradually increase the pd, either using a potential divider or a power pack
- Record the value at which the LED turn on or when the Ammeter reading isnt 0
- Record the wavlength of the colour used in the LED
- Repeat for different LEDs
- Draw a table of colour specific threshold voltages against there wavelength
When the appropriate data has been collected, how do you determine Plancks Constant
-Draw a graph of Threshold Voltage against the inverse of wavelength
- Determine the gradient which is equal to Vλ
- Multiply gradient by e/c, because eV = hc/λ
Formulate an equation that describes conservation of energy between photons and electrons during LED emission
eV = hc/λ
e - elementary charge
V - threshold voltage
h - plancks constant
c - speed of light in a vacuum
λ - wavelength of EM radiation
What is the relationship between the energy of a wave its
a) frequency
b) intensity
c) amplitude
a) E α f
b) E α I
c) E α A²
