Electromagnetic Radiation and Quantum Phenomena

Question 1

What is the photoelectric effect?

Answer 1

If an free electron on the surface of a metal absorbs enough energy, the bonds holding it to the metal break and the electron is released as a photoelectron

Question 2

For most metals, in what range is the necessary frequency to emit photoelectrons?

Answer 2

In the ultraviolet range

Question 3

State the 4 main conclusions of the photoelectric effect experiment

Answer 3

• For a given metal, no photoelectron are emitted below a certain frequency, the threshold frequency
• The photoelectrons are emitted with a variety of kinetic energies, ranging from zero to a maximum value. This max value varies depending on the frequency of radiation
• Intensity of radiation is amount of energy per second hitting an area of the metal, max kinetic energy is unaffected by variations in intensity
• Number of photoelectrons emitted per second is proportional to intensity of radiation

Question 4

Define Threshold frequency

Answer 4

Minimum energy needed to overcome the work function

Question 5

Define Work function

Answer 5

Minimum energy required for electron to leave the surface of a metal

Question 6

Describe an experiment to demonstrate the photoelectric effect

Answer 6

• 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 which repels the gold leaf, causing it to rise up
• UV light is then shone on the zinc plate, the light’s energy then causes electrons to be lost from the zinc plate via the photoelectric effect
• As zinc plate and metal lose their charge, gold leaf no longer repels, so falls back down

Question 7

What happens if the energy provided to an electron is less than the work function?

Answer 7

The electron will shake about a bit, then emit the energy as a photon

Question 8

Define photon

Answer 8

A quanta of energy

Question 9

Define stopping potential

Answer 9

The potential difference needed to stop the fastest moving electrons travelling with maximum kinetic energy. Work done by the p.d in stopping the fastest electrons is equal to the energy they were carrying

Question 10

Define the electronvolt

Answer 10

A unit of energy. The energy gained when an electron is accelerated by a p.d of 1V

Question 11

Describe discrete energy levels in atoms

Answer 11

• Electrons in an atom can only exist in certain energy levels
• Each level is numbered, with n = 1 being the ground state
• An electron is excited when it’s above ground state

Question 12

How can electrons move down an energy level?

Answer 12

By emitting a photon

Question 13

Define excitation

Answer 13

When an electron moves to a higher energy level by absorbing a photon with the exact energy difference between the two levels

Question 14

What is ionisation?

Answer 14

When an electron is removed from the atom. The energy of each energy level within an atom shows the amount of energy needed to remove an electron from that level

Question 15

Define ionisation energy

Answer 15

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

Question 16

Describe how fluorescent tubes works(6 marks)

Answer 16

• Contain mercury vapour, across which a high voltage is applied
• This voltage accelerates fast moving free electrons, ionising some of the mercury atoms, producing more free electrons
• When the flow of free electrons collide with the electrons in the mercury atoms, the atomic mercury electrons are excited to a higher level
• The excited electrons emit high energy photons in the UV range
• Photons emitted has varying energies and wavelength corresponding to different transitions of electrons
• A phosphorus coating on the inside of the tube absorbs the photons, exciting its energy levels to a much higher energy levels
• These electrons cascade down the energy levels and lose energy by emitting many lower energy photons of visible light

Question 17

What is an emission spectrum?

Answer 17

A spectrum of bright lines on a dark background corresponding to different wavelength caused by the de-excitation of electrons in the atoms of the gas

Question 18

What is an absorption spectrum?

Answer 18

A light spectrum with dark lines corresponding to different wavelength of light that have been absorbed by the atoms in the gas

Question 19

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

Answer 19

Black lines in the absorption spectrum match up to bright lines on the emission spectrum

Question 20

Define Wave-particle duality

Answer 20

All particles have both particle and wave properties. Waves can also show particle properties

Question 21

Give 2 examples of wave-particles duality

Answer 21

• Diffraction
• The photoelectric effect

Question 22

Describe how diffraction patterns can be observed

Answer 22

• Electrons are accelerated in high velocities in a vacuum and then passed through a graphite crystal
• As the electrons pass through the spaces between the crystal atoms, they diffract like waves passing through a narrow slit and produce patterns of rings

Question 23

According to wave theory, what factors increase the spread of the diffraction pattern?

Answer 23

• Increasing wavelength
• A smaller electron accelerating voltage

Question 24

What are the disadvantages of light microscopes compared to electron microscopes?

Answer 24

• Diffraction effects blur detail on an image
• Light blurs out more detail than electron waves do as the wave length is shorter. allowing it to resolve finer detail

Question 25

State the wavelength range of visible light

Answer 25

380nm-750nm

Question 26

State the colours of visible light, from the longest wavelength to the shortest wavelength

Answer 26

• Red
• Orange
• Yellow
• Green
• Blue
• Violet

Question 27

State the wavelength range of violet visible light

Answer 27

380nm-450nm

Question 28

State the wavelength range of blue visible light

Answer 28

450nm-495nm

Question 29

State the wavelength range of green visible light

Answer 29

495nm-570nm

Question 30

State the wavelength range of yellow visible light

Answer 30

570nm-590nm

Question 31

State the wavelength range of orange visible light

Answer 31

590nm-620nm

Question 32

State the wavelength range of red visible light

Answer 32

620nm-750nm