Turning Points: Waves Flashcards
What were the differences in Newton’s theory of light and Huygens?
Newton believed that light was made out of particles (known as corpuscles) whereas Huygens believed that light is a longitudinal wave travelling through ether.
What is the corpuscular theory?
Light was made out of corpuscles that travelled in a straight line from the source.
What did the corpuscular theory prove?
the formation of sharp shadow when an opaque object intercepted sunlight.
What are the key features of Huygens’ theory?
Light acts as a wave, every point of waveform acts as a source of secondary wavelets
How does a dark and bright fringes on screen support Huygens’ theory?
each point at slit produces secondary wavelets. The wavelets overlap the screen. Path difference etc..
Describe the process of Fizeau’s experiment to determine speed of light?
He shone a light at a partially reflecting mirror which directed between the teeth of rotating toothed wheel. The distant mirror reflects back towards the wheel and observer.
What happened as soon as Fizeau started to rotate the wheel?
the teeth broke up the light beams and into pulses of light is observed.
What happened when Fizeau started to increase the frequency of wheel?
A speed was discovered as light leaving through one gap returned at an instant as the next tooth blocked it path.
What is Fizeau’s equation of speed? and meaning of each
c = 4DNf
D - distance from wheel to distant mirror
N- number of teeth
f - frequency
What did Maxwell predicted about electromagnetic wave?
Electromagnetic wave exist when a changing magnetic field creates a changing electric field and so on…
What is m0 (muu nought) and what it relates to?
Permeability of free space. (H - Henry)
It relates to the magnetic flux density of a magnetic field to the electric current that creates it.
What is E0 (epsilon nought) and what it relates to?
permittivity of free space (F - Farad)
It relates to the electric field strength to the charge that creates it
Why did Hertz’s transmitted included an induction coil and a capacitor?
These produced an alternating high voltage to create a spark, which kept reversing direction and created radio waves.
What did Hertz do to detect the electromagnetic waves?
He used a spark gap detector made up of wire loop, the ends made up of brash sphered and seperated by small gap.
What did Hertz finally discovered?
EM waves spreading from the sparks created by transmitter which induces emf hence a current, in wire loop, creates sparks between brass spheres.
What else did Hertz do and concluded?
He created stationary radio waves from superposition of the incident and reflected waves. He measured the wavelength by node to node distance and used c = f * wavelength
List all EM waves from lowest wavelength to highest
Gamma, x-rays, ultraviolet, visible light, microwaves, radiowaves
What is black body radiation?
It is an electromagnetic radiation. Spectrum with peak depending on temperature
What is Wein’s displacement law?
The peak wavelength was inversely proportional to the kelvin temperature.
What led to the ultraviolet catastrophe? (predictions)
The wave theory states as the wavelength of radiation decreases, the intensity increases leading to infinite amount of ultraviolet radiation
What was Planck’s solution to the catastrophe? and equation
suggested that EM waves was quantised, EM waves travelled in packets called quanta. Hence E=hf
Give 1 reason why photoelectric effect can’t explain the wave theory (about threshold frequency)
Wave theory suggest that any frequency of light should cause photoelectric emission because the energy absorbed by each electron gradually increases whereas opposes the idea of threshold frequency
Give 1 reason why photoelectric effect can’t explain the wave theory (about immediate)
photoelectric effect is immediate whereas wave theory suggest time needed for electrons to reach work function
Give 1 reason why photoelectric effect can’t explain the wave theory (about intensity)
increasing the intensity of light does not increase the speed of the photoelectric emission (where the theory states)
Give 1 reason why photoelectric effect can’t explain the wave theory (about kinetic energy)
photoelectrons are released at range of kinetic energies
What did Einstein claimed the EM waves were
photons
What are the three points that Einstein proved the wave theory wrong?
- All electrons same amount of energy
- when photon interacts with electron, all the energy is transferred to it
- Intensity equal to number of photoelectrons released per second.
What happens when the potential supplied across metal surface is positive.
kinetic energy of photoelectrons will decrease as work must be done against the electrostatic force of attraction towards surface.
What is the stopping potential?
the potential difference need to supply to the metal to stop photoelectron with maximum kinetic energy
What is the equation relating stopping potential?
hf = work function + stopping potential*charge (e)
What is de-Broglie hypothesis and what equation relates?
all particles have a wave-like nature and particle nature.
wavelength = h/mv
What proved de-Broglie’s hypothesis
electron diffraction
When light passes through a diffraction grating, what happens with larger wavelength?
It gives larger angle of diffraction for each maximum hence more widely spaced concentric rings
What happens when accelerating the voltage increases?
speed of electrons increases and their de-Broglie wavelength decreases so fringe spacing decreases (concentric rings)
What is the resolving power of microscopes?
ability distinguish structures which are close to each other
How does TEM microscopes work?
electrons are accelerated by electron gun, passing though magnetic lenses, passing through a thin sample so electron don’t slow down. The lenses deflect electron towards the axis.
What are the three types of lenses of TEM and their uses?
- Condenser lens - deflects electrons so forms a wide parallel beam
- Objective lens - form an image of sample
- Projector lens - magnify the image and project on fluorescent screen
What are the reasons the resolving power are limited in TEM?
sample thickness,
electrons travelling at range of speed ( some electrons lose kinetic energy )
What is the diameter of atom
0.1nm
What does STM microscopes use?
uses quantum tunnelling of electrons in order to form an image on the surface of an object
How does STM microscopes operate?
a very fine tipped probe which moves across a surface of an object and stays constant potential. ( electrons travel one direction) As probe moves across, size of gap vary
What happens when size of gap is large in STM microscopes?
tunnelling current will decrease as tunnelling of electrons less likely to occur vice versa for small gap
What is tunnelling current?
The movement of electrons measured
2 things that needs to be constant in STM
height mode and current mode
