31.THE ATOMIC STRUCTURE -Experimental validation of de Broglie’s thesis.

Any scientific thesis,even if it is scrupulously proved to be true in theory, is not considered absolute , until it is proved by experimental data.As a theoretical physicist, Louis Victor de Broglie , was successful in proposing an idea , which seemed very plausible as it corroborated Niel’ Bohr’s quantum condition.However, without a valid proof, in the form of experimental data supporting the thesis, it would not be accepted with unanimity.

The experimental evidence for de broglie’s theory came in 1927, in Bell Labs in New Jersey.

Although,before studying this experiment, we should first try to understand a few concepts , which are characteristics of waves.

A] Interference  –

Two waves, travelling in the same medium , have a tendency to superpose i.e they get one above the other.The resultant wave formed due to this superposition could have an amplitude greater than or less than the two parent waves. This phenomenon is called as interference of waves.

Check out the following – File:Waventerference.gif  – (In this presentation, the green and blue are parent waves.The red wave is the resultant wave.)

There are two kinds of interferences –

1)Constructive Interference and
2)Destructive Interference.

311.jpg

B]Diffraction

The bending of a wave around objects is called Diffraction.When a wave encounters an obstacle or travel through a slit(small hole), the waves get bent around the corners of that obstacle or spread out after passing through the slit.Have you ever seen a CD in sunlight? We see different colors of light.This is due to diffraction! The light comes in contact with the CD and gets diffracted to produce lights of varied colors.

IMG_4624.jpg
Diffraction of light.

The length of the slit / gap through which the wave passes is inversely proportional to the amount of diffraction.

Smaller the gap, the greater the diffraction .
Check the following link –
https://en.wikipedia.org/wiki/File:Wavelength%3Dslitwidthspectrum.gif

Greater the gap,smaller the diffraction.
Check the following -https://en.wikipedia.org/wiki/Diffraction#/media/File:5wavelength%3Dslitwidthsprectrum.gif

Now let us consider a double -slit scenerio.If the waves encounter an obstacle/dam in a confined space,then there would be two gaps through which waves have to pass.

e.g.– Consider a water tank with a paddle.When this paddle rotates, it will produce waves in water.Let us try to study the double slit experiment with respect to this tank.We have to consider two possibilities –

Possibility 1- 

If we keep an obstacle/dam in the path of these water waves, equidistant from the two  sides of the tank which are  parallel to the propagation of the waves,then two large gaps will be formed.The space between the sides of the tank and the obstacle i.e. the length of the gap will be ‘d’ as follows –

313.jpg In this case, the obstacle will hinder the transmission of waves and thus cast a shadow.This means that the waves will continue to  propagate from the sides of the obstacle/dam ,except in the region of the dam. Thus,in the region beyond the obstacle/dam, on the other side of the paddle, no waves will be seen.Thus, a shadow will be obtained.This is comparable to the shadow of an opaque object formed when light waves fall on it.314.jpgThis is thus comparable to Ray Optics.

[NOTERAY OPTICS describes the propagation of light in terms of rays.e.g. rays of sunlight.The general assumptions are –

  • Rays travel in straight lines in a given medium.
  • Rays are a beam of particles.
  • Rays split in two when they encounter a hindrance.
  • Rays can be reflected from a surface.
  • Rays do not exhibit phenomenon of diffraction or Interference as their wave-like nature is not considered.]

Thus, this system can be modelled as a ‘water beam system’ as water is behaving as a beam of particles here- travelling in straight lines , and creating a shadow when it encounters an obstacle.

Possibility 2

If the spacing between the slits ‘d’  is much less than the wavelength(λ) of the water waves, a different phenomenon is observed.The waves diffract and interfere with each other giving a characteristic pattern of  interference fringes i.e alternate light and dark regions.THIS IS THE PHENOMENON CALLED DIFFRACTION.Here , we do NOT get a shadow, instead the whole screen has alternate light and dark regions as follows-

This cannot be explained by ray optics.ONLY WAVE LIKE BEHAVIOUR OF WATER CAN CAUSE SUCH A PHENOMENON. Thus, here the water particles behave as waves.They interefere with each other as shown in the figure, and thus cause constructive and destructive interference patterns in the form of light and dark regions on the screen respectively.Diffraction occurs with all waves, including sound waves, water waves and electromagnetic waves such as visible light, radio waves ,X-rays etc.The results obtained are as follows-

Now, let us begin our discussion with the actual experiment which was an experimental proof of de broglie’s thesis.

The Davisson and Germer Experiment – 

Two American physicists were conducting experiments to study the structure of Nickel by directing a beam of electrons at the Ni surface, in Bell Lab in New Jersey.

They were shooting accelerated electrons at Ni metal and studying how these electrons were bouncing off the Nickel’s surface. The experiment was conducted in a vacuum chamber to ensure that the electron beams directly hit the target. During their study, accidentally air entered the vacuum chamber and an oxide film was formed on the Ni surface. To correct this , they heated the Ni target to very high temperature in an oven.This lead to formation of large single crystal areas accidently.Davisson and Germer were unaware of this new development and when they reconducted their experiment, they got diffraction patterns with unexpected peaks !

They later got an inkling as to what might have happened and started their research in that direction.They first irradiated the Ni crystal with X-rays.X-rays were electromagnetic radiation with wavelength 10Å .The distance ‘d’ between two atoms in a NiCl crystal is 3.53Å.So, as seen earlier, as d< λ it was expected to get a diffraction pattern which consisted of concentric circles.

319.jpg

They conducted the same experiment with an electron beam.The source of electrons was a tungsten filament.The ballistic (free)electrons, which were produced, were accelerated and then made to hit the Nickel Chloride (NiCl)crystal.We can theoretically calculate that the wavelength of an electron is 12Å (using De broglie’s equation).The result obtained was a diffraction pattern similar to the X-ray diffraction pattern!There was no way of explaining this result, except accepting that the electron beam was behaving as a wave! Thus, this experiment proved experimentally the De broglie’s hypothesis that matter also behaves as waves.

318.jpg

Note that, X-rays were a representative of waves and electrons were a representative of matter.

In 1929,Louis Victor de broglie , was awarded the nobel prize.The Nobel committee stated that –

 “When quite young, you threw yourself into the controversy raging over the most profound problem in physics. You had the boldness to assert, without the support of any evidence whatsoever, that matter had not only a corpuscular nature but also a wave nature. Experiments came later and established the correctness of your view.”

Thus, the wave-particle duality was known to be true world wide.A new development was made in the field of nuclear physics.However, if matter behaved as waves, why don’t we see this as a reality in our daily lives?What else did the research in the field of atomic physics yeild? Were there any new developments? Yes , there were!! In our next post, we shall start talking about another giant in the field of modern physics and his thesis.Till then,

Be a perpetual student of life and keep learning!

Good day !

References and Further Reading –

1)https://en.wikipedia.org/wiki/Interference_(wave_propagation)

2)https://en.wikipedia.org/wiki/Geometrical_optics

3)Lecture 6,MIT Open courseware 3.091.

4)http://abyss.uoregon.edu/~js/21st_century_science/lectures/lec13.html

Image Source –

1)https://en.wikipedia.org/wiki/File:Waventerference.gif

2)http://www.uh.edu/engines/two_slits-b.jpg

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