G482 - Photons

Question 1

Wave Particle Duality

Answer 1

The theory which states that all objects can exhibit both wave and particle properties

Question 2

Photon Definition

Answer 2

Quantum packets of energy

Question 3

Energy of a Photon

Answer 3

E = hf

E = photon energy
h = planck's constant
f = frequency of the radiation the photon is associated with

Question 4

electron volt

Answer 4

eV
A unit of energy
One electron volt is the energy required to accelerate an electron through a p.d. of one volt

Question 5

Electronvolts to Joules

Answer 5

x 1.6x10^-19

Question 6

Joules to Electronvolts

Answer 6

÷ 1.6x10^-19

Question 7

Experiment to Estimate h

Answer 7

When an LED emits light each electron that passes through it loses energy
The f zed amount of energy lost corresponds to the photon energy of the emitted light (assuming no heating effect)
Energy Lost by Electron = e x p.d. across LED
e = elementary charge

hf = eV

The frequency is labelled on the LED so this equation can be rearrange to find h

Question 8

eV and Kinetic Energy

Answer 8

When energy is lost by an electron to give something else kinetic energy:

eV = 0.5mv²

Question 9

The Photoelectric Effect

Answer 9

The emission of electrons from a metal surface when radiation is incident on the surface

Question 10

Light

Evidence of Wave Behaviour

Answer 10

Diffraction

Interference

Question 11

Light

Evidence of Particle Behaviour

Answer 11

photoelectric effect

Question 12

Threshold Frequency

Definition

Answer 12

The lowest frequency if electromagnetic radiation that will result in the emission of a photoelectron from a specific metal surface

Question 13

Work Function

Definition

Answer 13

ϕ

The minimum energy required to release a photoelectron from the surface of a metal

Question 14

Photoelectric Effect

Kinetic Energy

Answer 14

Energy is conserved when a photon interacts with an electron i.e. it gives all of its energy to the electron
If this results in more energy than the work function of the metal then the extra energy will give the electron kinetic energy
Electrons that are closer to the nucleus of an atom will use more energy escaping the metal so will have less kinetic energy

Question 15

Maximum Kinetic Energy

Formula

Answer 15

hf = ϕ + KEmax

Question 16

Photoelectric Effect

Intensity

Answer 16

Kinetic energy is independent of intensity as each photon gives all of its energy to one electron
Current increases with intensity as more photoelectrons are emitted so more charges hit the collector plate per unit time
The number of electrons emitted is proportional to the intensity of the incident radiation

Question 17

Planck’s Constant

Value

Answer 17

h = 6.63 x 10^-34

Question 18

Energy of Photoelectrons

Positive Applied P.D.

Answer 18

All the photoelectrons emitted are collected
Negative electrons are emitted for the negative plate which repels them and they are attracted to the positive plate so a current flows

Question 19

Energy of Photoelectrons

Negative Applied P.D.

Answer 19

Negative electrons are emitted when radiation is applied to the positive plate
They are emitted with kinetic energy but are repelled by the negative charge on the negative plate
Only electrons with sufficient kinetic energy can reach the negative plate and record a current
If the p.d. is high enough no current will be recorded

Question 20

Stopping Potential

Definition

Answer 20

The potential difference required to completely stop electrons from reaching the anode

Question 21

Electrons

Evidence of Wave Behaviour

Answer 21

Electron diffraction

Question 22

Electrons

Evidence of Particle Behaviour

Answer 22

Deflection in magnetic / electric fields

Question 23

Electron Diffraction

Answer 23

Electrons from an electron gun are accelerate through a vacuum towards a layer of polycrystalline graphite
The atomic spacing in graphite is close enough to the wavelength of an electron that it diffracts it
However the graphite atoms are not lined up in the same direction so a circular electron diffraction pattern is produced

Question 24

The de Broglie Equation

Answer 24

λ = h / mv

λ = wavelength of particle m
h = planck's constant
m = mass  of particle kg
v = velocity of particle m/s

Question 25

Spacing of Atoms in a Solid

Answer 25

10^-10m

Question 26

Applications of Electron Diffraction

Answer 26

Can be used to determine at spacing as diffraction will only occur if the wavelength of the electron is similar to the spacing between the atoms
High speed electrons can be used to determine the arrangement of atoms within crystalline structures or to measure the diameter of a nucleus

Question 27

Emission Line Spectrum

Answer 27

A spectrum produced by a material that contains only certain frequencies due to electron transitions between energy levels

Question 28

Absorption Line Spectrum

Answer 28

A spectrum of dark line over a continuous colour spectrum produced when light passes through a gas and the gas absorbs certain frequencies depending on the elements present

Question 29

The Bohr Atom

Answer 29

…is an atom model

An electron can revolve around the nucleus in certain allowed orbits in which it doesn’t emit radiation and each orbit is associated with a definite amount of energy.

Question 30

The Bohr Atom

Quantum Jumps - Emitting Energy

Answer 30

An electron can jump from an orbit of higher energy, E2, to an orbit of lower energy, E1,
The energy lost is emitted as a photon the frequency of which is given by
f = (E2-E1)/ h
Each downward jump produces the emission of a photon of a definite energy, frequency and wavelength which is visible as a coloured line on an emission spectra
The combination of jumps that can take place inside an atom are unique to that element

Question 31

The Bohr Atom

Quantum Jumps - Absorbing Energy

Answer 31

Electrons in an atom will absorb energy equal to the gap between the energy level it is at and a higher energy level
The frequency of photons of this energy correspond to the dark lines on a absorption spectra for that element