2: Electromagnetic Radiation and the Quantum Phenomena

Question 1

What is the photoelectric effect?

Answer 1

when EM radiaton hits a material, free electrons that are on or near the surface of the metal absorb the energy from the radiation, and if given enough energy they will break from the metal and are released

Question 2

What conclusions can you make from the photoelectric effect experiement?

Answer 2

• for a given metal, no photoelectrons are emitted if the radiation has a frequency below a certain value (the threshold frequency)
• Photoelectrons are emitted with a variety of kinetic energies from 0 to a maximum value. This maximum value increases with the frequency of the radiation
• The intensity of radiation is the amount of energy per second hitting an area of the metal. The maximum kinetic energy is unaffected by varying the intensity of the radiation
• Number of photoelectrons emitted per second is proportional to the intensity of the radiation

Question 3

How can you demonstrate the photoelectric effect?

Answer 3

• A zinc plate is attached to the top of an electroscope (a box containing a piece of metal with a strip of gold leaf attached)
• The zinc plate is negatively charged
• The negatively charged metal repels the gold leaf, causing it to rise up
• UV light is then shone onto the plate
• The energy of the light causes electrons to be lost from the zinc plate via the photoelectric effect
• As the zinc plate and metal lose their negative charge, the gold leaf is no longer repelled and so falls back down

Question 4

What is meant by the work function?

Answer 4

The minimum energy needed to break the bonds so an electron can leave the surface of the metal

Question 5

What happens if the energy gained from the photon is greater than the work function?

Answer 5

An electron is emitted

Question 6

What happens if the energy is lower than the work function?

Answer 6

No electrons will be emitted, the electron will vibrate a bit and release the energy as another photon, the metal will also heat up

Question 7

What is the equation for threshold frequency?

Answer 7

f0= Φ/h

Question 8

What is the photoelectric equation?

Answer 8

hf = Φ + Ek (max)

Question 9

How can you measure maximum kinetic energy?

Answer 9

• Stopping potentials
• Photoelectrons emitted by the photelectric effect can be made to lose their energy by doing work against an applied potential difference.
• The work done by the potential difference in stopping the fastest electrons is equal to the energy they were carrying

Question 10

What is meant by the stopping potential?

Answer 10

The potential difference needed to stop the fastest moving electrons travelling with kinetic energy Ek(max)

Question 11

What equation relates stopping potentials to kinetic energy?

Answer 11

eVs = Ek (max)

where e is the charge on the electron
and Vs is stopping potential in Volts

Question 12

What is an electron volt?

Answer 12

The kinetic energy carried by an electron after it has been accelerated from rest through a potential difference of 1 volt

Question 13

What is 1 eV in joules?

Answer 13

1.6 x 10^-19

Question 14

What is meant by the ground state?

Answer 14

The lowest energy level an electron can be in

Question 15

How can electrons move down energy levels?

Answer 15

By emitting a photon

Question 16

What is meant by electron excitation?

Answer 16

When an electron is in an energy level higher than the ground state

Question 17

How can electrons move down energy levels?

Answer 17

By emitting a photon

Question 18

What is meant by ionisation energy?

Answer 18

The minimum amount of energy needed to remove an electron from the ground state

Question 19

How do fluorescent tubes use the excitation of electrons and photon emission to produce visible light?

Answer 19

• They contain mercury vapour, across which a high voltage is applied
• this high voltage accelerates fast-moving free electrons that ionise some of the mercury atoms, producing more free electrons
• When this flow of free electron collides with the electrons in the mercury atoms, the atomic electrons are excited to a higher energy level
• When these excited electrons return to their ground states, they lose energy by emitting high-energy photons in the UV range
• The phosphor coating on the inside of the tube absorbs these photons, exciting its electrons to much higher energy levels
• These electrons then cascade down the energy levels and lose energy by emitting many lower energy photons of visible light

Question 20

how do diffraction gratings and prisms work?

Answer 20

Diffracting light of different wavelengths at different angles
- A diffraction grating produces much clearer and more defined spectral lines than a prism

Question 21

What is a line emission spectrum?

Answer 21

• A series of bright lines against a black background. Each line corresponds to a particular wavelength of light emitted by the source
• because of this, line spectra provide evidence that electrons in atoms exist in discrete energy levels

Question 22

What is a continuous spectrum?

Answer 22

• split up white light using a prism, which makes the colours all merge into each other without any gaps
• all wavelengths are allowed because the electrons are not confined to energy levels in the object producing the continuous spectrum

Question 23

When do you get a line absorption spectra?

Answer 23

• When light with a continuous spectrum of energy (white light) passes through a cool gas
• At low temperatures, most of the electrons in the gas atoms will be at their ground states
• photons of the correct wavelength are absorbed by the electrons to excite them to higher energy levels
• these wavelengths are then missing from the continuous spectrum when it comes out the other side of the gas

Question 24

Evidence to suggest light was a wave

Answer 24

Diffraction

Question 25

Evidence to suggest light is a particle

Answer 25

The photoelectric effect

Question 26

What is the equation for de Broglie wavelength?

Answer 26

λ = h/mv

Question 27

What the de Broglie wavelength show?

Answer 27

wave properties (wavelength) can relate to moving particle properties (momentum)