The Photoelectric Effect

Question 1

threshold frequency

Answer 1

The lowest frequency of light that when shone on a metal will cause electrons to be released from it, ( by the photoelectric effect )

Question 2

work function

Answer 2

The minimum amount of energy required for an electron to escape a metals surface

Question 3

Equation with work function and threshold frequency

Answer 3

fo = Φ / h

Question 4

Max Plancks wave packets

Answer 4

Max Planck was the first person to suggest that EM waves can only be released in discrete packets, or quanta
E = hf = hc / λ
E = Energy of one wave-packet in J

Question 5

Max kinetic energy equation

Answer 5

hf = Φ + Ekmax
or Ekmax = Φ + 0.5mvmax squared

Question 6

What is the photoelectric effect?

Answer 6

If you shine radiation of a high enough frequency onto a surface of metal, it will instantly admit electrons.
For most metals, the necessary frequency falls in the ultraviolet range
Because of the way atoms are bonded together and metals, metals contain electrons that are able to move about the metal
The free electrons on or near the surface of the metal absorb energy from radiation making them vibrate
Electron absorbs enough energy the bond holding it to the metal can break and the electron can be released. This is called the photoelectric effect and the electrons admitted are called photoelectrons.

Question 7

Main conclusions to the photoelectric effect:

Answer 7

-For a given metal no photo electrons are emitted if the radiation has a frequency below a certain value - this is called a threshold frequency
-Electrons are emitted with a variety of kinetic energies ranging from 0 to maximum value. This value of maximum kinetic energy increases with the frequency of radiation.
-The intensity of radiation amount of energy energy per second hitting an area of the metal the maximum kinetic energy of the photo electrons is unaffected by the varying intensity of the radiation
-The number of electrons admitted per second is proportional to the intensity of the radiation

Question 8

Kinetic energy of photo electrons

Answer 8

The higher the intensity of the wave the more energy should transfer to each electron-the kinetic energy of the electrons should increase with intensity
Theory can’t explain the fact that the kinetic energy depends on the frequency in the photo Electric effect

Question 9

Einstein’s photons

Answer 9

Sign went further by suggesting that electromagnetic waves and the energy they carry can only exist in discrete packets. He calls these wave packets photons
He saw these photos of lights as having one and one, particle-like interaction with an electron in a metal surface
Each photon would transfer all its energy to one specific electron
The photon model could be used to explain the photo electric effect

Question 10

Demonstrating the photo Electric effect

Answer 10

The photo Electric effect can be demonstrated with a simple experiment
Zinc plate is attached the top of an electroscope (which is a box containing a piece of metal with a strip of gold leaf attached)
The zinc plate is negatively charged (which means the metal in the box is negatively charged). The negatively charged metal the gold leaf, causing it to rise up.
UV light is then shone onto the zinc plate
The energy of the light causes electrons to be lost from the zinc plates via the photoelectric effect.
As the sink plates and the metal lose their negative charge the gold leaf is no longer repelled and so falls back down

Question 11

Stopping potential

Answer 11

The maximum kinetic energy can be measured using the idea of stopping potential
Photo electrons admitted by the photo electric effect can be made to lose energy by doing work against an applied to potential difference
The stopping potential is the potential difference needed to stop the fastest moving electrons travelled it with kinetic energy
The work done by the potential difference in stopping the fastest electrons is equal to the energy they were carrying
eVs = Ek(max)
e = 1.6x10-19
Vs = stopping potential

Question 12

The electron volt

Answer 12

The kinetic energy carried by an electron after it’s been at accelerated from rest through a potential difference of one volt
The energy gained by an electron (eV) is equal to the accelerating voltage(V)
1eV = 1.6x10^-19

Question 13

Excitation

Answer 13

The movement of an electron to a higher energy level in an atom

Question 14

Ground state

Answer 14

The lowest energy level of an atom or the lowest energy level for an electron in an atom

Question 15

Discrete energy levels in an atom

Answer 15

Electrons in an atom can only exist in certain well-defined energy levels
Each level is given a number , with N=1 representing the low energy level and electron can be, the ground state

We say that’s an electron is excited when it’s in an energy level higher than the ground state

Question 16

Electrons moving down an energy level

Answer 16

Elections can move down an energy level by emitting a photon
Since these transitions are between definite energy levels, the energy of each photon emitted can only take a certain value

Question 17

Electron transitions

Answer 17

The energy carried by a photon emitted after a transition is equal to the difference in energies between the two levels of the transition
Electrons can also move up energy levels if they absorb a photon with the exact energy difference between the two levels
The movement of an electron to a higher energy level is called excitation

Question 18

Ionisation

Answer 18

When an electron has been removed from an atom, the atom is ionised
The energy of each energy level within an atom or shows the amount of energy needed to remove the electron from that level

Question 19

The ionisation energy

Answer 19

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

Question 20

Photon emission-fluorescent tubes

Answer 20

Fluorescent tubes use the excitation of electrons and photon emission to produce visible light
They contain mercury vapour which a high voltage is applied
The high voltage accelerates fast moving electrons that ionised some of the mercury atoms producing more free electrons
When this flow of electrons collides with the electrons in the mercury atoms The atomic mercury electrons are excited to a higher energy level
When these electrons return to their ground states they lose energy by emitting high energy photons in the UV range
The photons admitted have a range of energies and wavelengths that correspond to the different transitions of the electrons
Coating on the inside of the tube absorbs these photons, exciting its electrons to a much higher energy level
These electrons, then cascade down the energy levels and lose energy by many lower energy photons of visible light

Question 21

Line emission spectrum

Answer 21

Seen as a series of bright lines against the black background, each line corresponding to a particular wavelength of light emitting by the source
Spectrum provides evidence that the electrons in the atoms exist in discrete energy levels. Atoms can only admit photons of energies equal to the difference between two energy levels.
Since only certain photon energies are allowed, only see the corresponding wavelengths in the line spectrum

Question 22

Getting a line spectrum

Answer 22

If you split the lights from the fluorescent tube with a prism or a diffraction grating you get a line spectrum
Diffraction gratings and prisms work by diffracting light of different wavelength at different angles
The diffraction grating produces much clearer and More defined spectral lines than the prism

Question 23

Line absorption Spectra
Continuous spectra

Answer 23

The spectrum of white light is continuous
If you split the light up with a prism, the colours all merge into each other
There aren’t any gaps in the spectrum
Hot things admit a continuous spectrum in the visible and infrared
All the wavelengths are allowed because the electrons are not confined to energy levels in the object producing the continuous spectrum
The electrons are not bound to atoms and are free

Question 24

What are line absorption spectra?

Answer 24

When light with a continuous spectrum of energy(white light) passes for a cool gas.
At Low temperatures most of the electrons in the gas atoms will be in their ground states
Photons of the correct wavelength are absorbed by the electrons to excite them to higher energy levels
These are missing from the continuous spectrum when it comes out the other side of the gas
You can see a continuous spectrum with black lines corresponding to the absorbed wavelengths

Question 25

Comparing line absorption, spectra and emission spectra

Answer 25

If you compare the absorption and emission spectrum of a particular gas the black lines in the absorption spectrum match up to the bright lines in the emission spectrum

Question 26

Diffraction

Answer 26

When a beam of light passes through a narrow gap, it spreads out
This is called diffraction
Diffraction can only be explained using waves
If the light was acting as a particle particle in the beam would either not get through the gap(if they were too big), or just straight and the beam would be unchanged

Question 27

The photo electric effect

Answer 27

The results of photo Electric effect experiments can only be explained by thinking of light as a series of particle like photons
If photon of light is a discrete bundle of energy and it can interact with an electron in a one-to-one way
All the energy in the photon is given to one electron

Question 28

Wave particle duality

Answer 28

The photoelectric effect and diffraction show that light behaves as both particle and wave
This is an example of a phenomenon known as wave particle duality

Question 29

De broglie equation

Answer 29

Lambda = h/mv
Lambda = de Broglie wavelength(m)
M = mass in KG
V = velocity
h = plancks constant

Question 30

Electron microscopes

Answer 30

A shorter wavelength gives smaller diffraction effects
This fact is used in the electron microscope
Diffraction effect blur detail on an image
If you want to resolve tiny detail on an image, you need a shorter wavelength
Light blur out detail more than electron waves so an electron microscope can resolve final detail than a light microscope
They can let you look at things as tiny as a single strand of DNA