Module 4 - Quantum (Photons)

Question 1

What was Max’s Plancks Theory

Answer 1

That radiation was emitted in discrete packets of energy (quanta)

Question 2

What is a photon

Answer 2

A quantum of energy emitted by electromagnetic radiation

Question 3

Describe the relationship between the energy of the radiation and the frequency of the radiation

Answer 3

The greater the frequency of the radiation the greater the energy emitted:
E α f

Question 4

Describe the relationship between the energy of the radiation and the wavlength of the radiation

Answer 4

The greater the wavelength of the radiation the less the energy emitted:
E α 1/λ

Question 5

What is the rest mass of a photon and hence what is its speed

Answer 5

• 0kg
• Thus it travels at the speed of light

Question 6

What is the equation for the energy of a photon, in two forms

Answer 6

E = hf
E = hc/λ

Question 7

What is the value of Plancks Constant

Answer 7

6.63×10⁻³⁴

Question 8

What is the unit for Planck’s constant

Answer 8

• Joule seconds [Js]

Question 9

What is meant by one Electronvolt

Answer 9

• 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

Question 10

What is the value of one eV in Joules

Answer 10

1.6 x 10⁻¹⁹ J

Question 11

What is the Electronvolt and why is it used

Answer 11

• It is an alternative unit of energy, appropriate for measuring energy at a quantum scale

Question 12

What is the equation of J to eV

Answer 12

J = eV x (elementary Charge)

Question 13

If an electron is accelerated through a pd of n Volts, what is the gain in Ek in eV

Answer 13

n (eV)

Question 14

What is meant by the threshold voltage of an LED

Answer 14

• The value of potential difference below which no current will flow throgh a diode

Question 15

What is the threshold voltage of an LED dependent on and what is the trend

Answer 15

• The colour of the LED
• Lowest to Highest:
• R-G-B

Question 16

What does a high threshold voltage entail

Answer 16

A higher energy per unit of charge in order to emit photons

Question 17

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

Answer 17

• 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

Question 18

Why is it that below the threshold voltage, there is no photons or current

Answer 18

• LEDs are made of semiconductors, which have a valence and conduction band.
• When electrons get enough energy they get excited and cross the band gap from valence to conduction band (leaving positive holes in the valence)
• Over time it will relax back to valence and emit a photon equal to the energy it was supplied to excite
• If the voltage is not high enough the electrons cannot initially excite

Question 19

What is the setup of the Plancks Constant experiment

Answer 19

• Put an LED in parallel to a voltmeter and in series with an ammeter

Question 20

What is the methodology for the Plancks Constant experiment

Answer 20

• 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

Question 21

When the appropriate data has been collected, how do you determine Plancks Constant

Answer 21

-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

Question 22

Formulate an equation that describes conservation of energy between photons and electrons during LED emission

Answer 22

eV = hc/λ
e - elementary charge
V - threshold voltage
h - plancks constant
c - speed of light in a vacuum
λ - wavelength of EM radiation

Question 23

What is the relationship between the energy of a wave its
a) frequency
b) intensity

Answer 23

a) frequency details energy of photon
b) Intensity details the total number of photons emitted