3.6 - wave particle duality

what was light initially considered to be? why?

a wave, because visible light was part of the electromagnetic spectrum

what did scientists initially use to explain all concepts in physics?

• newton’s laws of motions

• theory of electromagnetic waves

how did scientists (before einstein) initially explain the photoelectric effect?

they couldn’t, but they believed that it would eventually be explained by newton’s laws of motion and the electromagnetic wave theory

how did einstein successfully explain the photoelectric effect?

by theorising that light was composed of photons, particles that were ‘packets’ of EM waves, therefore giving light a wave-particle duality

light has a dual nature - what does this mean?

that it can behave as either a wave or as a particle, depending on the situation

is light a wave or a particle?

it has a wave-particle duality, meaning it can behave as either a wave or a particle depending on the situation

what is a photon?

a particle that is a ‘packet’ of EM electricity

how is it possible for light to have a wave-particle duality?

it’s possible if we consider light as being composed of photons, which are particles made up of EM energy

what is the evidence for light’s wave-like nature?

diffraction of light

what is the evidence for light’s particle-like nature?

photoelectric effect

how does light diffract?

• when light passes through a narrow slit, the emerging light spreads out

• narrower gap / longer wavelength → diffraction

DIAGRAM

what happens when light passes through a narrow slit?

it spreads out in the same way water waves would

what increases the amount of light diffraction?

• longer wavelength

• smaller gap

what decreases the amount of light diffraction?

• shorter wavelength

• wider gap

how does the diffraction of light demonstrate a wave-like nature in light?

light, when passing through a narrow slit, spreads out in the same way water waves do when moving through a gap. this means light must have a wave-like nature as only waves diffract when passing through small gaps

how does the photoelectric effect demonstrate a particle-like nature in light?

• existence of threshold frequency; as if light were a wave then the electrons would gain energy regardless of frequency, except they don’t

• photoelectric effect occurs immediately; if light were a wave then ASK ABOUT THIS :(

aside from light, what else has a dual wave-particle nature?

matter

evidence that matter has a dual wave-particle nature

• light does, so matter might

• electrons in a beam can be deflected by a magnetic field

• electrons in a beam can be diffracted when pointed at a thin metal foil

how can electrons in a beam be deflected?

by a magnetic field

what happens when a magnetic field is directed towards electrons in a beam?

the electrons are deflected

what was de broglie’s theory regarding the wave-particle duality of matter?

• the idea that duality from photons can be extended to matter

• therefore matter has a dual wave-particle nature

• the wave-like nature of matter is characterised by it’s de broglie wavelength, λ

what is the wave-light nature of matter characterised by?

it’s de broglie’s wavelength

what is de broglie’s wavelength, λ?

• λ = h / p = h / mv

λ = de broglie wavelength

h = planck’s constant, 6.63 × 10^-34

p = particle’s momentum

m = particle’s mass

v = particle’s velocity

a representation of the wavelength of the particles of matter, depending on the particle’s momentum

de broglie wavelength (λ) = ?

• λ = h / p = h / mv

λ = de broglie wavelength

h = planck’s constant, 6.63 × 10^-34

p = particle’s momentum

m = particle’s mass

v = particle’s velocity

what is the only way to alter the de broglie’s wavelength of a particle? why is this the case?

the only way to alter it is to change the velocity of the particle, as the rest of the values used to calculate the de broglie’s wavelength (planck’s constant and mass) are constant

why does changing the speed of a particle change it’s de broglie’s wavelength?

because λ = h / p,

and p = mv,

λ = h / mv

since the other values (planck’s cosntant and particle mass) are constant, changing the speed changes de broglie’s wavelength

how does de broglie’s wavelength change when particle speed is increased?

de broglie’s wavelength decreases, because speed is part of the denominator and the larger the denominator, the smaller the result

how does de broglie’s wavelength change when particle speed is decreased?

de broglie’s wavelength increases, because speed is part of the denominator and the smaller the denominator, the larger the result

s ↑, λ ?

s ↓, λ ?

s ↑, λ ↓

s ↓, λ ↑

what proved the wave-like nature of electrons, and therefore proved de broglie’s wavelength and the dual wave-particle nature of matter?

the discovery that a beam of electrons can be diffracted when pointed at a thin metal foil

diffraction of electrons

DIAGRAM

1. narrow beam of electrons in a vacuum tube directed at thin metal foil

2. fixed rows of positive ions in the metal foil cause electrons to be diffracted in the same manner light is when passing through a slit

3. diffracted electrons form a ring pattern on a fluorescent screen at the end of the tube

what is the set up for an electron diffraction experiment?

DIAGRAM HERE

• narrow beam of electrons, moving at a constant speed

• thin metal foil composed of tiny crystalline regions, arranged in fixed positions in rows

• fluorescent screen in which diffracted electrons pattern is visible

what is each ring in the electron diffraction pattern due to?

due to electrons diffracted by the same amount from grains of different orientations, at the same angle to the incident beam

are electrons diffracted by the same or different amounts as each other?

electrons are diffracted by the same amount

are electrons diffracted at the same or different angles as each other?

the same angle from grains of different orientations to the incident beam

why is the beam of electrons narrow?

ASK

how is the metal in an electron diffraction experiment composed?

lots of tiny crystalline regions / grains consisting of positive ions arranged in fixed positions in rows of a regular pattern

what causes the electrons to be diffracted?

the beam of electrons passing through the fixed rows of atoms

in what pattern are the electrons diffracted in?

• diffracted in certain positions only

• form a pattern of rings

in the electron diffraction experiment, how is the beam of electrons produced?

attracting electrons from a heated filament wire to a positively charged metal plate with a small hole at it’s centre in which electrons pass through and form the beam

where do the electrons that form the beam come from?

a heated filament wire

what are the electrons from the heated filament wire (in the electron diffraction experiment) attracted to?

the positively charged metal plate

how can we increase the speed of the electrons that form the beam?

by increasing the potential difference between the filament and the metal plate

what does increasing the potential difference between the filament and the metal plate do?

increase the speed of the electrons that form the beam

what does increasing the speed of the electrons that form the beam do to the diffractive pattern?

make the diffraction rings smaller

why does increasing the speed of the electrons that form the beam make the diffraction rings smaller?

because the increase of speed makes the de broglie wavelength smaller (λ = h / mv), so less diffraction occurs and the rings become smaller

is there more or less diffraction with a larger de broglie wavelength?

greater diffraction with larger de broglie wavelength

is there more or less diffraction with a smaller de broglie wavelength?

less diffraction with smaller de broglie wavelength

why does an electron have a fixed energy level that depends on the shell it occupies?

because it’s de broglie’s wavelength has to fit the shape and size of the shell

for an electron moving around the nucleus, what must the circumference of it’s orbit be equal to?

a whole number of de broglie wavelengths

does the de broglie wavelength of an electron increases or decrease when it moves to an orbit where it travels faster?

it decreases, because de broglie wavelength decreases with increasing speed (incerasing momentum)

what does de broglie wavelength decrease with?

• increasing speed

• therefore increasing momentum

what does de broglie wavelength increase with?

• decreasing speed

• therefore decreasing momentum

what are some practical applications of quantum technology?

• PET scanner

• STM (scanning tunnelling microscope)

• TEM (transmission electron microscope)

• MR (magnetic resonance) body scanner

• SQUIDs (superconducting quantum interference devices)

what is an STM (scanning tunnelling microscope) used for?

mapping atoms on solid surfaces

how is the wave-like nature of matter utilised in STMs?

the wave-like nature of electrons allows them to tunnel between surface and the metal tip by a few nanometres

what is a TEM (transmission electron microscope) used for?

to obtain very detailed images of objects too small to see with optical microscopes

how is the wave-like nature of matter utilised in TEMs?

electrons are accelerated to high speed so their de broglie wavelength is small enough to give off very detailed images

what is a MR (magnetic resonance) body scanner used for?

detects the radio waves emitted when hydrogen atoms in a patient in a strong magnetic field flip between energy levels

how is the wave-like nature of matter utilised in MR?

the hydrogen’s flip of energy levels emits radio waves that can be detected

what is a SQUID used for?

detect very weak magnetic fields, such as the magnetic fields produced by electrical activity in the brain

how is the wave-like nature of matter utilised in SQUIDs?

COME BACK TO THIS but i tthink smth to do with the amgnetic field produced