The basic concept of the Uncertainty principle (we can know the velocity of a particle or its position to certain degrees of accuracy but we cannot know both at the same time) was something I knew for a while but I didn't know how to explain it. So, today, I'd like to start to explain to you about the how this principle was derived by Werner Heisenberg. But this week is just the background.
First, we must brush up on the wave-particle duality and the DeBroglie Hypothesis. As we have seen in last week's post, Max Planck theorized that light was delivered in packets of energy, which he called quanta. These light particles are now called photons. However, another brilliant scientist, before Planck, had conducted an experiment that demonstrated that light was a wave-like phenomenon. Thomas Young did this through studying interference, which is when two waves are added together.
In the experiment shown above, the light source shines a light through a hole in the first screen, which, through diffraction, causes the light to spread through the two holes in the second screen. After the second screen, the light is diffracted again through the two slits and then falls on the last wall showing an interference pattern. This pattern displayed is one of alternating bands of light and darkness. Young explains this phenomena by saying that the bands of light were created by the light waves which were exactly in step, with no phase difference so the peaks of the waves were in sync, causing them to reinforce each other and produce a bright band. However, the bands of darkness, he said, were created by waves which were exactly out of step (had a phase difference of 1/2) and so the peak of one wave coincided with the trough, leading to both waves being cancelled out. As to how
Next came the French physicist Louis de Broglie, who thought that if light could have this wave-particle duality, then maybe so could matter. E.g. a stream of electrons could behave like a wave. This theory was tested in the 1920s by Clinton Davisson and Lester Germer, when they passed an electron beam through a crystal of graphite and they observed an interference pattern. Later on, this property was confirmed for heavier particles such as neutrons so we now accept that the wave particle duality is a universal property of all matter. But in the case of light, the frequency of the wave was directly proportional of the energy of the quantum but the frequency of the matter was hard to define. So De Broglie said that the wavelength of the wave was connected to the momentum of the particle. He set out the equation: wavelength=h/momentum. (the 'h' is Plank's constant).
I'm going to try to reference from now on in order to get some practice for when I'm in Uni...
References:
1. Rae, A.I.M. (2006) Quantum physics: A beginner’s guide. Oxford, England: Oneworld Publications.
2.Davis, C.L. (no date) Light and optics - double slit interference - physics 299. Available at: http://www.physics.louisville.edu/cldavis/phys299/notes/lo_interference.html (Accessed: 28 November 2016) (for photo)
First, we must brush up on the wave-particle duality and the DeBroglie Hypothesis. As we have seen in last week's post, Max Planck theorized that light was delivered in packets of energy, which he called quanta. These light particles are now called photons. However, another brilliant scientist, before Planck, had conducted an experiment that demonstrated that light was a wave-like phenomenon. Thomas Young did this through studying interference, which is when two waves are added together.
Next came the French physicist Louis de Broglie, who thought that if light could have this wave-particle duality, then maybe so could matter. E.g. a stream of electrons could behave like a wave. This theory was tested in the 1920s by Clinton Davisson and Lester Germer, when they passed an electron beam through a crystal of graphite and they observed an interference pattern. Later on, this property was confirmed for heavier particles such as neutrons so we now accept that the wave particle duality is a universal property of all matter. But in the case of light, the frequency of the wave was directly proportional of the energy of the quantum but the frequency of the matter was hard to define. So De Broglie said that the wavelength of the wave was connected to the momentum of the particle. He set out the equation: wavelength=h/momentum. (the 'h' is Plank's constant).
I'm going to try to reference from now on in order to get some practice for when I'm in Uni...
References:
1. Rae, A.I.M. (2006) Quantum physics: A beginner’s guide. Oxford, England: Oneworld Publications.
2.Davis, C.L. (no date) Light and optics - double slit interference - physics 299. Available at: http://www.physics.louisville.edu/cldavis/phys299/notes/lo_interference.html (Accessed: 28 November 2016) (for photo)
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