Quantum Phenomena

Question 1

What is the photoelectric effect?

Answer 1

Photoelectrons are emitted from a metal after light above the threshold frequency is shone on it.

Question 2

What factor affects the threshold frequency?

Answer 2

Different metals have different threshold frequencies.

Question 3

What is the photon model of light?

Answer 3

EM waves travel in discrete packets called a photon, which have energy directly proportional to the energy of the wave.

Question 4

How do the electrons absorb photons?

Answer 4

• If the wave is above the threshold frequency
• Each electron can absorb a single electron
• The number of photelectrons emitted per second is proportional to the intensity of the light but the kinetic energy of the emitted electrons stay the same.

Question 5

What is the work function of a metal?

Answer 5

The minimum energy required for electrons to be emitted from the surface of a metal. (φ)

Question 6

How do you calculate the energy of a photon?

Answer 6

E = hf
or
E = hc/λ

h = Planck’s constant (6.63 x 10^-34 JHz^-1)
f = frequency (Hz)
c = speed of light (3 x 10⁸ ms^-1)
λ = wavelength (m)

Question 7

How do we determine the kinetic energy of a photoelectron emitted from the surface of a metal?

Answer 7

KE = hf - φ

Question 8

When does photoemission occur?

Answer 8

As soon as light with a frequency greater than the threshold frequency is shone onto the metal surface, regardless of intensity.

Question 9

What is the stopping potential?

Answer 9

The potential difference required to apply across the metal to stop the photoelectrons with the highest kinetic energy (at the surface) from escaping.

Question 10

What do we use the stopping potential for?

Answer 10

E = eV
where
E = kinetic energy of the electron (J)
e = charge of the electron (1.6 x 10^-19 C)
V = stopping potential (V)

Question 11

What is eV a measure of? How can you convert it into a simpler unit?

Answer 11

• Unit of energy
• Convert to Joules by multiplying by charge of an electron

Question 12

Where do electrons exist in the atom?

Answer 12

Discrete energy levels

Question 13

What is excitation?

Answer 13

Excitation occurs when an electron absorb a photon with enough energy in order to move it up to a higher discrete energy level (known as an excited state).

Question 14

What occurs in de-excitation?

Answer 14

A photon is emitted with the same amount of energy as the energy difference between the energy levels that the electron moves between.

Question 15

What is ionisation?

Answer 15

Ionisation occurs when an electron gains energy by absorbing a photon with enough energy to allow to escape from the outer shell. This causes the atom to have a net positive charge.

Question 16

What experiment proves wave properties of electrons?

Answer 16

Diffraction grating experiment

Question 17

Why is it easier to demonstrate wave properties of electrons than protons?

Answer 17

It is easier to obtain a stream of electrons.

Question 18

Why do some electrons that collide without atoms maintain their kinetic energy whilst others lose energy?

Answer 18

• One electron does not have enough kinetic energy to excite an electron.
• The other electron provides enough energy to allow excitation of the electron to occur.

Question 19

What is meant by an excited atom?

Answer 19

An atom with an electron in a higher energy level than their ground state.

Question 20

What is the process of fluorescence in a fluorescent tube?

Answer 20

1) A partially evacuated tube with low density mercury gas and is coated with a phosphor powder coating on the inside of the tube has a high voltage applied across the tube.

2) This causes electrons to flow from the cathode to the anode - producing an electron beam.

3) These electrons collide with mercury atoms causing the electrons in them to be excited and move to a higher energy level.

4) These electrons then de-excite to their ground state due to the instability of their excited state and emit a photon in the UV wavelengths.

5) These photons collide with the phosphor powder coating atoms on the surface of the glass causing them to get excited.

6) When these electrons de-excite, they emit photons in the visible light wavelengths.

Question 21

What is the ionisation energy of an atom?

Answer 21

The minimum energy required to remove an electron from an atom from its ground state. It is usually measured in eV.

Question 22

How doe the energy levels change as we move away from the ground state?

Answer 22

Decreasingly negative energies until the ionisation level where the energy value is 0 V.

Question 23

Why do electrons de-excite after they are excited?

Answer 23

They are most stable in their ground states so de-excitation makes them more stable.

Question 24

Explain how light emission spectra work. (6 marks)

Answer 24

1) Electrons can only exist in discrete energy levels.

2) If an electron gains energy, it can move to a higher energy level, as it is excited.

3) The electron then de-excites back to its ground state. It emits a single photon of EM radiation.

4) The photon energy is equal to the difference of energy in the excited and ground states.

5) Photon energy = hf, so the frequency depends on the energy level differences.

6) Each element has a specific set of energy level differences, hence causing them to emit a unique set of frequencies.

Question 25

What are line absorption spectra and how do they differ from line emission spectra?

Answer 25

- They test which frequencies of light are absorbed by a element. - A **single electron** can gain energy from a **single photon**, meaning it **only absorbs certain frequencies of light**. - The photons released from **de-excitation** are emitted in all directions, causing the **intensity in the original direction** to reduce.

Question 26

Why do we use line spectra?

Answer 26

No two elements produce the same set of spectral lines, so we can **uniquely identify elements by their line spectrum**.

Question 27

How do we know that light behaves as both a particle and a wave?

Answer 27

Particle - **photoelectric effect** Wave - **diffraction** or **interference** in a **Young's Slits Experiment**

Question 28

How does the photoelectric effect prove that light behaves as a particle (photons)?

Answer 28

- EM waves carry energy in **discrete** packets called photons. Photoelectrons are only emitted when photons are above the **threshold frequency**. - Waves do not explain this as wave theory suggest the **energy absorbed** by each electron would allow photoelectrons to be emitted if the **exposure time** is long enough. - It also expects that **kinetic energy of emitted electrons** increase with **intensity** but this is not true.

Question 29

How did de Broglie prove that electrons can be have as a wave?

Answer 29

**Diffraction** pattern produced when a **beam of electrons** is directed at a **thin, graphite film**. If it were acting as a particle, we would see **even distribution uniformly** across the screen.

Question 30

What do we observe when we diffract a beam of electrons?

Answer 30

We see **concentric circles** form. A **larger accelerating voltage** reduces **diameter** of a ring.

Question 31

Why do we use a graphite screen for electron diffraction? Why do we see a diffraction pattern form on the screen?

Answer 31

- **Crystalline structure** allows many slits between atoms allowing a diffraction pattern to form - **Fluorescent screen** with **phosphor coating** emits photons to see the circular pattern.

Question 32

What is the equation that relates wavelength and momentum?

Answer 32

p = h/λ p = momentum h = Planck's constant λ = wavelength

Question 33

Why does de Broglie's wavelength prove the diffraction pattern?

Answer 33

- As momentum increases, the wavelength decreases - This causes the concentric rings to get closer together - This aligns with the electron diffraction with smaller radius with increasing voltage

Question 34

How do find kinetic energy from de Broglie's equation?

Answer 34

λ = h/√2mE λ = wavelength h = Planck's constant m = mass E = kinetic energy

Question 35

How does peer review validate a theory?

Answer 35

**Other** scientists check if the experiment is **repeatable** and the results are **reproducible**. This allows us to check validity, originality and significance.

Question 36

What would we be expecting if the electrons were acting only as particles and are passed through a diffraction grating?

Answer 36

They would scatter **uniformly**.

Question 37

Why does the fluorescent screen prove that incident electrons are acting as particles?

Answer 37

- Energy must be provided INSTANTLY or in ONE INTERACTION to excite - particles collide and transfer kinetic energy in one collision - wave could only transfer energy gradually

Question 38

Why are energy levels negative?

Answer 38

- This is the energy that the electron needs to be free - Free is 0J

Question 39

What is a suitable material to give an observable diffraction pattern. Why?

Answer 39

- Graphite - Gaps between atoms act as slits with **similar width as wavelength of electron**