Turning points 2 Flashcards
What was Newton’s theory for light
Made up of tiny particles called corpuscles
How did Newton explain why light reflects
- When corpuscles collide with a surface, a repulsive force pushes them back
- Force causes perpendicular component of velocity to change direction but parallel component remains constant
How did Newton explain why light refracts
- When corpuscle enters a denser medium at an angle, short range attractive forces act perpendicular into the surface
- Causes velocity of corpuscle perpendicular to surface to increase while parallel component stays constant
- Causes it to bend towards the normal as the particle is now more perpendicular to the surface than before
What did Newton believe about the speed of light as it passes into a denser medium
Perpendicular component of velocity is greater whilst parallel component is the same so speed increases
What was Huygens principle
- Point source produces waves joined by a wave front
- Every point on the wave front becomes another point source of secondary “wavelets” which form a new wave front (cycle continues)
What did Huygens believe about the speed of light as it enters a denser medium
Slows down (and bend towards the normal)
Give 3 reasons why Newtons theory of light preferred over Huygens
- Newton had a very high reputation at the time
- Diffraction had not been observed
- Speed of light had not been measured
How did Young’s double slit experiment prove Huygens theory of light
- When light was shone through a double slit with each slit acting as a coherent point source, bright and dark fringes formed due to diffraction and interference
- These are wave properties
What would have occurred in Young’s double slit experiment if Newton was right
There would be only 2 bright fringes corresponding to the 2 slits instead of an interference pattern
When did scientists start agreeing with Huygen’s wave principle
- Years after Young’s experiment when the speed of light in water was measured to be slower than in air
What is an EM wave formed of
- Alternating magnetic and electric field travelling in phase and perpendicular to each other and the direction of wave propagation
- The electric field goes up and down and magnetic field is side to side when drawing
What was Maxwell’s formula for the speed of an EM wave in a vacuum
c = 1 / sqrt (μ0ε0)
where μ0 is the permeability of free space which relates the magnetic flux density of a wire to the current flowing through it
and ε0 is the permittivity of free space, relating the electric field strength to the charge of the object producing it
Explain Fizeau’s method for measuring the speed of light
- Light source causes beam of light which is passed through a rotating wheel
- The wheel has teeth and gaps so the light only passes through if it passes through a gap
- There is a mirror about 8km away so the light comes back at the wheel after reflecting off the mirror
- Alter frequency of wheel until the light will pass through the gap to the mirror and get blocked by the next tooth as it comes back
- The distance travelled is about 16 km and time taken can be calculated using the time period of the wheel at the frequency and speed = distance/ time
What calculations did Fizeua do after his experiment to calculate the speed of light
- Distance travelled = 2d as it goes to the mirror and back
- If a wheel has n teeth and n gaps, 1/ 2n of a revolution a tooth will replace a gap so the light is blocked
- Time period = 1/f so a tooth will replace a gap every 1/2nf seconds
- c = 2d / (1/2nf) = 4dnf
Why did people believe light was an EM wave after Fizeau’s experiment
His value for speed of light was very close to Maxwell’s value for speed of EM wave
What was the apparatus used in Hertz’s experiment to find the speed of radio waves
- 2 charged plates called electric dipoles
- Induction coil to produce a high pd (powered by battery)
- Spark gap connected between the plates where the pd is applied
- Loop of wire with gap in it
- Metal plate for radio waves to reflect off
How did Hertz produce radio waves in his experiment to find the speed of them
When the induction coil produced a high pd causing sparks to jump across the gap of air radio waves are produced
How did Hertz detect the magnetic field of the radio waves
- When the radio waves enter the loop of wire, wire experiences a changing magnetic field causing EMF to be induced
- Another spark is produced across the gap in the loop
How did Hertz detect the electric field of the radio waves
Instead of the loop of wire, use another dipole parallel to the first and the radio waves will create an alternating current in the second dipole
Explain how Hertz calculated the speed of radio waves
- Place metal sheet so that radio waves are reflected back producing a stationary wave
- Detect strength of wave at different points by using the loop of wire as a detector i.e. no sparks due to B field at nodes
- Measure the distance between 2 nodes and double for wavelength
c = fλ
How did Hertz’s experiment prove that radio waves were EM waves
They travelled at the same speed that Maxwell predicted for the speed of EM waves
How could Hertz show that radio waves were polarised
Rotate the loop of wire from 0 to 90 degrees and the signal varies from a max to min value
Max value as detector was perpendicular to oscillations and min value when detector was parallel
Define a black body
A body that can absorb all wavelengths of EM radiation and can emit all wavelengths
e.g. the sun is a black body but a mirror is not as it reflects most wavelengths instead of absorbs
What was the predicted graph of intensity of EM radiation emitted against wavelength of EM radiation emitted according to classical wave theory
As wavelength decreases, intensity increases graph is in the form y = k/x
What was the ultraviolet catastrophe
- Graph of intensity against wavelength for a black body according to experimental data showed:
- Until UV range, as wavelength decreases intensity increases but after reaching UV, the intensity decreases with
wavelength - Wave theory couldn’t explain why and predicted an infinite amount of UV radiation which is impossible
How does the graph of intensity against wavelength for a black body change at different temperatures
- At low temperatures, most of the intensity and the peak intensity is in the infrared range so graph is shifted down and to the right
- At high temperatures, most of intensity and peak is in visible light range (so very hot things glow) and graph is shifted up and to the left
How did Planck solve the UV catastrophe
EM waves travel in discrete packets (quanta) which have energy proportional to frequency
Why could wave theory also not explain the photoelectric effect
- Wave theory would mean no threshold frequency as even at low frequencies, energy would gradually increase until electron absorbs enough
- Wave theory predicts that increasing intensity would increase energy of electrons but, intensity is the number of photons hitting the metal per second so increasing it only increases number of photoelectrons released
- Wave theory could not explain why photoelectrons exposed to the same light had different energies but this was because each electron only absorbed 1 photon and electrons deeper lose energy through collisions when leaving so had less KE
What is the stopping potential (Vs)
Pd applied across the metal required to stop photoelectrons with max KE
Ekmax = eVs
How can you relate stopping potential to Einstein’s photoelectric equation
E = hf = ϕ + Ekmax
E = hf = ϕ + eVs
Vs = (h/e) f - (ϕ/e)
Describe the graph of stopping potential against frequency
Vs = (h/e) f - (ϕ/e)
so straight line with;
gradient of (h/e)
y intercept of -(ϕ/e)
x intercept of threshold frequency
What is the de Broglie hypothesis
- All particles have a wave-like nature and particle-like nature e.g. electrons has momentum but can diffract
- the wavelength of any particle = h / mv
Describe the experiment which shows electron diffraction
- Electron gun accelerates electrons towards a crystal lattice
- Electrons interact with small gaps between atoms, causing diffraction and producing interference pattern on a fluorescent screen behind the crystal lattice
Derive a relationship between the de Broglie wavelength and the pd used to accelerate the electrons towards the crystal lattice
1/2 mv^2 = eV
m^2 v^2 = 2meV
mv = sqrt (2meV)
λ = h / sqrt(2meV)
How does increasing the accelerating voltage affect the interference pattern
- λ = h / sqrt(2meV) so as V increases λ decreases
- smaller λ means the fringe spacing decreases and the concentric rings get closer together
Why does an electron microscope have a much higher resolving power than a light microscope
- The wavelength of an electron beam is much smaller than light
- Means less diffraction and diffraction blurs out details
- Therefore electron microscope can distinguish structures which are extremely close to each other and small i.e. higher resolving power
What is the first stage of how a transmission electron microscope (TEM) works
Accelerate electrons using electron gun through condenser lens which deflects the electrons so they form a wide parallel beam
What is the second stage of how a transmission electron microscope (TEM) works
Electron beam pass through the extremely thin sample so that it doesn’t slow down affecting the wavelength and it passes through objective lens which forms an image of the sample based on how the beam of electrons interacted with the sample
What is the final stage of how a transmission electron microscope (TEM) works
Electron beam now passes through projector lens which magnifies the image made by the objective lens and projects it on to a fluorescent screen
How does accelerating voltage of the electron gun affect the resolving power
If V is increased, λ decreases so resolving power increases
What are limitations of the TEM microscope
- If the sample is too thick, the beam of electrons will slow down causing their wavelength to increase so resolving power decreases
- Electrons travel at different speeds out of the electron gun due to loss of KE due to collisions while leaving the metal. This means different wavelengths so different electrons diffracted by different amount causing blurring of the image i.e. aberration
What is quantum tunnelling
Occurs due to the wave nature of electrons so when they are trying to cross a barrier or gap which is small enough, electrons can move across it. The smaller the gap, the more likely it is for tunnelling to occur
How does a scanning tunnelling microscope (STM) work
- Very fine tipped probe moves across the surface of the object with constant potential
- Electrons may tunnel across gap forming a tunnelling current
- As gap size gets smaller probability of tunnelling increases so current increases and vice versa
- Record changes in current or gap size to produce images
How does constant height mode for an STM work
The probe is kept at a constant height and the changes in current are recorded and used to produce images
How does constant current mode for an STM work
The current is kept constant by adjusting the height of the probe and changes in height are recorded and used to produce images
