Davisson–Germer Experiment


Thomas Young, in 1805 performed his famous double slit experiment and showed that light actually behaves like a wave. These results were baffling and created a turmoil in physics. Big question which everyone asked was : WHAT IS LIGHT? A WAVE OR A PARTICLE ? DOES LIGHT ACT AS BOTH PARTICLE AND WAVE?

       Young’s Double Slit Experiment

The matter settled when Louis de Broglie gave a hypothesis. He said that light and every other moving object shows both particle and wave like characteristics simultaneously.  But which one of the two characters dominate depends upon velocity (precisely momentum) and mass of the object. Here is de Broglie equation:

\lambda = \frac{h}{p}

λ= wavelength, h= planks constant(6.626×10−34   ), p= momentum

In 1927, Davisson and Germer performed an experiment similar to Young’s but this time it was not with light but with electron. And like light, electron also produced an interference pattern. This showed that individual electron is also a wave ( However an electron act as particle when it is not observed. This is due to uncertainty principle. If not understood ,  it can be ignored)

Now the question. In water wave, a group of water molecules goes up and some of them goes down and thus forming crests and troughs, which is typical characteristic of wave. If an individual electron is a wave then what is it that is waving?

Erwin Schrodinger gave first guess. He said that electrons constituents smeared out in space and smeared electron essence does the waving. This was later disapproved. If electron smeared out in space then we will observe some part of its charge over here and some of its mass over there. But this is not true. When we observe electron, its mass and charge is concentrated at a single point.

In 1927 something unusual was said by Max Born. The wave is not a smeared out electron. It was something which was never encountered in science before. The wave which Max Born was talking about is a probability wave.

The probability wave envisioned by electron have regions of high intensity and low intensity. But the meaning which Max Born said about the shape of wave was totally different. He said ” the size of wave at a given point in space is proportional to the probability that the electron is located at that point in space”. Places where the probability wave is large are locations where the electron is likely to be found. The places in space where it is small is place where electron is less likely to be found.

Now let us look at double slit experiment with electron. When electron was forced to pass through each slit, it has a choice either to pass through right slit or through left slit. But the probability wave of electron through each slit has equal amplitude. So instead of electron passing through either one, electron pass through each of the slits, commingles its probability wave (?) and produce an interference pattern.Quantum mechanics once again was successful in explaining the behavior of micro particles. It always have. It explains what we see but it does not explain why we see. If not understood take a look at the video.

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2 thoughts on “Davisson–Germer Experiment

  1. Good post. Some questions in general to think about:

    1. How accurate is plank’s constant? Does it ever change? How do you know it never changes? What instrumentation was used to come up with it and are we sure that it’s accurate to the infinite decimal place? If not, then we may be experiencing phenomena that seem at odds with classical theories becuase our equations are not exactly perfect.

    2. How did they know that they only sent a single photon through the two slit experiment? In order to know it was single wouldn’t you have to measure it and thereby induce energy or extract it from the experiment itself giving you false data? How would you know that the photon imprint on the photopaper on the wall was the same photon that you shot and not a different one that the first one collided with or that it collided with millions and that the total amount of energy at the end was sufficient to send the last one smacking into the photopaper?

    3. Does all matter emit photons or some kind of energy? Doesn’t this interfere with all the other energy around? Maybe the photopaper or the body of the laser, or the walls of the room supplied energy that reacted with the photon or electron to make it do millions of different things and at the end it just works itself out that you are left with a single spot on the photopaper.

    4. If the mass of an electron is concerntrated at a single point, then it has no dimensions. If it has no dimensions then it is non-physical, and if it is non-physical then why use physics to study it?

    • 1. I think you are expecting to much from the planks constant. No it does not changes as it determines the size of quantum of energy. It can only change with change in quanta itself (which does not happen).Yes we can determine its value till infinity (or how much decimal places you want). However that value cannot be used as it would be too unrealistic.
      2. It used a complicated machines. I am not the right person to tell you about it , nor I am focusing on it. And this experiment can be performed at home with light. Results will be same
      3. It is not like we can create a photon out of void. It is only produced when an excited electron collapse to lower orbital state. Matter do not manufacture photons out of nothing. They utilize the energy from surrounding to create a photon. In fact, a photon is type of matter wave. It all depends on how you define a ‘matter’. The interaction of system with surrounding is bound to happen according to laws of thermodynamics. However effects are negligible, but are present.
      4. Now since you got indulged in dimensions of an electron, you cannot ignore the string concept of electron. You see, dimensions originate from a point. If there are no points there would be no dimensions.

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