CGP AS Section 2 - Electromagnetic Radiation and Quantum Phenomena

Question 1

How does the photon model explain the threshold frequency?

Answer 1

-if the energy gained by an electron (on a metal’s surface) from a photon is greater than the work function, the electron is emitted
-if it isn’t the metal will heat up but no electrons will be emitted
FOR ELECTRONS TO BE RELEASED [hf >= work function]

Question 2

What is the energy transferred to an electron by a photon?

Answer 2

hf

Question 3

What will the kinetic energy of an electron be when leaving a metal?

Answer 3

hf - any energy it has lost on its way out

-electrons deeper down in the metal lose more energy than those on the surface (explaining the range of energies)

Question 4

What is the minimum amount of energy an electron can loose whilst leaving the metal?

Answer 4

the work function
-so the maximum kinetic energy of a photon is given by [hf = work function + Eₖ ₍ₘₐₓ₎] where [Eₖ ₍ₘₐₓ₎ = 0.5mvₖₘₐₓ²]
(given on formula sheet)

Question 5

What can be said about the kinetic energy of the electrons and the intensity of the photons in the photoelectric effect?

Answer 5

the kinetic energy of the electrons is independent of the intensity as they can only absorb one photon at a time, increasing the intensity just means more photons per second each photon still has the same energy as before

Question 6

What is the intensity of the photons?

Answer 6

the number of photons per second on an area

Question 7

What can measure the maximum kinetic energy?

Answer 7

The maximum kinetic energy can be measured using the idea of stopping potential

Question 8

What is the stopping potential?

Answer 8

-the emitted electrons are made to lose their energy by doing work against an applied potential difference
-the stopping potential is needed to stop the fastest moving electrons
[charge on the electron x stopping potential = Eₖ ₍ₘₐₓ₎]

Question 9

Where do electrons exist in atoms?

Answer 9

Electrons only exist in well-defined energy levels (each level given a number)

Question 10

What is the energy level n=1 called?

Answer 10

the ground state

Question 11

How can electrons move down energy levels?

Answer 11

By emitting a photon
-these transitions are between definite energy levels, the energy of each photon emitted can only take a certain value (the energy of the photon produced =the difference in energies between the energy levels)

Question 12

Define electronvolt:

Answer 12

symbol: eV
The kinetic energy carried by an electron after it has been accelerated through a potential difference of 1 volt
[energy gained by electron = accelerating voltage]

Question 13

What does 1eV equal in joules?

Answer 13

1.60 x 10⁻¹⁹J

Question 14

How can electrons move up an energy level?

Answer 14

By absorbing a photon with the exact energy difference between the two levels

Question 15

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

Answer 15

excitation

Question 16

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

Answer 16

It is ionised

Question 17

What is the ionisation energy of an atom?

Answer 17

the amount of energy needed to completely remove an electron from the atom from the ground state

Question 18

How do fluorescent tubes work?

Answer 18

• mercury vapour has a high voltage applied this accelerates fast-moving free electrons that ionise some mercury atoms, producing more free electrons
• the free electrons collide with mercury atoms so the electrons in the atoms become excited to higher energy levels
• excited electrons return to their ground state and emit photons in the UV range

Question 19

What do fluorescent tubes contain?

Answer 19

contain mercury vapour

Question 20

What do fluorescent tubes have on the inside to stop dangerous UV radiation being emitted?

Answer 20

A phosphorus coating on the inside of the tube absorbs the UV photons

Question 21

What happens when the UV radiation is absorbed by the coating in fluorescent tubes?

Answer 21

• absorbing the UV radiation photons
• this excites its electrons to much higher orbits
• electrons then cascade down the energy levels, emitting many lower energy photons in the form of visible light

Question 22

What happens if you split the light from a fluorescent tube with a prism or a diffraction grating?

Answer 22

you get a line spectrum

Question 23

What is a line spectrum seen as?

Answer 23

A series of bright lines against a black background

Question 24

What do the lines on a line spectrum represent?

Answer 24

Each line corresponds to a particular wavelength of light emitted by the source
(since only certain photon energies are allowed you can only see the wavelengths corresponding to these energies

Question 25

What does shining white light through a cool gas do?

Answer 25

it gives a line absorption spectrum

cool gases remove certain wavelengths from the continuous spectrum

Question 26

What can the spectrum of white light be described as and what is an example of this description?

Answer 26

continuous

-if you split light up with a prism the colours all merge into each other (there aren’t any gaps in the spectrum)

Question 27

What E-M waves do hot things emit?

Answer 27

a continuous spectrum in the visible and infrared

Question 28

What does a continuous spectra contain?

Answer 28

• all the wavelengths are allowed because the electrons aren’t confined to energy levels in the object producing a continuous spectrum
• the electrons are not bound to atom and are free

Question 29

What happens to gases at low temperatures?

Answer 29

• most of the electrons in the gas atoms will be in their ground states
• so the electrons can only absorb photons with energies equal to the difference between two energy levels
• photons of corresponding wavelengths are absorbed by the electrons to excite them to higher energy levels (these wavelengths are missing from the continuous spectrum when it comes out the gas)

Question 30

What do the black lines in a continuous spectrum correspond to?

Answer 30

It corresponds to the absorbed wavelengths

Question 31

What happens if you compare the absorption and emission spectra of a particular gas?

Answer 31

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

Question 32

What features show light as a wave?

Answer 32

interference and diffraction

Question 33

What are interference and diffraction patterns?

Answer 33

alternating bands of dark and light

Question 34

How can interference and diffraction of light be explained?

Answer 34

can only be explained using waves interfering constructively (two waves overlap in phase) or interfering destructively (two waves are out of phase)

Question 35

What does the photoelectric effect show about light?

Answer 35

photoelectric effect shows light behaving as a particle

Question 36

How did Einstein explain the mysterious behavior of light from the photoelectric effect?

Answer 36

• by thinking of the beam of light as a series of particle-like photons
• if a photon of light is a discrete bundle of energy, it can interact with an electron in a one-to-one way so all of the energy in the photon is given to one electron

Question 37

What did Louis de Broglie come up with?

Answer 37

Wave-Particle Duality Theory

Question 38

What is Wave-Particle Duality Theory?

Answer 38

if ‘wave-like’ light showed particle properties (photons), ‘particles’ like electrons should be expected to show wave-like properties
-experiments have confirmed the wave nature of electrons

Question 39

What equation did de Broglie come up with?

Answer 39

de Broglie wavelength = h ÷ (mass x velocity)

-mass x velocity = momentum

Question 40

What can the de Broglie wave of a particle be interpreted as?

Answer 40

can be interpreted as a ‘probability wave’

Question 41

What does electron diffraction show?

Answer 41

It shows the wave nature of electrons

Question 42

When are diffraction patterns observed?

Answer 42

when accelerated electrons in a vacuum tube interact with the spaces in a graphite crystal
(this confirms electrons show wave-like properties)

Question 43

What does wave theory mean in terms of the spread of lines in a diffraction pattern?

Answer 43

That the spread of lines in the diffraction pattern increases if the wavelength of the wave is greater

Question 44

In electron diffraction experiments what does a smaller accelerating voltage give?

Answer 44

it gives more widely-spaced rings

Question 45

What happens if you increase the electron speed (and therefore the momentum) what happens to the diffraction pattern? And what does it mean in terms of the de Broglie equation?

Answer 45

• the increase in momentum causes the diffraction pattern circles to squash together toward the middle
• this fits with the de Broglie equation above as the momentum is greater the wavelength is shorter

Question 46

What part of the E-M spectrum is the electron wavelength in a vacuum similar to?

Answer 46

the x-ray part of the spectrum

Question 47

What would the diffraction pattern be of a particle with a greater mass was travelling at the same speed as an electron?

Answer 47

• the diffraction pattern would be a more tightly packed pattern
• this is because the mass increasing gives a shorter de Broglie wavelength

Question 48

When do particles not show diffraction patterns?

Answer 48

-if a particle interacts with an object which isn’t near the size of its de Broglie wavelength