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  • hey it's professor Dave, I want to tell you about wave-particle duality

  • so let's take a break from atoms look at something else

  • light. for some time we had known light to behave as an

  • electromagnetic wave, for this reason light is also called electromagnetic

  • radiation

  • light waves have both a wavelength, the distance from tip to tip

  • and a frequency, the number of waves that pass by a point per unit time

  • they are inversely proportional since the shorter the wave

  • the more of them that can go by per second resulting in a higher frequency

  • these variables are related by the following equation:

  • in which c is the speed of light, about 300 million meters per second

  • the speed limit of the universe. the electromagnetic spectrum

  • shows light of all the different wavelengths from gamma rays to radio

  • waves

  • including this tiny sliver in the middle which is visible light

  • the light we can detect with our eyeballs. the wave theory of light

  • was very useful for a time but it could not explain

  • the photoelectric effect. this was the observation

  • that if a certain metal plate is irradiated with light an electron is

  • ejected

  • which can be detected when it interacts with a positively charged wire or

  • plate sensor.

  • the incongruity was that the ability of the light to eject an electron

  • depends only on its frequency and not its intensity

  • so if the beam was below a certain frequency even a very intense beam

  • with a lot of light could not eject an electron whereas

  • above a certain frequency the faintest beam possible

  • could do it. this seem strange and a famous fellow named

  • albert Einstein solved the problem by extending a concept developed by max

  • planck

  • just five years prior. planck proposed that energy is not continuous

  • but rather is quantized, that is all energies

  • are multiples of the smallest fundamental unit of energy

  • the plank energy. in fact quantum theory went on to show that everything is

  • quantized

  • space and time too, so you can't infinitely subdivide them

  • you'll eventually get to a smallest thing that can't be divided further

  • but more on that in the physics course.

  • einstein extended what planck said about energy

  • and rationalized that light must also be made of quanta

  • which he called photons. these are particles of light

  • this explains the photoelectric effect because an electron is ejected

  • when struck by a singular photon with sufficient energy

  • it only takes one photon so even the faintest beam

  • above a certain frequency will be able to do it but if none of them are of that

  • minimum energy

  • no matter how many photons strike the sample an electron will not be ejected

  • it was found that the energy of the photon can be given by the following

  • equation:

  • according to the frequency of the photon and planck's constant

  • we now have to accept that light obeys

  • wave-particle duality, meaning it is both a particle

  • and a wave at the same time. confused? you should be

  • this is the first discovery in a series that brought about the quantum

  • revolution

  • completely changing the landscape of physics and how we view the universe

  • Newton's Laws no longer reign supreme

  • in the world of the very small and the universe became much stranger

  • almost overnight thanks to Einstein and others

  • that were inspired by him. thanks for watching guys, subscribe to my channel

  • for more tutorials

  • and as always feel free to email me

hey it's professor Dave, I want to tell you about wave-particle duality

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B2 中高級 美國腔

波-粒子二象性與光電效應 (Wave-Particle Duality and the Photoelectric Effect)

  • 70 6
    Ken Wang 發佈於 2021 年 01 月 14 日
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