Turning Points- Wave-Particle duality (unfin)

Question 1

What was Newton’s corpuscular theory?

Answer 1

Newton’s theory that light exists as discrete packets called corpuscles that travel in straight lines and have momentum

Question 2

How did Newton’s corpuscular theory explain reflection?

Answer 2

Corpuscles collide with surface and experience repulsive force perpendicular to surface so component of velocity perpendicular to surface changes and component parallel stays the same

Question 3

How did Newton’s corpuscular theory explain refraction?

Answer 3

Corpuscles approach optically denser medium and experience short-range attractive forces causing velocity component perpendicular to surface to increase while parallel component stays the same so light bends towards the normal.
This means light travels faster in denser mediums

Question 4

What was Huygens’ wave theory of light?

Answer 4

Huygens’ theory that light was a wave and every point on a wavefront is a point source to secondary wavelets which spread to form the next wavefront (Huygens’ principle)

Question 5

How did Huygens’ wave theory explain reflection?

Answer 5

The wavefront reaches the boundary at different times (unless moving perpendicular to it) and wavelets spread away from the surface forming reflected wavefront

Question 6

How did Huygens’ wave theory explain refraction?

Answer 6

Assumed that light travels SLOWER in an optically denser medium, therefore slower wavelets are produced and form a wavefront closer to the normal

Question 7

Why was Newton’s theory accepted over Huygens’?

Answer 7

-Newton had a very high reputation at the time
-Diffraction could not be explained by Newton however it had not yet been observed either
-Speed of light hadn’t yet been measured and compared in optically denser mediums

Question 8

What is Young’s double slit experiment?

Answer 8

Coherent light shone through two slits, each acting as a coherent point source. The light interferes producing bright and dark fringes, constructively where they meet in phase and destructively where they meet in antiphase

Question 9

What was the significance of Young’s double slit experiment?

Answer 9

Proved wave nature of light as corpuscular theory predicted two bright fringes but interference pattern demonstrated diffraction and interference, both wave properties. This led to Huygens’ wave theory of light being accepted. Newton’s theory disregarded after light was found to be slower in water

Question 10

What are electromagnetic waves?

Answer 10

Transverse waves formed of alternating magnetic and electric fields travelling in phase and at right angles to each other. Direction of wave travel is perpendicular to direction of oscillations

Question 11

What did Maxwell predict?

Answer 11

The existence of EM waves and made an equation predicting their speed in a vacuum, c = 1/√μoεo

Question 12

What is μo?

Answer 12

The permeability of free space, relating magnetic flux density produced by a wire to the current in the wire

Question 13

What is εo?

Answer 13

The permittivity of free space, relating electric field strength to the charge on the object which formed the field

Question 14

How did Hertz discover radio waves?

Answer 14

Used apparatus which allow high voltage sparks to jump across a gap of air and be detected using a receiver.
Spark gap transmitter produced a spark, receiver was a loop of wire with a gap, detects wave’s alternative magnetic field causing flux cutting, inducing a pd across the gap causing a spark across it

Question 15

How did Hertz measure the speed of light?

Answer 15

By placing a metal sheet in front of the spark transmitter, stationary waves are formed. Using the loop of wire, distance between adjacent nodes and antinodes can be measured (no spark or spark) to calculate wavelength. Using frequency of the waves, speed can be found and it confirmed Maxwell’s predictions

Question 16

How did Fizeau measure the speed of light and what was its significance?

Answer 16

Pulsing light through a half-silvered mirror and rotating cog with a mirror far behind the cog and an eyepiece next to half-silvered mirror.
Frequency of rotation of the cog increased until pulsed beam can no longer be seen in eyepiece, at which point light passing through a gap returns and is blocked by the neighbouring tooth
Value for c calculated was close to Maxwell’s prediction, showing that light was an EM wave

Question 17

How did Fizeau work out the speed of light from his experiment?

Answer 17

c=2L/t, L is distance between mirror and cog
speed of cog, v = 2πrf
distance between teeth, d = 2πr/n, n number of teeth
time for rotation between teeth = v/d = 1/nf
time for rotation between tooth and gap = 1/2nf
c=2L/ t= 2L/ 1/2nf = 4Lnf

Question 18

What is a black body?

Answer 18

A body that absorbs and emits all possible wavelengths of radiation and emits all wavelengths of radiation dependent on its temperature

Question 19

What was the ultraviolet catastrophe?

Answer 19

Classical wave theory predicted that as wavelength of radiation decreases, intensity increases leading to an infinite amount of UV being emitted which was impossible and did not match with experimental evidence

Question 20

How was the ultraviolet catastrophe resolved?

Answer 20

Planck’s interpretation of EM waves which stated that they were discrete packets of energy called quanta, with energy E = hf

Question 21

How did classical wave theory fail to explain the photoelectric effect?

Answer 21

-Suggests any frequency of light could cause photoelectric emission over a long enough time, no threshold frequency
-Suggests it would take time for emission as energy is supplied over time, contradicts immediate emission seen
-Suggests intensity increases speed of emission but instead increases rate of emission
-Suggests photoelectrons released with one energy but are emitted with a range

Question 22

What is the stopping potential and what is the equation for it?

Answer 22

The pd needed to be applied across two metal plates to stop photoelectrons with maximum kinetic energy from crossing, Ek max = eVs, Vs is stopping potential

Question 23

What was de Broglie’s hypothesis and what is the equation?

Answer 23

All particles have a wave-like nature and a particle-like nature, wavelength of any particle found with λ = h/ mv

Question 24

How did electron diffraction provide experimental evidence for de Broglie’s hypothesis?

Answer 24

Showed that electrons, which are particles, can undergo diffraction which is a wave property

Question 25

How is electron diffraction carried out and what is the equation for it?

Answer 25

Electron gun used to produce electrons which are accelerated through crystal lattice where electrons diffract and form diffraction pattern on fluorescent screen behind it. eV = 1/2mv^2, 2eV = mv^2, 2meV = m^2v^2 so mv = √2meV, λ = h/ mv

Question 26

How did the change of electron speed in electron diffraction show wave-like properties?

Answer 26

When electron's energy increased, momentum increases so λ decreases, leading to closer fringe spacing in pattern which agrees with wave theory

Question 27

What is resolving power?

Answer 27

The ability to distinguish structures close to each other

Question 28

Why do electron microscopes have higher resolving power than optical microscopes?

Answer 28

Wavelength of electron beam much smaller than that of light, so have much higher resolving power than optical microscopes. As wavelength decreases, resolving power increases

Question 29

What is a transmission electron microscope?

Answer 29

A microscope that passes a beam of electrons through magnetic lenses and across a thin sample to form an image on a fluorescent screen. The resolution is around 0.1nm

Question 30

What is the condenser lens?

Answer 30

The first lens in a TEM responsible for deflecting the electrons into a wide parallel beam, that is then directed at the sample

Question 31

What is the objective lens?

Answer 31

The second lens in a TEM microscope. It is responsible for magnifying and focusing the image of the sample

Question 32

What is the projector lens?

Answer 32

The third lens in a TEM microscope that magnifies the image made and projects it onto the fluorescent screen

Question 33

What are the two problems that limit resolving power of transmission electron microscopes?

Answer 33

-Sample thickness: as electrons pass through sample they lose energy and slow down, increasing wavelength and so decreasing resolving power -Range of speeds for electrons: electron gun produces electrons of different speeds as some collide during emission, causes different wavelengths and diffraction causing blurry images, known as aberration

Question 34

How do you find the voltage required to give wavelengths of the order of the size of an atom?

Answer 34

use λ = √2meV, with λ of 0.1nm, m e of electrons and rearrange for V

Question 35

What is a scanning tunnelling microscope?

Answer 35

A microscope that uses a probe controlled by piezoelectric transducers to scan the surface of a sample. It uses electron tunnelling and has a resolution of around 0.001nm

Question 36

What is quantum tunnelling?

Answer 36

For a sufficiently small gap (barrier), the amplitude of a matter wave passing through it will not become zero and so it can 'tunnel' across the gap. Allows electrons to jump the gap between tip of probe and sample when close enough

Question 37

How does a scanning tunnelling microscope work?

Answer 37

Fine tipped probe moves along surface of an object and has a constant potential so electrons only move one way.

Question 38

How does a constant-height scanning tunnelling microscope work?

Answer 38

Probe kept at constant height above sample, as it moves across size of gap will vary so amount quantum tunnelling varies so tunnelling current varies. Tunnelling current used to produce an image

Question 39

How does a constant current scanning tunnelling microscope work?

Answer 39

Current of the probe remains constant and probe's height changes with changes in the sample's surface. Movement of probe used to produce an image.