Section 2 - EM Radiation and Quantum Phenomina

Question 1

What can shining light on a metal do?

Answer 1

Release electrons

Question 2

How can you release electrons from the surface of a metal?

Answer 2

By making sure the light is a high enough frequency (Normally in UV range)

Question 3

State the process for electrons to be released from the surface of a metal

Answer 3

• Free electrons of the surface of the metal absorb energy from the light
• If enough energy is absorbed, the bonds holding it to the metal break, and the electron is released
• Called photoelectric effect, electrons emitted are called photoelectrons

Question 4

What is the name of the electrons emitted in the photoelectric effect?

Answer 4

Photoelectrons

Question 5

What are the 3 conclusions from the photoelectric effect (brief)

Answer 5

• There is a threshold frequency
• There are a range of kinetic energies, varying on frequency and not intensity
• No. of photoelectrons emitted/second is proportional to intensity

Question 6

What is the range of kinetic energies for the photoelectrons being emitted?

Answer 6

0 to some maximum value

Question 7

Why can’t wave theory explain photoelectric effect?

Answer 7

Wave theory predicted that:

• Energy carried is proportional to intensity
• Energy carried would be spread evenly across wavefront
• Each free electron would gain a bit of energy
• Slowly, each electron would gain enough energy to escape

Question 8

What problems does wave theory have the photoelectric effect?

Answer 8

• No explanation for threshold frequency, as wave theory states that regardless of frequency electrons should be emitted
• KE should increase with intensity, but it does not
• Intensity should increase energy transfer to each electron, but it increase no. emitted/second

Question 9

State Einstein’s photon model of light

Answer 9

• EM waves exist in packets, called photons
• Energy carried by one of these photons is E=hf
• Photons have one-to-one interaction, like particles with an electron and the metal surface. A photon would transfer all energy to one electron
• Before an electron can escape, it needs enough energy to break bonds holding it there - called work function

Question 10

What does the photon model explain which wave theory cannot?

Answer 10

The threshold frequency

Question 11

State how the photon model can explain the threshold frequency

Answer 11

• If E gained by electron from a photon > work function, electron is emitted
• If it isn’t, no electrons are emitted, but metal heats up
• For electrons to be released, hf>= work function, so hf = work function

Question 12

How does the photon model explain the maximum kinetic energy of a photoelectron

Answer 12

• Energy transferred to electron = hf
• KE electron carries when leaving metal = hf - energy lost on way out. Electrons deeper in the metal lose more energy than ones on surface which explains range
• Minimum energy it can lose is work function
• Max KE of photoelectron is given by hf = work function + Ekmax, where Ekmax = 1/2mv^2
• KE of electrons is independent fo intensity due to one-to-one interaction. Increasing intensity leads to more photons emitted/second

Question 13

What is the use of the stopping potential?

Answer 13

Gives the max KE

Question 14

Explain how you can use the stopping potential to find out the maximum KE

Answer 14

• Emitted electrons are losing energy by doing work against an applied PD
• Stopping potential, Vs is the PD needed to stop fastest electrons
• WD by PD stopping fastest electrons = energy they were carrying -> eVs = EKmax

Question 15

Explain eVs = Ekmax

Answer 15

e = charge of electron
Vs = stopping potential
Ekmax = max KE

Question 16

How do electrons exist in atoms?

Answer 16

In discrete energy levels

Question 17

What is the level of the ground state?

Answer 17

n=1

Question 18

How can electrons move down an energy level?

Answer 18

By emitting a photon

Question 19

Why can the energy of a photon only take certain values?

Answer 19

Because the transitions for photons to be emitted are between definite energy levels

Question 20

Define an electron volt

Answer 20

KE carried by an electron after it has been accelerated through a PD of 1V

Question 21

What is the energy carried by a photon equal to?

Answer 21

hf, or difference in energies between 2 levels

Question 22

How can electrons move up an energy level?

Answer 22

If they absorb a photon with the exact energy difference between the 2 levels

Question 23

What is the movement of electrons to a higher energy level called?

Answer 23

Excitation

Question 24

What is it called when an electron is removed from an atom?

Answer 24

Ionisation

Question 25

What is the ionisation energy equal to?

Answer 25

Energy needed to completely remove an electron from the ground state

Question 26

How do fluorescent tubes produce light?

Answer 26

By using excited electrons

Question 27

State the process for fluorescent tubes to produce light

Answer 27

• Contain mercury vapour, which accelerate free electrons under a high PD which ionise mercury atoms, producing more electrons
• When free electrons collide with mercury atoms, electrons are excited to higher energy levels
• When excited electrons return to ground states, they emit photons in UV range
• Phosphourus coating inside the tube absorbs these photons, exciting its electrons to much higher levels
• These electrons cascade down energy levels, emitting many lower energy photons in the form of visible light

Question 28

What type of spectrum do fluorescent tubes produce?

Answer 28

Line emission spectra

Question 29

What spectrum does shining white light through a cool gas give?

Answer 29

Absorption spectrum

Question 30

What type of wavelengths do continuous spectra contain?

Answer 30

All types of wavelength

Question 31

What spectra is white light?

Answer 31

Continuous

Question 32

How can you test if the spectra emitted is continuous?

Answer 32

Split the light emitted up via a prism, and the colours should merge without any gaps in the spectrum

Question 33

Why does a continuous spectrum contain all types of wavelength?

Answer 33

Electrons are not confined to energy levels in the object. They electrons are also not bound and are free

Question 34

What do cool gases do to a continuous spectrum?

Answer 34

Remove certain wavelengths

Question 35

In a gas at low temperatures, what state will most electrons be in an atom?

Answer 35

Ground state

Question 36

State how a cool gas removes certain wavelengths from the continuous spectrum

Answer 36

• As most electrons will be at ground states, photons of corresponding wavelengths can be absorbed by electrons to excite them to higher energy levels
• These wavelengths are missing from the continuous spectrum when it comes out of the other side of the gas

Question 37

What do black lines in a spectrum show?

Answer 37

The absorbed wavelengths

Question 38

What is the similarity between an emission and absorption spectra of a particular gas

Answer 38

The black lines in the absorption spectrum match up to the bright lines in the emission spectrum

Question 39

How can light be shown to be a wave?

Answer 39

By interference and diffraction

Question 40

How can light be shown to be a particle?

Answer 40

Photoelectric effect

Question 41

What does electron diffraction show?

Answer 41

Wave nature of electrons

Question 42

How can electrons show diffraction patterns?

Answer 42

When accelerated in a vacuum tube and they interact with the spaces in a graphite crystal - confirms wave like properties

Question 43

Do all particles show wave like properties?

Answer 43

No, not all the time