Module 7: Nature of Light Flashcards
Explain, using Einstein’s thought experiments, why the length changes for the observer
Einstein predicted from his postulates that the laws of relativity must hold for all experiments and that the speed of light is the absolute reference point for all investigations. This means that the speed of light must be invariant for all observers. For this to be true the length of the spacecraft which moves with a speed of __ will be different as measured by the astronaut and an outside observer. The journey predicts that the length will contract a significant amount as measured by the observer while the proper length is measured by the astronaut. The length of the spacecraft must contract for the observer for the speed of light to remain invariant for both the observer and the astronaut. As the spacecraft approaches the speed of light the length will become increasingly shorter
Cathode Ray Experiment, Describe TWO observations that were made in this experiment which contradicted the Wave Theory of Light.
The first observation which contradicted the wave model was that increasing the intensity of light did not increase the photocurrent in all situations, as would have been expected. The second observation was that a when a continuous supply of energy was placed on the surface of the cathode it did not eventually produce the emission of a photocurrent – a definite threshold frequency was required for the photocurrent to start. These observations were not predicted by the wave model but supported the particle model.
Outline how a continuous (black body) spectrum can be produced in a classroom
To produce a continuous spectra, a voltage was applied to an incadescent filament so it heated up and visibly glowed. This can the be observed using a spectroscope.
Outline how an emission spectrum can be produced in a classroom
To produce an emission spectra, a high voltage was applied to a spectral tube containing gas. Which then can be observed using a spectroscope or a diffraction grating.
Define and describe muons
Muons are short-lived subatomic particles created in the atmosphereby cosmic ray collisions. They have a mean life of 2.2 microseconds and travel near the speed of light.
Explain how muons support Einstein’s theory of Special Relativity
Using their lifespan and travel speed, the calculated distance they travel is not enough to reach the Earth’s surface from the atmosphere.
However, detectors on Earth’s surface have detected muons, meaning they are able to travel a much larger distance than predicted.
Since muons travel near the speed of light, In the reference frame of an observer on Earth’s surface, the rate of time for the muon would be slower. Hence, muons have a much longer lifespan.
In the reference frame of muons, Earth is traveling near the speed of light towards them. Thus, muons will observe the length contraction of the distance between the atmosphere and Earth’s surface, meaning
they travel a smaller distance.
The detection of muons at Earth’s surface has provided evidence that supports Einstein’s theory of special relativity including time dilation and length contraction.
Evaluate the significance of the Hafele-Keating experiment regarding special relativity
Hafele and Keating aimed to validate the theory of time dilation by flying atomic clocks around the world. 2 clocks were inside planes and 2 clocks remained on Earth’s surface (the control) for reliability. The
planes travelled east and then west
Greater time dilation was observed in the clocks on the westward journey. Eastward traveling clocks lost 59 ± 10ns while the westward traveling clocks gained 273 ± 7ns
Greater time dilation in west direction was due to greater relative motion between the Earth and plane as it traveled in the opposite direction to Earth’s rotation
The differences in time reading of the clocks that travelled and the stationary ones agreed with predictions using time dilation. Hence,
these results supported Einstein’s theory of special relativity.
Describe the observatons about the photoeletric effect that could not be explained by classical electromagnetic theory
Existence of threshold frequency
No photolectrons are emitted from a metal suface if the frequency of incident light is below the threshold frequency, this could not explained by the wave theory of light as waves are a transfer of energy and energy in the metal would build up overtime regardless of frequency and emit electrons which is not the case.
Max KE of electrons (Ke= HF-work function)
Classical wave theory failed to explain this as it relates the intensity of the wave to the amplitude, wrongly predicted the energy of photoelectrons is related to the intensity of light and independent of frequency
Instant emission of electrons
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Explain the prediction and propagation of electromagnetic waves and relate them to Maxwell’s predictions
Hertz found EM waves could be produced by an oscillating electric field. He confirmed their existence, production and that they travelled at the accepted speed of light.
Mawell’s prediction of electromagnetic waves resulted from his formulation of a complete and symmetric theory of electricity and magnetism known as Maxewell’s equation.
The waves predicted by Maxwell would consist of oscillating electric and magnetic fields perpendicular to each other, defined to be an electromagnetic wave (EM waves). Electromagnetic waves would be capable of exerting forces on charges great distances from their source, and they might thus be detectabe. They could be produced by an osciallating electric charge. Maxwell’s equations calculates the speed of EM waves correctly.
Describe how the study of spectra of a star can give us information about its surface temperature, motion and chemical composition
Wein’s law states that the wavelength of the maximum energy emitted from a perfect black body is inversely proportional to the temperature of the body. The spectra if a sar can be used to determine the wavelength of maximum intensity then substituted into Wein’s equation.
If the spectral lines in a star’s spectrum are uniformly shifted to the red end (receding), shifted to the blue end (moving towards) it can tell us the relative motion and translational motion of the star.
The presence of a spectral line correspondng to the specific energy transition for an ion, element or molecule is present in that star. This allows spectral class to be determined.
By matching its luminosity curve with that of a blackbody, surface temperature can be determined as a stellar spectrum is composed of a blackbody radiation curve superimposed upon a vast number of dark absorption lines.
Identify properties of stars which can be determined from their spectra
- surface temperature
- chemical compositional
- rotational velocity
- translational velocity (approach/recession of a star)
Explain why the intensity of the second-order inteference maximum is less than the intensity of first order maximum
As light is diffracted from each slit, the diffraction angle differs for each order where the greater the angle of diffraction the lower the intensity. The second maximum occurs at a greater angle of diffraction and hence will be less intense than the first maximum.
Explain how the inteference pattern would change if the double slit was replaced with a diffraction grating that had a slit seperation that was greater than the separation between slits
Maxima would become much brighter and sharper, but the angular separation between maximum would be reduced. The interference pattern on the screen would be compressed but the maxima would appear sharp in the form of bright lines on the screen.
Explain how spectroscopy is used to indentiy elements
Each element emits and absorbs specific wavelengths of light that act as a signature of that element, by observing emission and absorption spectra elements can therefore be determined.
Describe how you could determine if a beam of light was polarised or unpolarised
A polarising filter could be used to determine if the light was polarised because the intensity of polarised light that is passed through a polarising filter depends on the angle between the electric field and the incident light and the polarisation axis of the filter (Imax= Icos^2)
If unpolarised light is passed through a polariing filter, rotating the filter will not change the intensity of the transmitted light.
If polarised light is passed through a polarising filter, rotating the filter will change the intensity of light between a maximum and zero each 90 degrees of rotation.
