Unit 5 Flashcards
(34 cards)
Explain why stationary waves are formed and explain how the wavelength of the radio waves can be determined by moving the detector (6)
- Radiowaves from the transmitter are reflected back towards the transmitter
- Stationary waves are created by the superposition of 2 progressive waves with the same frequency and amplitude moving in opposite directions
- Nodes and Antinodes are formed
- Distance between nodes is 1/2 a wavelength
- Detector signal is Zero at the nodes
What was the outcome of Hertz’ investigation? (2)
- Speed of Radiowaves is the same as the speed of light
- Speed of EM waves is the same as the speed of light so Radiowaves are EM waves
State de Broglie’s hypothesis
-Matter particles have wavelike properties
TEM- Explain why greater image detail is seen when the anode voltage is increased (2)
- Because the potential difference is increased and therefore the speed of the electrons is increased
- This means hat the de Broglie’s wavelength is reduced
- Less refraction occurs for a reduced wavelength so greater resolution of image is produced
TEM- State and explain 1 reason why it is important that the electrons in the beam have the same speed
- Electrons of different speeds have different de Broglie wavelengths
- Therefore the resolution would be reduced
STM- explain why electrons can cross the gap between the probe tip and the surface
- The electrons transfer from - to +
- The electrons can cross due to their wave nature
- Their de Broglie wavelength is sufficiently long to stretch across
- The narrower the gap the greater the number of electrons per second that cross the gap
STM- Describe one way in which an STM is used to investigate a surface
- In constant height mode, the tunnelling current is recorded as the tip scans the surface in a fixed plane
- As it does so the tunnelling current is recorded to map the height of the surface
- As the height increases the current decreases
Use Huygens wave theory of light to explain the formation of these fringes by the double slits
- Waves are emitted by each slit
- Each slit diffracts light
- Bright fringes occur when 1 light reinforces the other
- Dark fringes occur when they cancel out
- The 2 slits are coherent light sources
Explain what Newtons theory of light would predict for the same experimental arrangement (double slit)
- Light consists of corpuscles
- Corpuscles would not be diffracted and would pass straight through the slits
- So only 2 bright fringes would be seen
Give one reason why Huygens wave theory of light did not replace Newtons’.
- There was no evidence that light travelled slower in water as predicted by Huygens’ theory
- Huygens’ theory considered light waves as longitudinal and therefore could not explain polarisation
Use Newton’s theory of light to explain the refraction of the light Ray at the air/glass boundary
- When light is refracted from air into glass, corpuscles are attracted towards the glass surface so travel faster
- The component of velocity perpendicular to the boundary of each corpuscle is increased
- The component of velocity parallel to the boundary is unchanged
- Direction is the same after leaving the glass as before the entry to glass
State and explain one piece of evidence that supports Huygens’ theory.
- A piece of evidence is Young’s double slit experiment
- It’s shows that light passing through double slits produces an interference pattern
- Interference is a wave property
State Huygens’ principle.
Every point on a wavefront may be considered to be a point source of secondary wavelets that have the speed of the wave
Explain why the photoelectric effect occurs when a metal plate is illuminated by blue light but not by red light
- Light consists of photons
- An electron in the metal absorbs the energy from a photon
- An electron needs a minimum amount of energy to escape
- Blue photo shave a greater energy than red photons
- The red light didn’t have enough energy to overcome the threshold frequency
Outline why Huygens’ wave theory of light fails to explain that blue light causes photoelectric emission but red light does not
- Every electron would gain suffice incident energy from the waves
- No matter what the frequency of light is
Describe in terms of electric and magnetic fields, the nature of electromagnetic waves travelling in a vacuum.
- Electric wave is 90’ to the magnetic wave
- Electric wave and magnetic wave are in phase
- The direction of travel (propagation) is perpendicular to both waves
Describe in terms of electric and magnetic fields, a plane polarised electromagnetic wave travelling in a vacuum
Vibrations of the electric wave and magnetic wave are:
- perpendicular to each other
- perpendicular to the direction of propagation
- In phase with each other
Explain why the theory of special relativity does not allow matter particles to travel as fast as light
- As the relative speed of an object approaches c, the mass approaches infinity which is physically impossible
- Since E=mc^2, as the energy of the particle increases the mass increases
Einstein’s postulates
1) the speed of light in free space is invariant (constant regardless to the speed of the observer or the light source)
2) Physical laws have the same form inertial frames of reference
What is meant by an inertial frame of reference?
-An inertial frame of reference is one in which Newton’s first law is obeyed
E.g. Object moving at constant velocity
Explain why interference fringes are seen
- A bright fringe is caused by the 2 beams arriving in phase with one another
- Dark fringes occur when they are 180’ out of phase
The interference fringe pattern did not shift when the apparatus was rotated by 90’. Explain the significance of this null observation
- Rotation by 90’ realigns beams relative to the direction of the Earth’s direction of motion
- No shift means no change in optical path difference between the two beams
- Therefore time taken by the light to travel each mirror is unchanged by rotation
- Distance to mirrors is unchanged by rotation
- No shift means speed of light is unaffected
Isotopes
Different forms of the same element, with the same proton number but a different nucleon number
Radioactive Decay
When an unstable atom breaks down to become more stable by releasing energy
