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Quantum Physics - The Double Slit Experiment (wave - particle duality)

Updated on January 19, 2012

Paint balls through double split

Creates 2 bands on dection wall at back directly behind splits
Creates 2 bands on dection wall at back directly behind splits

Wave Particle Duality

Now to understand this experiment we first need to look at 2 things.

1. Matter

2. Waves

We will look at this with reference to the experiment we are studying.


If we take a macro (something large like paint balls) sized piece of matter, and we shoot it through a sheet of metal placed in front with a single slit of it and a screen behind to show where they land, now common sense tells you that they will form a straight line behind the slit...Correct, this is exactly what happens.

Now lets put a second silt in next to the other slit, what does the screen show now, again it matches with common sense it produces 2 lines of paint behind each of the slits, as well as a lot of mess on the metal sheet where the slits are; See image inset on right.

Waves through Single Slit

Waves through a double slit


If we look at waves for, example waves in a pond, and we do the same as we did for the paint balls. We put a screen with one slit in the middle and a screen to detect where they land at the back, then what pattern do you think would form? That’s right the wave spreads out after going through the slit with the most intense band just behind the slit spreading out as we go see image on right.

Now let’s add a second slit, now what happens? Where the top of the wave meets the bottom of the other wave, the waves cancel each other out, but where the top of a wave meets the top of the other wave, or bottom to bottom, we get a reinforced wave of extra bright ness and all those in-between, so we develop what is known as an interference pattern, bands of light and dark, where the waves have been constructive (reinforced) and destructive (cancelled each other out). See image to the right.

Quantum Effects

Ok let’s look at the Quantum Effects of this experiment. In this example we will be using electrons. Electrons are tiny particles of matter in fact it has a mass of just 9.10938215(45) ×10−31 kg, that’s very very small.

What happens when you fire these electrons through a single slit, well exactly what you would expect they form a band directly behind the slit, just as we expect with matter.

Now let’s add the second slit, expecting it to form 2 bands as with the paintball experiment we begin...whoa! It forms and interference pattern, matter forming an interference pattern, what’s going on? Physicists being smart thought about this and thought the electrons must be interfering with each other i.e. bouncing off each other to create this pattern. How can we solve this? The answer is let’s fire one electron at a time through the slit, then there is no way they can interfere with each other. This experiment was then left for an hour or so, when the scientists returned they saw an interference pattern! So we got an interference pattern just like waves, but from matter!

Interference pattern, with single electron at a time

A cross section of the interference pattern formed when electrons pass through a double split one at a time.
A cross section of the interference pattern formed when electrons pass through a double split one at a time.

The conclusion is the electron leaves the source as a particle, becomes a wave of potentials and interferes with itself when it hits the wall. Don't worry if this boggles the mind, it goes against everything common since tells us yet this IS what happens. This is known as the wave-particle duality, the concept that matter and energy exhibits both a wave-like and matter-like properties.

The electron once leaving the source goes into what is known as superposition, that’s is the electron takes every conceivable and unconceivable path, through both slits, through one slit, through no slits rebounding against the sheet, around the experiment. This is known as the electrons wave function and upon measuring it the wave function collapses and that is where the electron is found, in this example on the detector screen at the back.


Superposition - The electrons take every possible path, interfering with themselves, through 1, 2 or no slits!
Superposition - The electrons take every possible path, interfering with themselves, through 1, 2 or no slits!

However physicists thought what happens if we try to detect which slit the electron is "really" going through, so they set up a detector at one of the slits, if it is detected it went through that slit if not it went through the other.

Now they released the electron and it went back to behaving like a particle and creates two bands, like in the first experiment with paint balls. Not the interference pattern! The very act of observing this caused it to only go through one slit.

So I here you say what did observing it have to do with anything? Well the fact is that observing the electron collapsed the wave function and forced it to go through a single slit, weird! Take a look at the video to summise what we have been disscussing, before moving on.

Summary of experiment

When you fire you electron source, what actually comes out is a Ψ (called psi, pronounced sigh) wave (i.e. a wave function). It is not random: it travels according to the perfectly predictable laws of wave propagation, moving out in all directions and interfering with itself and all that good stuff. According to the Copenhagen interpretation, the Ψ wave represents the probability of the photon being at any particular place. So at this stage, the question "Where is the electron?" does not have an answer—there is only a wave of probabilities traveling outward.

In terms of the math, we can represent Ψ with a drawing very similar to the drawing we used for a light wave at the beginning of the paper. But it's important to remember that the drawing means something very different! In a light wave, the height of the drawing corresponds to the strength of the electric field. In a Ψ wave, the height of the drawing represents the probability of finding the particle at a particular point. To make it more confusing, Ψ can be negative or positive, but the probability is always positive—the probability only cares how far from zero Ψ is, it doesn't matter at all which direction! So you might see something like this:


This shows the probability was, wave function.
This shows the probability was, wave function.

So how does this apply to the experiment, well in the double split where we get the interference pattern, this is actually a probability pattern and of course, lots of electrons choose high-probability areas, only a few choose low-probability areas, and none of them choose zero-probability areas. That's why we see the interference pattern.

But when we put a measuring device to measure which slit the electron goes through we collapse the wave function and cause it to be 100% at one slit and 0% at the other hence why we get two bands and no interference pattern.

But of course, the theory also raises a lot of questions including;

  • What is this Ψ thing? What does it mean to say that ‘probability waves’ are flying through space, interfering with each other, and ‘collapsing’ into certainty?

  • What does it mean to "measure" it? Presumably the electron is bumping into, and interacting with, all kinds of things on the way to the metal sheet, but they apparently don't count as "measurements." At the same time, a measurement doesn't need to involve a person; you can stick a measuring device at one of the slits and never have a human being look at it, and the interference pattern on the back wall still disappears. So what is good enough to count as a "measurement"?

  • When you put a measuring device on a single slit, how does the wave going through the other slit "know" that it's supposed to collapse?

  • Isn't there some possible explanation that corresponds equally well to all the data, but makes more sense than this nonsense?

Some of these questions are completely unanswered, and some have had some thoughts and experiments however, understand this no one really “understands” this stuff not your teachers, not physicists no one, no one knows why Ψ can successfully predict experimental results but it does.

I will be writing more articles on quantum physics including some more mathematical interpretations of the experiments for those of you who are interested. There are also some great materials on the subject check out some of the books listed below.

Great material for further reading


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    • profile image

      Todd Lohnes 

      23 months ago

      Does it not matter how thin the slits are and how far they are from each other?

      What else is present with the electrons as they are passed through? I didn't think that there was any such thing as "nothing" unless there were a vacuum and why can't a "black hole" be made to exist?

    • dipless profile imageAUTHOR


      7 years ago from Manchester

      @Arthur Vilain & Athur, why do you post under two different names, come and join hubpages, you can do this through my profile. You're not throwing a spanner in the works at all, with regards to the photon detectors there are so many out there and this is a very active area of research and more and more are being developed with ever greater precision. Here's a link to a recent publication from the American institute of Physics

      Why are you talking about pair production, the photons can indeed create electrons and positrons but these need to be have an energy over 1.022 MeV which is substantially more than would be used for these experiments. The plates detect the arrival of the photon

      All particles can behave like waves and particles even you do as human, but your wavelength is so so small.

      Arthur photons exhibit wave and particle like properties which is why it called the wave-particle duality. It depends on the experiment and the measurements being made on them.

      There was a publication about the theory of them in magnetic fields

      It is very mathematical but the abstract will give you an idea of what is going on. Another one stemming from gauge field theory

      No worries just give me a shout with questions.

    • profile image


      7 years ago


      So the fact that photons act like a wave when they pass through something is impossible, means that photons are particles and waves? one thing is true in physics and life alike, and probably isn't confined to the structure of things that we see in this part of space, is change. Entropy, how amusing.

      Since I mentioned entropy, Is anyone able to point me in the direction of interactions between Neutrinos and photons?

      or have no studies been conducted on this as of yet?

      or have I met a scaling factor I have overlooked?

      I think I'll have to google that one!

      Please take into account I am no expert in this field.

      Thanks Again

    • profile image

      Arthur Vilain 

      7 years ago

      Sorry to throw a spanner in the works,

      Can someone direct me to information on what a photon detector consists of?

      Sure the plate on the back shows an interference pattern when the photon is unobstructed. Photon = electron-positron pairs, or quark-antiquark pairs (among others), what exactly does this "plate" detect?

      Sorry I am new to this stuff and I thought I would spark some thought into other directions.

      I founds some articles of interest,

      In this article it goes on to say that it has been solved,

      This is one of the Partial theories, Where there is a so called symmetry in space:

      and in the same article on how the different frequencies of photons act in different ways:

      Also in some Bose-Einstein Condensates there has been a wave like formation from matter:–Einstein_c...

      Any Reply is welcome. Thank you for your time

      Still much more to come!

    • dipless profile imageAUTHOR


      7 years ago from Manchester

      Thank you Donn glad you thought so, Quantum Physics is fascinating and a subject which flexes those mind muscles. It is very exciting and something the more you read about the more you begin to understand what happens, the big question is why does it happen, this we do not yet have an answer. If you are really interested check out the start hub which is

      Enjoy and any questions please ask.

    • Donn Olson profile image

      Donn Olson 

      7 years ago from Out of Nowhere

      A most interesting hub. I have read a small amount on the subject, and do not pretend to understand much about it, yet.

    • dipless profile imageAUTHOR


      7 years ago from Manchester

      @Arthur it is the whole system that is impossible A photon when it leaves the source and arrives at the detector it can take any possible path and the likelihood of these paths is shown as the wavefuntion. Once this is measured at whatever point in the experiment, be it the detector at the plate or at the slits, this causes the collapse of the wavefuntion and the photon to behave like a particle. If this is done at the slits the interference pattern disappears.

    • profile image


      7 years ago

      This is maybe not worded correctly but it's a simple interpretation. Not just absorbed but also reflected. I would be interested to see this experiment repeated using different materials for the substrate of the slits. Is it possible that the interference pattern is created by the matter containing the slits interacting with the electrons? Even simple things have their complications! What a great example!

    • profile image

      Arthur Vilain 

      7 years ago

      It's simple, as a similar experiment to put it into terms, an orange squeezed through a single hole will come out the other side mashed in that shape. When the packets of light pass through the 'Slits' the energy is spread between the two slits, as this is not normal for a photon to be split in this way it creates a wave as it tries to come back together. The reason this cannot be obsevered with instruments is because the excess 'wave' energy is absorbed by the detectors. Pure energy in the form of photons is easily absorbed.

    • dipless profile imageAUTHOR


      7 years ago from Manchester

      @slc this makes two of us glad you liked it, I am finishing off the series at the moment.

      @funny Guy thank you

    • profile image


      8 years ago

      Thanks for your work, I find it very informative.....

    • slc334 profile image


      8 years ago from Canada

      Way to go! Love the topic, always been a big fan of quantum theory.

    • dipless profile imageAUTHOR


      9 years ago from Manchester

      wildt good luck with the writing, now I am back I will check some of your hubs out

    • profile image

      broken egyptian slave 

      9 years ago

      Talking head lies! Use one slip and an ordinary hand laser in a dark room, you see bands on each side. The insistence that you don't is to discourage you from seeing the phenomenon alone; the other light goes to the Pharaoh down the little hole in the Pyramid. Try it. At home alone.

    • ss sneh profile image

      ss sneh 

      9 years ago from the Incredible India!

      Hi! Wonderful! Very well written hub!

      One of the greatest mysteries of quantum world reveled by the famous double slit experiment!

      Yes there are many unanswered questions to the "accepted science"

      * * * * *

      Never say "NO" to science!

      There may be someone out there...quietly developing a theory than can explain the double slits, and the quantum tunnelings, and the bose-einstein condensates...

      John G Cramer has already explained the double slit experiment in 1986...though it's not accepted science yet.

      And I categorically say here that John G Cramer is right and his explantions by....

      retarded - forward-in-time! - and advanced - backward-in-time! -- waves the truth! (Just wait and see ourselves) --Thanks

    • ConsciousObserver profile image


      9 years ago

      Great Job and Excellent explanations of what is currently known. How would the electrons have interference during observation unless there was consciousness of being observed?

    • mike1242 profile image


      9 years ago from London

      Have you read the book "Quantum Success" or the film "Down the Rabbit Hole" very interesting subject. The Observer(ie us) also affects the outcome of quantum experiments.Nice Hub

    • wildt profile image


      9 years ago

      amazing hub! enjoyed reading it. quantum theory mechanics is great. just started by myself to write about.

      keep up the good work!

    • dipless profile imageAUTHOR


      9 years ago from Manchester

      No problem glad you enjoyed it

    • spartanking1978 profile image


      9 years ago from Earth

      This definitely opened my eyes the first time I researched the topic...thank you for your synopsis.

    • profile image


      10 years ago

      Thanks, nice explanation!

    • dipless profile imageAUTHOR


      10 years ago from Manchester

      Thanks very much for the comment :)

    • Dame Scribe profile image

      Dame Scribe 

      10 years ago from Canada

      Very informative, detailed and well expressed. :) great Hub!


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