M7 Flashcards
Describe Hert’z’s experimental setup that enabled him to back Maxwell’s findings on light as an EM wave.
Hertz connected an induction coil with a high voltage and rapidly oscillating electric field to a transmitting circuit with a small gap. The current caused great sparking to occur between the spherical electrodes between which the gap exists. He then set up another receiving coil (receiver) nearby which was a looped wire (no power source) where there was a gap of similar size to the transmitting circuit. When the receiving coil was brought near the transmitting coil, a similar sparking occurred. The sparking was due to the electric field oscillation of the EM wave.
Hertz found that the frequency of the vibration in receiving was the same as the Efield in the transmitting coil; the fact that the wave was transmitted from one coil to another was the first evidence of the propagation of an EM wave. He found that the invisible waves behaved like visible light, and the speed of the waves was the same as the speed of light. Further experiments using reflecting surfaces showed that radio waves can be reflected and refracted, which is consistent with the behaviour of visible light.
Thus supporting Maxwell’s theories.
Describe one historical method of determining the speed of light
Fizeau shone a bright light through a toothed cogwheel. As the wheel turned, sometimes light would be blocked by the cogs. Fizeau set up a mirror 8 km away from the cog in order to measure the time-of-flight for light to travel the 16 km path. As the pulse of light also needed to pass again through the teeth on the way back for it to be observed, knowing the speed of rotation of the wheel and the distance between the cogwheel and the mirror, Fizeau was able to calculate the speed of light to be approximately 315 000 km s (s = d/t). Foucault then refined this method by using a rotating mirror to block light’s path, he found the speed of light to be 2.98 x 10^8 m/s.
Explain the significance of Einstein’s mirror and train thought experiment
Einstein considered what would happen to his reflection in a mirror if he was in a train travelling at c
If the train moved at c, according to the aether model, there would be no relative motion between the inertial frame of the train and the light, as light travelled at constant c relative to the aether itself.
This meant that light would never catch up with the mirror to return as a reflection, and if it did then it would have to be travelling at twice the known speed of light, as observed by a stationary observer outside the train, which would be highly impossible as light is a constant speed c
The action of holding up a mirror in this inertial frame of reference would indicate the velocity of the frame, which would violate the principle of relativity
Ultimately he proposed that the velocity of light is a constant value ‘c’ regardless of the relative motion of the source and observer.
This ultimately led him to propose the theory of special relativity, suggesting that distance and time became relative instead.
Eventually, the existence of an aether was also overruled by the Michelson-Morley experiment.
What was the purpose and result of the Michelson-Morley experiment?
Aim: Measure the relative velocity of Earth through the aether
Method:
A beam of light from a monochromatic light source was directed at the 45° half-silvered mirror which splits it into two perpendicular beams.
One beam travels to mirror M1 and the other to mirror M2.
The beams are reflected by M1 and M2 and rejoin after passing through Ms (half mirror)
It will produce an interference pattern due to the phase differences in the time taken for the separate light beams to travel their separate paths. This time difference would be due to the aether wind, slowing down or speeding up one or both of the light beams. They rotated the sandstone block around, expecting the interference fringes to move side to side (i.e. shift)
Result:
No change in interference pattern was observed, null result
This demonstrated that there was no motion of the earth relative to the aether. Suggesting the aether model was flawed as it could not account for this.
Outline an experiment showing the relativity of simultaneity
Imagine that there are two observers: Anne and Ben, standing inside a train and a third observer Claire outside on the platform
The train has a light bulb at its very centre
Anne and Ben observe that the flash of light reaches the front and back walls at the same time, and this makes sense as the light bulb is at the very centre of the train.
Clare observes the same flashes, except she sees something different, As the train has inevitably moved forward in the instant of time between the light bulb being turned on and it reaching the wall, the distance to the back wall is less than the distance to the front wall. As d =s/t and speed is constant, Clare observes the time taken being greater for light to reach the back wall.
In essence, what we are seeing here is that events that are simultaneous in one frame of reference appear not to be in another frame of reference
This is the relativity of simultaneity and is a consequence of Einstein’s theory of special relativity which can be proven to be true with great accuracy today.
Time dilation experiment
Einstein performed a simple thought experiment involving lights and mirrors on a train moving at a constant relativistic velocity relative to a stationary observer. When the light clock switches on, the man on the train, sees the light beam travel up to the mirror and back at rest (linear path). The man on the platform sees the train move quickly and as a result, the light beam’s path is diagonal. The man on the platform sees light travel a longer distance at the same speed so it must take a longer time to do so. Therefore, time appears to move slower for people travelling closer to the speed of light.
What is one piece of evidence for time dilation?
Muons are a type of subatomic particle formed in the upper atmosphere through the interaction between atmospheric and cosmic radiation. They have a very short proper lifetime of about 2 microseconds which means that they cannot reach the Earth’s surface even though they travel at relativistic velocities.
However, using advanced scientific equipment has allowed the detection of these particles on the Earth’s surface. This led to the measurement of their lifetimes to be 16 microseconds from the external reference frame of Earth. The factor difference between 16 and the actual speed of muons was the Lorentz factor. Hence this phenomenon is evidence for time dilation and is evidence for the validity of Einstein’s theory of special relativity.
Explain why mass dilates at the speed of light and why an object will never reach the speed of light.
As an object approaches the speed of light, due to momentum dilation, its momentum tends towards infinity. Any force acting on the object will act to increase the object’s momentum, however, this force will act to increase mass rather than its velocity. So, the increase in velocity of an object will tend towards 0. And an infinite force is required to increase its velocity to c, this is impossible and hence an object will never reach c.
Outline the steps that led to the development of the theory of special relativity with reference to scientists who contributed to the debate. (SCAFFOLD)
- Galilean relativity (cannonball exp, introduced cannot determine velocity without looking outside your Frame of reference)
- Newton (applied Gal. law to inertial frames)
- Aether (following Maxwell’s discovery that light was an EM wave, scientists believed it required a medium to propagate, as sound waves do)
- Einstein & mirror/train thought experiment (wanted to test that no such thing as an absolute velocity it is ‘relative’, outline experiment, and state 2 postulates; c is constant for ALL, laws of physics apply to all frames. )
- Michelson Morley (experiment, result) no relative motion between Earth and aether
- Einstein suggested that it was within the nature of light that needed to be re-considered, as speed = distance/time, if speed were to be constant then distance and time must become relative and they were later seen to be true (Time dilation and length contraction).
Outline one modern piece of experimental evidence allowing the speed of light to become an S.I unit
In 1958 Keith Froome utilised a device known as a radio wave interferometer which allowed him to study interference patterns of radio waves, determine their wavelengths and thus knowing frequency, calculate the velocity. The development of the laser in the 1970s provided a light source with a known, fixed wavelength. This meant more variables in the experiment (refraction, diffraction) could be better controlled allowing for more accurate calculation of the speed of light.
Finally, by 1983, the measurements for the speed of light had become so reliable that c became an SI standard unit, fixed to precisely 299 792 458 m s−1.
How does an incandescent filament work, and what type of spectrum is produced?
An incandescent filament produces light by heating a metal filament to a very high temperature which produces EMR at a LARGE range of wavelengths. This produces electromagnetic radiation at a range of wavelengths; that is, the light from the incandescent globe produces a continuous spectrum. Some of the light produced is in the infrared part of the spectrum which is invisible to human beings but is detected as heat.
How do discharge tubes work and what type of spectrum is produced?
A discharge tube contains low-pressure gas through which a current is produced. The electrical current excites this low-pressure gas and upon relaxation, these atoms release UV radiation. This UV radiation excites the phosphor which is the material that coats the inside surface of the glass tube, the phosphor is specifically chosen to ensure only visible wavelengths are produced and get the fluorescent effect
Upon excitation, the phosphor then releases light over the entire visible spectrum, and this makes it useful for everyday use as we can now use it as a standard lightbulb. When the light emitted from a gas discharge tube is dispersed through a glass prism, the light is split into its constituent wavelengths. This produces a spectra composed of only a few discrete wavelengths of light (discontinuous compared to incandescent filament).
What kind of spectra does reflected sunlight produce, and what can this inform us of?
Planets and moons do not produce light of their own but they appear luminous as they reflect the sun’s light. Essentially what the planet/moon reflects they haven’t absorbed. So by analysing what they reflect we can figure out what exactly they absorbed. They will produce an absorption spectrum, matching this absorption spectra to laboratory spectra on Earth we can tell a lot of info regarding the types of atoms and molecules in the atmosphere of these planets/moons. For example, Saturn’s moon Titan is known to have an atmosphere of mostly nitrogen with methane in higher atmospheric regions.
