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Wave particle duality Flashcards

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1
Q

What is Huygen’s Wave theory?

A

believed that light travelled as wave
- developed Huygen’s principle
- it explains reflection, refraction, diffraction
BUT since no evidence of diffraction -> doubt abt his theory

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2
Q

What is Huygen’s principle?

A
  • Every point on a wavefront is itself a source of secondary wavelets.
  • Each wavelet will spread out as a new wave at a point source.
  • A new wavefront propagates as a tangent to the old wavefront.
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3
Q

According to Huygen what is the relationship bet new and old wavefronts?

A

new wavefront is tangential

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4
Q

What was Newton’s theory?

A

corpuscular theory of light
believed that light was made of small particles called corpuscles.

His theory relied heavily on his widely accepted theories of motion.

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5
Q

Whose theory was more preferred by scientific community?

A

Newton’s theory was more intuitive.
Newton had an established reputation.

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6
Q

What was Newton’s evidence for corpuscular theory of light?

A

believed his theory because light seemed to always travel in straight lines.

relied lot on his theories of motion

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7
Q

What did Newton’s corpuscular theory explain?

A

explained reflection as particle bouncing from surface.

explained refraction by saying that particles travel faster in a medium of higher density.

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8
Q

What effect did YDSE have?

A

provided evidence to support Huygen’s wave theory as showed that light could behave as a wave

BUT Huygen’s theory was still not accepted, due to Newton’s reputation

After a decade, Young formalised that light was transverse wave to explains these properties, & scientific community began to agree.

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9
Q

What is YDSE?

A

Young performed an experiment where he shone two coherent light sources through two slits.
An interference pattern was shown on the screen behind the slits, which suggested that the light was interfering.

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10
Q

What did Young discover decade after DSE?

A

discovered that light is a transverse wave, which explains diffraction

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11
Q

What was Fizeau’s experiment? CHECK

A

set up an experiment using a rotating cog + beam of light to measure speed of light.

shone a beam of light through gap bet 2 teeth of cog.

beam reflected back from a mirror, & back through a gap in the cog.

He adjusted speed of cog until reflected light was blocked completely by a tooth.

He could then use rotational frequency & distance that light had travelled to calculate speed of light.

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12
Q

What was Maxwell’s theory?

A

predicted that electromagnetic waves existed, and also predicted their speed.
He believed that the speed of electromagnetic waves in a vacuum was:
check eqn.
​
c = 3 × 108 ms-1

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13
Q

How did Fizeau and Maxwell link?

A

Fizeau’s observed speed of light was very close to Maxwell’s estimation.
This was evidence that light was an electromagnetic wave.

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14
Q

How did Fizeau calc speed of light?

A

Used rotational f & distance light had travelled

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15
Q

What did Hertz discover 1887?

A

discovered radio waves

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16
Q

How did Hertz measure the speed of radio waves?

A

set up radio wave detectors at regular intervals, & managed to construct a stationary radio wave.

measured wavelength, & by using a fixed resonant frequency measured speed using:
v = fλ

The speed lined up with Maxwell’s prediction, and provided evidence that radio waves were another type of electromagnetic wave.

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17
Q

How did Hertz know radiowaves were EM?

A

using an induction coil, showed that sparks could cross a gap of air - found these waves could induce a pd & so had a magnetic component

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18
Q

Hertz’s method

A

set radio wave at fixed resonant f and then into stationary wave
measure wavelength then use eqn for speed

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19
Q

What is black body?

A
  • object which absorbs + emits 100% of radiation incident upon it.
  • have a very specific emission spectrum.
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20
Q

What is the ultraviolet catastrophe?

A

Wave theory could not explain all of the intensity - wavelength graph for black bodies.

Wave theory predicted that power output would continue to rise as wavelength approached 0.

The observation was that power output curved back towards zero.

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21
Q

What was made in order to overcome ultraviolet catastrophe?

A

Einstein’s photon model of light

Max Planck suggested EM waves might only be emitted in quanta.

Einstein further suggested that EM waves can only exist in quanta, called photons.

The photon model was able to explain photoelectric effect + ultraviolet catastrophe.

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22
Q

What properties of light show that it behaves like a wave?

A

produce interference patterns
can be diffracted

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23
Q

What properties of light show that it behaves like a particle?

A

photoelectric effect?

24
Q

What was de Broglie’s hypothesis?

A

suggested wave-particle duality - all particles can behave as a wave and vice versa

25
Q

evidence that electrons behave as waves?

A

electron diffraction
When electrons are accelerated through a vacuum tube and interact through space in a graphite crystal, they can form an interaction pattern.

26
Q

What is the electron diffraction pattern?

A

has a bright central disc, and circular bands around it.
As the electron speed increases, the pattern gets tighter.

27
Q

How do electron microscopes work?

A

use electrons to scan an object instead of electromagnetic radiation such as light.

28
Q

How do we ensure electron microscopes are accurate?

A

Electron microscopes need to emit electrons with a small de Broglie wavelength to be accurate enough. This requires a potential difference on the anode of at least 150V.

29
Q

What is a transmission electron microscope?

A

used for v thin specimens.

Electrons are projected through specimen, & those that are able to pass through specimen form an image.

Thinner parts of the image will allow more electrons to pass.

30
Q

What is a scanning tunneling microscope?

A

uses a small probe which moves along sample.

A pd bet sample & probe allows a current to flow.

The distance between probe & sample will affect the resistance, & hence current.

These differences in current can be used to paint an image of the surface on the material.

31
Q

What do electron microscopes need?

A

must have small db wavelength
need a high voltage

32
Q

What are electron microscopes used for?

A

used to look at small objects

33
Q

How are electron microscopes different?

A

use electrons to scan object instead of EM radiation such as light

34
Q

What is Wien’s displacement law?

A

peak wavelength is inversely proportional to kelvin temp

35
Q

What did Wien do?

A

Wien wanted to create approx of ideal black body - measured radiation coming out oven through hole at different temp

Continuous spectrum of wavelengths so measured one which peak intensity of thermal radiation occurred

36
Q

What is a black body?

A

perfect absorber + emitter of thermal radiation

won’t reflect or allow radiation to be transmitted through

37
Q

What did Rayleigh-Jeans equation predict?

A

correct prediction of observed thermal E at longer wavelengths

but predicted at short that radiated power/unit wavelength becomes infinite - not observed experimentally

i.e. at ordinary temp objects emit intense UV & more intense X-rays

eqn - theoretical approach to Wien’s cavity radiation - assumed it forms standing EM wave

38
Q

How did Planck help?

A

suggested energy of EM waves quantised and not continuous

39
Q

How did Plank’s idea become widely accepted?

A

idea that EM energy quantised only once Einstein applied theory to explain observations on photoelectricity

40
Q

What does phrase UV catastrophe describe?

A

failure of classical physics to correctly predict energy spectrum of black body at short wavelengths

41
Q

How did Einstein apply Planck’s quantum theory?

A

as wave theory of light unable to explain instantaneous emission of photoe- + existence of threshold f

proposed e- in metal absorbs only whole quantum of incident EM energy

42
Q

How was De Broglie’s prediction verified? What happened if anode voltage increased?

A

e- gun accelerated by anode voltage in vacuum tube towards material of nickel

speed of e- increases - de Broglie wavelength increases - smaller angle of diffraction for maximum - tightly spaced concentric ring pattern

43
Q

Why can optical microscopes not be used to analyse lattice spacing in crystal?

A

particle/photon can only prove down to distances = their wavelength

spacing smaller than wavelength of visible light

44
Q

How can we use e- to probe structures to small to be probed by x-rays?

A

reduce e- wavelength by increasing e-‘s gun anode votage

45
Q

How is focusing of e- beam achieved in TEM?

A

using magnetic lenses

46
Q

How can we achieve better resolution for TEM?

A

smaller db wavelength so better image detail

47
Q

Why do we need the voltage bet the tip & surface in STM?

A

e- cross gap in only 1 direction
(-ve->+ve)
voltage must be kept constant

48
Q

Why does quantum tunnelling occur?

A

wave nature of e-

creates small, finite prob that e- can jump across the 1nm gap bet prove tip + sample surface

[db wavelength of e- at room temp is 1nm)

49
Q

How does STM work to produce image? (assuming constant height mode)

A

size of gap affected by sample surface
smaller this distance - greater # e- move across gap/s
increased current
variations in tunnelling current -> image of sample’s surface

50
Q

Why is it imp that the voltage bet surface and probe tip is kept constant - STM?

A

only changes in gap width affect current

51
Q

How does STM work in constant current mode?

A

heigh of probe varied so tunnelling current constant
variations in vertical position of probe tip w time used to image sample

52
Q

What did Newtons theory of light fail to explain?

A
  • narrow slit diffraction
  • predicted that double slit would produce 2 bright fringes but Young found series of fringes (interference - WAVE)
  • predicted speed of light in water increases than air - proved wrong by Fizeau (measured w cogs in air) + Foucault
53
Q

What did Huygen’s theory fail to explain?

A

sharp shadow formation

54
Q

How did Newtons explain refraction of light?

A

corpuscles velocity parallel to boundary unchanged
component perp increased
medium exerts attractive f on corpuscle to do so

(for reflection its a repulsive to produce v change+reversal)

55
Q

What did Hertz discover?

A

transverse nature - discovered radio waves polarised
- confirmed maxwells speed of light from stationary radio wave pattern

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