Page 18 - Winter 18-19

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We see the wave spreading out in all directions away from the slit. His apparatus was such that the wave of light would appear on a screen as a large lit area because the light is spread out and not focused to be a straight line. Alternatively, if light was made up of particles that would shoot through the slit and continue in a straight line, it would appear on the screen like this.
The line of light on the screen is not real sharp because some of the light particles are bouncing
off the edges of the slit it is going through.
What if there were two slits? This would be a way of confirming the wave nature of light because it would be like the dropping of two pebbles and the resultant waves would interfere with each other. It would look like this.
When Dr. Young tried this, there was indeed an interference pattern of light bands as we see here. This seemed to settle the question and there it sat for another century — until the birth of quantum physics.
Quantum Mechanics
This discovery of the quantum nature of substance was one of the most earth-shaking discoveries ever made in physics, which is why there is often the division between “classical” and “quantum” in the history of physics.
occurs in discrete packets. This was both totally unexpected and contrary to all previous thought. However, it was the birth of a new physics and a new understanding.
The significance of this is bringing us back to the two-slit experiment with the question — “if energy is occurring in packets then why is there the appearance of wave activity, including interference phenomena”? One would think they would act like little bullets shooting through the slits instead of wave forms. In recent times, with more sophisticated equipment it is possible to set up the two – slit experiment so that only one electron (or photon) is sent forth at a time.
Again, the expectation is that the single “energy packet” will go through one slit or the other so that the detection of the result, as many millions of them are sent through, will look like this.
If energy was in the form of little particles
But this is not what happens. Yes, a single electron will appear on the detection screen at one point, but as many of these are transmitted there is built up a pattern that
This discovery was
made by Max Plank
who was trying to solve
what was one of the
very few remaining
puzzles, the solution of
which would pre-
sumably complete the
science of physics.
Briefly, there was a
discrepancy between
the mathematical pre-
diction of energy propa-
gation — as light from
the sun, or heat from
mathematically predicted that the energy should rapidly deplete — like in seconds or minutes — and it did not. The expectation was that energy was an unbroken continuous stream flowing from source to detector (or person feeling it).
In 1900, after many years of attempting unsuccessfully to solve the contradiction, Planck found a way to explain it. In doing so he said it was "an act of despair ... I was ready to sacrifice any of my previous convictions about physics."
The answer was the recasting of the nature of matter and energy. Energy does not exist in an unbroken stream but
Max Planck
a hot device — in that it was
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