The Infamous Double Slit
Kirtan Purohit 40764145
• The double slit experiment was one of the
most famous experiments in quantum
mechanics. Carried out by Thomas Young in
the 1800’s it revolutionized physics.
• The basis for Young’s experiment was firing
electrons through 1 slit then 2 slits and
observing the resulting pattern on the wall
behind the slits. We can first consider what
would happen with paintballs and water
waves in such an experiment.
• If one fires paintballs through appropriate
slits, the wall behind the paintballs will appear
to have 2 steady lines of paintball splashes, as
such. Thus the paintballs can be said to
behave as particles
• If one were to repeat the experiment, but with waves hitting the slit instead the following
could be observed.
• The top of one wave and the bottom of an another would hit each other and cancel each
other out. We have learned about this already in physics in our waves unit. This would result
in a diffraction pattern where the middle would have the highest intensity.
Young attempted this ‘double-slit’ experiment at
first with one slit. Since electrons are like tiny
balls of matter they lined up perfectly behind
the slit on the wall. Thus helping the hypothesis
of behaving as particles.
Then he attempted the experiment with two
slits. Something marvelous was observed.
The electrons went through the two slits and a interference pattern of many bands was observed.
Thus one could conclude reasonably that electrons behave as waves, although surprising it could
have been possible. However the experiment was taken further by . . .
Observing what the electron does as it goes through the two slits. A measurement
device was set up to see how the electrons went through the slits. The result was
extraordinary. Electrons began behaving as particles again. That is to say they lined up
perfectly behind the two slits instead of an interference pattern. This was incredible!
The simple act of measuring or observing what happens to the electrons as they pass
through the slit had changed the outcome. An intuitive although slightly incorrect
example of this would be something impossible like measuring something with your
eyes closed, yet after opening your eyes having the measurement be something
This picture is a great example of the results of the experiment. The picture on the left shows the
result with no measuring done. A diffraction pattern is observed and the electrons behave as a
wave. Yet with measurements done the electron forms a 2 band pattern and behaves as a
• This experiment showed that electrons
behave as both WAVES AND PARTICLES! This
revolutionized the field of quantum