PWS – Quantum mechanics Joris Visser, Siar Moradi V6
“The odd behavior of light”
1. Introduction
1.1 Background information
Most people think atoms are the smallest particles that exist, as this is what they get told during the chemistry lessons they got at school. However, while everyday matter is made of atoms, atoms themselves consist of multiple particles as well.
These so-called elementary particles are particles whose substructure is unknown. Right now it is unclear whether elementary particles are the smallest particles in the universe, as nobody knows what these particles are made of, yet.
An elementary particle is any particle that cannot be split in multiple particles. An elementary particle is either a matter particle, or fermion, or an anti-matter particle, also known as a boson.
According to the standard model, elementary particles are classified as gauge bosons, leptons, or quarks.
In an atom, only electrons are elementary particles, while neutrons and protons contain quarks themselves.
There are three properties that describe elementary particles: mass, speed, and charge.
Mass:
Every particle with mass produce gravity. A particle only has mass when it takes energy to accelerate it. Photons do not have mass as they do not create gravity nor does it take energy to accelerate them.
Spin:
The turning of a particle has a certain value, called a “spin number”. An elementary particle is always spinning at a certain speed.
Charge:
Protons have a charge of +1, as they consist of 2 quarks with a 2/3 charge and 1 quark with a -1/3 quark, adding up to 1.
A neutron consists of 2 quarks with a -1/3 charge and 1 quark with a 2/3 charge, therefore it has an average charge of 0
An electron is a particle with a charge of -1
– What is quantum mechanics?
Contrary to classical physics, modern physics provides a wider and more accurate description of the universe and the behaviour of particles. Quantum mechanics is one of the biggest fundamentals of modern science. Scientific research into the wave nature of light began in the 17th and 18th centuries, when scientists such as Robert Hooke, Christiaan Huygens and Leonhard Euler proposed a wave theory of light based on experimental observations. Their findings led to the believe that light was a wave and not a collection of particles (photons) when originally light was believed to be a collection of photons as Newton had proposed before. This is because Newton could not explain interference and light polarization.
– How does an elementary particle like a photon behave in the quantum world relative to the normal world?
Light shows certain behaviors that are identical to a wave and it would be difficult to explain with a purely particle-view. Light reflects in the same way that any wave would reflect. Light refracts in the same way that any wave would refract. Light diffracts in the same way that any wave would diffract. Light interferes with itself in the same manner that any wave would interfere. And light exhibits the Doppler effect just as any wave would exhibit the Doppler effect. Light behaves in a way which is consistent with our mathematical understanding of waves. Since light behaves like a wave, one would have good reason to believe that it might be a wave.
A wave doesn't just stop when it reaches the end of the medium. Rather, a wave will show certain behaviors when it encounters the end of the medium.There will be some reflection off the boundary and some transmission into it. The transmitted wave bends if it approaches the boundary at an angle. If the boundary is only an obstacle in the medium, and if the obstacle is smaller than the wavelength of the wave, then there will be little noticeable diffraction of the wave around the object. This results in the diffraction pattern and the interference patterns in our experiment.
So light behaves like a wave, as the results of our experiment are consistent with the characteristics of a wave.
– Double slit experiment introduction
In 1801 Thomas Young tried to derive an equation relating the wavelength of a light source to a reliably measured distances associated with a two-point source light interference pattern. The equation is known as the Young’s equation: λ = y • d / (m • L):
Slit Separation (d) in mm
Distance from Slits to Screen (L) in m
Distance from AN0 to AN4 (y) in cm
Order value (m) in integers
Young's method involved using sunlight that entered the room through a pinhole in a window shutter. A mirror was used to direct the pinhole beam horizontally across the room. To obtain two sources of light, Young used a small paper card to break the single pinhole beam into two beams, with part of the beam passing by the left side of the card and part of the beam passing by the right side of the card. Since these two beams emerged from the same source (the sun) they could be considered coming from two coherent sources. Light waves from these two sources (the left side and the right side of the card) would interfere. The interference pattern was then projected onto a screen where measurements could be made to determine the wavelength of light.
Our version of the same experiment is performed using a laser beam as the source. Rather than using a note card to split the single beam into two coherent beams, a carbon-coated glass slide with two closely spaced etched slits is used. The slide with its slits is most commonly purchased from a manufacturer who provides a measured value for the slit separation distance – the d value in Young's equation. Light from the laser beam diffracts through the slits and emerges as two separate coherent waves. The interference pattern is then projected onto a screen where reliable measurements can be made of L and y for a given bright spot with order value m. Knowing these four values allows a person to determine the value of the wavelength of the original light source.
1.2 Research question
Does an elementary particle, such as a photon, show the characteristics of a wave or a particle?
1.3 Sub-questions
– How does a change in the amount of slits change the interference pattern?
– In what way is the distance between the light source and the surface related to the size of the interference pattern?
– In what way is the distance between the light source and the slits related to the size of the interference pattern?
1.4 Hypothesis
An elementary particle will show the characteristics of both a wave and a particle because, when the particle is in a quantum superposition it behaves like a wave of all possibilities and when the particle is observed or in a state of quantum decoherence it behaves like a classical particle.
Quantum superposition:
An image of light when both wave Quantum superposition; two energy levels Particle in superposition when going
and photon. through double slit
Quantum decoherence:
Quantum decoherence; when Particle being observed at double slit Decoherence time; the time it takes for the
observed the particle acts “normal” acts normal and goes through one quantum object to “collapse” to one state
When the laser beam is aimed at two slits in an unobserved quantum superposition (wave of possibilities), it acts like a wave going through two slits and will show an interference pattern of two waves of possibilities.
When the laser beam is aimed at a single slit in a quantum superposition (wave of possibilities) it will show a single slit diffraction, a pattern of light bands with the center having the highest intensity and smaller bands following with lower intensities and spaces between the bands. It will act like an interference pattern too because of the properties of a wave of possibilities. The waves going through the slit will interfere with each other and at 1/2λ difference cancel each other out and leave the black spaces and intensify each other leaving the light bands. The higher the distance between slits and the screen, the higher the distance between the bands of light.
Intensity graph single slit diffraction
2. Method
2.1 List of materials
For our experiment we used various materials, including:
– A Spectra-Physics model 155A Helium-Neon laser including a stand
– An LD Didactic GmbH diaphragm with 1-,2-,4-slits
– A ruler
– A dark room with a white wall
– A pencil
– A camera
– A notebook
2.2 Process
For our experiment we used an entirely darkened classroom at school, where we set up our design. We placed the laser on its stand exactly 100 centimeters from a white wall. We made the laser shine through every combination of slits on the 1-,2-,4-slits diaphragm.
We used various distances between the diaphragm and the light source, as we started off with a distance of 10 centimeters, then a distance of 5 centimeters, and finally a distance of 2 centimeters between the diaphragm and the light source.
Each time we measured the length of the main band, the length of the bands in the diffraction pattern, the faint spots between each band in the interference pattern, and finally the total length of the interference pattern.
We placed pencil marks next to the main bands, the full pattern and the other bands in the interference pattern, so we could measure each distance when the light was turned on again.
We wrote down all our results in a notebook for further processing.
3. Results
Wavelength (nm)
Slit seperation (d)
Distance from slits to screen (L)
Distance from middle band to others
Order value (m)
632.8 nm
0.90m
7
632.8 nm
632.8 nm
4. Discussion
4.1 Conclusion and explanation
Moet een lopend verhaal zijn, daarom deze 2 punten samen. Vergelijk met de hypothese!
4.2 Errors
Errors can occur in any experiment, just like in ours. For instance, once we did not measure the whole interference pattern because we measured with the lights still on, that’s why we decided to use pencil marks and measure everything twice to prevent measurement errors in our results. Also, the laser was very easy to move so we had to keep putting it on the right place.
4.3 Follow-up
After having conducted this experiment, it might be interesting to do another experiment related to this one. We might use a different color of light, a higher amount of slits, or a different distance between the light source and the surface.
Another idea would be to use an electron cannon with marked electrons and an electron measurement device to determine how the electrons in the interference pattern are divided.
5. Account
5.1 Sources
http://www.falstad.com/mathphysics.html
http://slideplayer.nl/slide/2865560/
http://slidegur.com/doc/1473871/quantummechanica
http://abyss.uoregon.edu/~js/21st_century_science/lectures/lec13.html
http://www.zuriky.com/Doubleslit.html
5.2 Log file
Taakverdeling en tijd aan besteed
5.3 Reflection