electromagnetic radiation and quantum phenomena

Question 1

What is the photoelectric effect

Answer 1

if radiation of high enough frequency is shone onto the surface of a metal
free electrons absorb energy and vibrate
if enough energy is absorbed bonds holding it to the metal are broken and the electron is released

Question 2

what are the electrons emitted called

Answer 2

photoelectrons

Question 3

explain threshold frequency

Answer 3

no photoelectrons are emitted if radiation incident on a certain metal is below a certain frequency

Question 4

what effect does increasing the frequency of incident radiation have

Answer 4

photoelectrons are emitted with kinetic energies varying up to the maximum
the maximum increases with the frequency

Question 5

what effect does changing the intensity of incident radiation have on the max kinetic energy of photoelectrons

Answer 5

unaffected.
increasing intensity increases the rate at which photoelectrons are emitted proportionally

Question 6

what is the intensity of radiation

Answer 6

amount of energy hitting an area of metal per second

Question 7

why are wave theory and threshold frequency incompatible

Answer 7

wave theory suggest energy supplied by the EM wave should be proportional to the intensity
over time electrons would gain enough energy to be released regardless of the frequency of incident radiation
however, electrons are never released unless the wave is above the threshold frequency

Question 8

what is a photon

Answer 8

discrete wave packet of EM radiation

Question 9

how can we demonstrate the photoelectric effect

Answer 9

electroscope
thin metal zinc plate
gold leaf
plate is negatively charges, therefore so is the box
UV light shone onto the plate
electrons are lost from the zinc plate, so negative charge lost. gold leaf falls

Question 10

energy of a photon
how does this correlate to the electron which collides with this photon

Answer 10

E = hf
electron absorbs this energy
if E > work function, it can be released
any extra energy becomes kinetic energy correlating to the speed at which it is released

Question 11

what is work function

Answer 11

minimum energy an electron needs tp break the bonds holding it to the metal
dependent on the metal

Question 12

work function {}

Answer 12

work function = threshold frequency * h

Question 13

how do you calculate the maximum kinetic energy of a photoelectron

Answer 13

E = hf
hf - work function = ke max

Question 14

how do you calculate the speed of a photoelectron

Answer 14

calculate the maximum kinetic energy
k.e = 1/2 * m * v^2
m= 9.11 * 10^-31 ( mass of an electron )
rearrange for v

Question 15

what is stopping potential

Answer 15

potential difference needed to stop the fastest moving electrons ( k.e max)
E = QV
E = k.e max
QV = k.e max
Q = 1.6 * 10^-19 ( 1ev )
v = stopping potential

Question 16

define electron volt

Answer 16

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

Question 17

what does it mean to say an electron is excited

Answer 17

when it is in an energy level higher than ground state

Question 18

what is ground state

Answer 18

the lowest energy level an electron can be in

Question 19

how can electron move down an energy level

Answer 19

by emitting a photon of specific energy
must be specific since energy levels are fixed

Question 20

what is excitation

Answer 20

the movement of an electron to a higher energy level

Question 21

what is ionisation energy

Answer 21

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

Question 22

how do fluorescent tubes work

Answer 22

excitation of electrons to higher energy levels
when they return to their ground state they lose energy by emitting (uv range) high energy photons
phosphor absorbs these photons , more excitation.
they then lose energy again by emitting lower energy photons of visible light

Question 23

what is line spectra evidence for

Answer 23

evidence for transitions between discrete energy levels in atoms.
atoms can only emit photon with specific energy ( difference between energy levels )
specific energy means specific wavelengths

Question 24

what is the spectrum of white light like

Answer 24

continuous
all wavelengths appear
since electrons aren’t confined to energy levels and are free

Question 25

how are line absorption spectra formed

Answer 25

light with continuous spectrum
passes through a cool gas
photons of specific wavelength are absorbed by electrons which are then excited
these wavelengths are then missing (black lines)

Question 26

what are two experiments which show wave-particle duality of light

Answer 26

diffraction
photoelectric effect

Question 27

de Broccoli (Broglie) {}

Answer 27

λ = h/mv

Question 28

describe electron diffraction

Answer 28

electrons accelerated to high velocities in a vacuum then passed through a graphite crystal
they diffract like waves would
produces a pattern of rings

Question 29

when will an electron beam diffract

Answer 29

when it’s de broglie wavelength is roughly the same size (order) as the gap it’s being sent through

Question 30

how would the diffraction pattern vary with speed of the electrons

Answer 30

slower electrons -> widely spaced rings
larger wavelength
faster electrons ( higher velocity ) -> shorter wavelength -> closely spaced rings

de broglie wavelength

Question 31

how would the diffraction pattern vary with the mass of the particle

Answer 31

particles with greater mass they would show a more tightly packed pattern
greater mass -> shorter de broglie wavelength

Question 32

when is an atom ionised

Answer 32

when an electron is removed
the ionisation energy is equal to the energy required to remove an electron from an atom when it is in ground state