Chapter 5.4 - Wave-particle duality

Question 1

What is Huygens’s principle

Answer 1

Huygens principle is for predicting the future movement of waves. Giving we know the current position of the wave front.

Question 2

What must be considered of every point on a wave front in Huygens’s principle

Answer 2

That every point on the wave front is a new source of circular waves travelling forwards from that point

Question 3

What is found when the superposition is considered in the myriad circular waves of Huygens’s principle

Answer 3

The resultant wave will be the new position of the original wave front

Question 4

What is the evidence that light is a wave?

Answer 4

Young’s two slit experiment and the interference pattern produced via diffraction both require superposition of wave displacements to generate the standing wave pattern seen.

This is only possible if light is behaving as a wave and has the appropriate repeating cycles of displacement that cause ongoing superposition to maintain constructive and destructive interference. Producing nodes and antinodes.

Particles cannot superpose In this way

Question 5

What is a phenomenon exhibited by electromagnetic waves that can only be explained in classical physics by using the idea of waves?

Answer 5

Polarisation

Question 6

Formula to work out the photon energy

Answer 6

Planks constant x frequency

Question 7

If an electron is made to travel at high speeds through a gap what can be seen and what does this prove?

Answer 7

The electrons provide a diffraction pattern proving that electrons can behave as waves

Question 8

What are the evidences for wave movement in:
Light

Electrons

Answer 8

Light - diffraction, interference, polarisation

Electrons - Diffraction, interference

Question 9

What are the evidences for particle movement in:
Light

Electrons

Answer 9

Light - Photoelectric effect

Electrons - ionisation

Question 10

What are photons?

Answer 10

Packets of electromagnetic radiation energy.

Question 11

What is quantisation?

Answer 11

The concept that there is a minimum smallest amount by which a quantity can change.

Question 12

What is wave-particle duality?

Answer 12

The principle that the behaviour of electromagnetic radiation can be described in terms of both waves and photons

Question 13

The photoelectric effect experiment

Answer 13

Ultraviolet light is shone onto -tively charged zinc plate

Plate looses charge - light causes electrons to leave the metal removing the charge.

Gold leaf falls immediately to the zinc plate

Question 14

If electromagnetic radiation only travelled as waves what could be observed in the photoelectric effect? What instead would be observed

Answer 14

If the wave was a small amplitude it could shine for longer and slowly pass energy to the electrons until they gained the energy to leave the zinc

This is not observed however, no mater how long or at what amplitude red light is shone on the zinc plate, no photo electrons will be emitted.

Question 15

What is the setup of the photoelectric effect?

Answer 15

A gold leaf electroscope with a negatively charged zinc plate and a UV light source

Question 16

What are the observations of the photoelectric effect?

Answer 16

Light travels as photons, with a photons energy level being proportional to the frequency

When a photon encounters and electron, it transfers all its energy to the electron (the photo creases to exist)

If an electron gains sufficient energy - more than the work function - it can escape the surface of the metal as a photoelectron

Brighter illumination (amplitude) means more photons per second which means greater number of photoelectrons per second

If electron doesn’t gain sufficient energy from an encounter with a photon to escape the metal surface it will transfer the energy gained from the photon to the metal as a whole before it can interact with another photon. Thus, if the photon energy is too low no photoelectrons are observed

Question 17

Photoelectric effect equation

Answer 17

Work function = (planks x freq) - 0.5xmassxvelocity^2

Question 18

What is the photoelectric cell experiment and how does it work?

Answer 18

Photoelectric cell can be used to measure planks and the work function for a metal.

In a vacuum place the metal as an anode in the cell with a gap t othe cathode. Shine light onto anode and current will be registered n ammeter. Turning up the pd will eventually make anode positive to the point photo electrons are attracted back giving stopping voltage (max Ek)

Question 19

How can a graph be used to find planks constant?

Answer 19

Gradient of the graph where y axis= photoelectrong max Ek and x axis= Frequency

Question 20

What are photoelectrons?

Answer 20

Electrons released from a metal surface as a result of its exposure to electro magnetic radiation

Question 21

What is the work function?

Answer 21

Minimum level of energy needed by electron at the surface of a metal to escape from the metal

Question 22

What is the threshold frequency?

Answer 22

For a given metal is the minimum frequency of electromagnetic radiation that can cause emission of photoelectrons

Question 23

What is the stopping voltage

Answer 23

In an appropriately illuminated photoelectric cell is the minimum voltage needed to reduce current to zero

Question 24

What is the de Broglie equation?>

Answer 24

Electron wavelength = Planks / momentum

Question 25

How can we investigate electron diffraction?

Answer 25

Utilising an electron beam diffraction tube.

This accelerates a beam of electrons through a high voltage and passes the beam through a sliver of graphite. The array of carbon atoms in the graphite act as diffraction grating in two dimensions which produces a circular diffraction pattern. The front of tube has a phosphorescent screen that will show up the diffraction pattern

Question 26

Radiation intensity formula

Answer 26

Intensity = power / area

Question 27

What is the ground sate?

Answer 27

The lowest energy level for a system

Question 28

What is excitation

Answer 28

An energy state for a system that is higher energy that the ground state

Question 29

What is ionisation energy?

Answer 29

Minimum energy required by an electron in an atom’s ground state in order to remove the electron completely from the atom

Question 30

What is a line spectrum

Answer 30

A series of individual lines of colour showing the frequencies present in a light source