Turning points 2

Question 1

What was Newton’s theory for light

Answer 1

Made up of tiny particles called corpuscles

Question 2

How did Newton explain why light reflects

Answer 2

• 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

Question 3

How did Newton explain why light refracts

Answer 3

• 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

Question 4

What did Newton believe about the speed of light as it passes into a denser medium

Answer 4

Perpendicular component of velocity is greater whilst parallel component is the same so speed increases

Question 5

What was Huygens principle

Answer 5

• 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)

Question 6

What did Huygens believe about the speed of light as it enters a denser medium

Answer 6

Slows down (and bend towards the normal)

Question 7

Give 3 reasons why Newtons theory of light preferred over Huygens

Answer 7

• Newton had a very high reputation at the time
• Diffraction had not been observed
• Speed of light had not been measured

Question 8

How did Young’s double slit experiment prove Huygens theory of light

Answer 8

• 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

Question 9

What would have occurred in Young’s double slit experiment if Newton was right

Answer 9

There would be only 2 bright fringes corresponding to the 2 slits instead of an interference pattern

Question 10

When did scientists start agreeing with Huygen’s wave principle

Answer 10

• Years after Young’s experiment when the speed of light in water was measured to be slower than in air

Question 11

What is an EM wave formed of

Answer 11

• 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

Question 12

What was Maxwell’s formula for the speed of an EM wave in a vacuum

Answer 12

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

Question 13

Explain Fizeau’s method for measuring the speed of light

Answer 13

1. Light source causes beam of light which is passed through a rotating wheel
2. The wheel has teeth and gaps so the light only passes through if it passes through a gap
3. There is a mirror about 8km away so the light comes back at the wheel after reflecting off the mirror
4. 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
5. 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

Question 14

What calculations did Fizeua do after his experiment to calculate the speed of light

Answer 14

1. Distance travelled = 2d as it goes to the mirror and back
2. 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
3. Time period = 1/f so a tooth will replace a gap every 1/2nf seconds
4. c = 2d / (1/2nf) = 4dnf

Question 15

Why did people believe light was an EM wave after Fizeau’s experiment

Answer 15

His value for speed of light was very close to Maxwell’s value for speed of EM wave

Question 16

What was the apparatus used in Hertz’s experiment to find the speed of radio waves

Answer 16

• 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

Question 17

How did Hertz produce radio waves in his experiment to find the speed of them

Answer 17

When the induction coil produced a high pd causing sparks to jump across the gap of air radio waves are produced

Question 18

How did Hertz detect the magnetic field of the radio waves

Answer 18

• 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

Question 19

How did Hertz detect the electric field of the radio waves

Answer 19

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

Question 20

Explain how Hertz calculated the speed of radio waves

Answer 20

• 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 adjacent nodes and double for wavelength
  c = fλ

Question 21

How did Hertz’s experiment prove that radio waves were EM waves

Answer 21

They travelled at the same speed that Maxwell predicted for the speed of EM waves

Question 22

How could Hertz show that radio waves were polarised

Answer 22

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

Question 23

Define a black body

Answer 23

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

Question 24

What was the predicted graph of intensity of EM radiation emitted against wavelength of EM radiation emitted according to classical wave theory

Answer 24

As wavelength decreases, intensity increases graph is in the form y = k/x

Question 25

What was the ultraviolet catastrophe

Answer 25

- 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

Question 26

How does the graph of intensity against wavelength for a black body change at different temperatures

Answer 26

- 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

Question 27

How did Planck solve the UV catastrophe

Answer 27

EM waves travel in discrete packets (quanta) which have energy proportional to frequency

Question 28

Why could wave theory also not explain the photoelectric effect

Answer 28

1. Wave theory would mean no threshold frequency as even at low frequencies, energy would gradually increase until electron absorbs enough 2. 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 3. 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

Question 29

What is the stopping potential (Vs)

Answer 29

Pd applied across the metal required to stop photoelectrons with max KE Ekmax = eVs

Question 30

How can you relate stopping potential to Einstein's photoelectric equation

Answer 30

E = hf = ϕ + Ekmax E = hf = ϕ + eVs Vs = (h/e) f - (ϕ/e)

Question 31

Describe the graph of stopping potential against frequency

Answer 31

Vs = (h/e) f - (ϕ/e) so straight line with; gradient of (h/e) y intercept of -(ϕ/e) x intercept of threshold frequency

Question 32

What is the de Broglie hypothesis

Answer 32

- 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

Question 33

Describe the experiment which shows electron diffraction

Answer 33

1. Electron gun accelerates electrons towards a crystal lattice 2. Electrons interact with small gaps between atoms, causing diffraction and producing interference pattern on a fluorescent screen behind the crystal lattice

Question 34

Derive a relationship between the de Broglie wavelength and the pd used to accelerate the electrons towards the crystal lattice

Answer 34

1/2 mv^2 = eV m^2 v^2 = 2meV mv = sqrt (2meV) λ = h / sqrt(2meV)

Question 35

How does increasing the accelerating voltage affect the interference pattern

Answer 35

1. λ = h / sqrt(2meV) so as V increases λ decreases 2. smaller λ means the fringe spacing decreases and the concentric rings get closer together

Question 36

Why does an electron microscope have a much higher resolving power than a light microscope

Answer 36

- 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

Question 37

What is the first stage of how a transmission electron microscope (TEM) works

Answer 37

Accelerate electrons using electron gun through condenser lens which deflects the electrons so they form a wide parallel beam

Question 38

What is the second stage of how a transmission electron microscope (TEM) works

Answer 38

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

Question 39

What is the final stage of how a transmission electron microscope (TEM) works

Answer 39

Electron beam now passes through projector lens which magnifies the image made by the objective lens and projects it on to a fluorescent screen

Question 40

How does accelerating voltage of the electron gun affect the resolving power

Answer 40

If V is increased, λ decreases so resolving power increases

Question 41

What are limitations of the TEM microscope

Answer 41

1. If the sample is too thick, the beam of electrons will slow down causing their wavelength to increase so resolving power decreases 2. 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

Question 42

What is quantum tunnelling

Answer 42

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

Question 43

How does a scanning tunnelling microscope (STM) work

Answer 43

- 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

Question 44

How does constant height mode for an STM work

Answer 44

The probe is kept at a constant height and the changes in current are recorded and used to produce images

Question 45

How does constant current mode for an STM work

Answer 45

The current is kept constant by adjusting the height of the probe and changes in height are recorded and used to produce images