Unit 12 Turning Points in Physics Flashcards
What was Newton’s corpuscular theory (1670s)?
That light is made of ‘corpuscles’ = particles, which are perfectly elastic, rigid, weightless and travel in straight lines
Name two processes that Newton’s corpuscular theory explained.
Reflection - corpuscules bounce off surfaces
Refraction - corpuscules attracted to boundary, travel faster in denser material (incorrect)
Name three processes that Newton’s corpuscular theory could not explain.
Diffraction, interference, polarisation
What was Huygen’s wave theory (1690s)?
That light is made of waves, with wave fronts constructed from secondary wavelets
Name two processes that Huygen’s wave theory explained.
Reflection - wavelets reflected
Refraction - waves travel slower in denser medium (correct)
What was produced in Young’s double slit experiment?
Pattern of bright and dark fringes on a screen, where waves superpose either constructively to form a bright fringe or destructively to form a dark fringe
Why was Newton’s theory accepted despite not explaining some observations?
Newton was a respected scientist.
What is a black body?
An object that will absorb all types of electromagnetic radiation, none will be reflected and also capable of emitting all types of electromagnetic radiation with equal efficiency
What is the ultraviolet catastrophe?
Where scientists had previously predicted that as the temperature of the object increased, the object would emit an infinite amount of ultraviolet radiation. However the spectrum of radiation emitted by a black - body at different temperatures, had a peak in the shorter wavelengths.
What did Planck theorise in order to address the ultraviolet catastrophe?
He theorised that the energy emitted was quantised, and could only be emitted as packets/quanta of energy which were called photons.
The energy of the photon was dependent on the frequency of the radiation.
Therefore there is no catastrophe because higher frequency photons carry more energy,
What does the number of photoelectrons emitted per second from a metal surface depend on?
Intensity of light (number of photons incident per second), so more photons are absorbed by electrons, so more electrons are emitted from the surface of the metal however maximum kinetic energy stays the same for all electrons.
What two things determine the maximum kinetic energy of a photoelectron ejected from a metal surface?
Frequency of the incident photon and work function of the metal - the released electrons have a range of kinetic energies up to a maximum.
How can you explain the instantaneous emission of electrons in the photoelectric effect?
The electrons are emitted as soon as photons are absorbed.
What does wave theory suggest about the photoelectric effect, that is incorrect?
That all wavelengths of light cause electron emission eventually as the energy delivered to the metal ‘builds up’
More intense light would mean a larger maximum kinetic energy as more energy would be delivered to an electron per unit time
How did Fizeau determine the speed of light?
Fizeau spun a toothed wheel with n teeth a distance d between mirrors. When spinning at frequency f, light just gets through the gap so c = 4dnf .
How did Hertz first detect radio waves?
Seeing them make a spark across a gap in a loop, after using an induction coil and capacitor to produce the radio waves
How did Hertz find the radio wave’s speed?
By finding the wavelength by producing standing waves and the frequency using a rotating mirror - c = f λ
Found they were polarised by a change in signal strength when he rotated the dipole
In low energy electron diffraction experiments, what happens to the de Broglie wavelength as the accelerating potential difference increases?
Decreases, as this increases the energy thus increasing the speed of the electrons
For the rings produced in electron diffraction, what two observations would be made as the accelerating potential difference increases?
Rings are brighter - diameter is smaller
What focuses electrons in a transmission electron microscope?
Magnetic fields
How does a scanning tunnelling microscope work?
Fine-tipped conducting probe scans the surface by applying a potential difference.
What did the Michelson- Morley experiment disprove the existence of?
The ether as there was a null result as there was no shift in the interference pattern caused by a difference in speed caused by the difference in time it took for the light to travel between the mirrors after the experiment was rotated 90 degrees. Thus concluding that there is no ether, the speed of light is not affected by the Earth’s motion.
What is an inertial frame of reference?
A frame of reference that moves with constant velocity/ not accelerating and obeys all of Newton’s laws of motion.
In addition to the law of physics having the same form in all frames of reference, what is the second postulate of special relativity?
Speed of light is invariant - the same in all frames of reference
The observation of which particles provide evidence for time dilation and length contraction?
Muons - as more muons were detected at the bottom of a mountain than would be expected from their half life. This was because as muons moved closer to the speed of light, their half life would increase when observed by someone stationary on Earth which allowed muons to reach the surface of the Earth before decaying.
What happens to the mass of a particle as it moves close to the speed of light?
Significantly increases
How did Bertozzi show that the energy of electrons moving close to the speed of light is relativistic?
Measured speed of the electrons and the increase in temperature of a metal plate that the electrons hit.
