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Interference as evidence of waves.
Is light a particle or a wave?
What happens when particles meet head on?
Choices...
It seems that two pulses can pass through each other. Where they meet, the resulting shape is like adding the height of one pulse plus the height of the other pulse, like this...
What will the height of the pulse be when these two pulses meet each other?
Check out this simulation of what's supposed to
happen:
Here's a very short movie of pulses on a slinky with two pulses on the same side of the slinky moving toward each other.
Sequential frames from this movie show two pulses advancing towards each other, turning into a "higher" pulse where they meet, and then moving away from each other...
At each moment in time, a moving boat disturbs the water, sending a wave pulse out to the left, and another to the right. This is called the boat's wake.
If two boats go beside each other, what should be the height where the two wakes run into each other? Take a look: two-boat wake-surfing video. (FF to about halfway in to the video...)
Where two crests meet, the height of the wave should be twice as high as one. Where two troughs meet, twice as low.
Where a trough and crest meet, total cancellation.
In the accompanying diagram:
Have the waves from just one source hit a screen with two openings in it. Each opening acts like a "source" in its own right. But now the two "sources" are always in synch with each other, and are guaranteed to have the same frequency (that of the original plane wave coming at the wall).
There is a formula for the angular distance $\Delta \theta$ between destructive interference lines, (or between constructive interference lines, for that matter) in terms of the distance $d$ between the two slits (holes in the wall) which is:
$$\lambda=2d\sin\Delta\theta$$
The wavelength of the waves, $\lambda$ is a constant for what you've been looking at. Is this formula compatible with what you observed?
Initial parameters:
$ 0 189 10 2 true false 8 25 477 1 s 20 21 s 208 21 c 13833 0 w 16 0 l 29 w 61 0 l 111 w 13 0 c 15 0 w 20 0 l 10 c 59 0 l 1 w 30 0 l 62 w 5 0 l 10 w 7 0 c 43 0 l 2 w 4 0 l 5 c 139 0 w 14 0 c 5 0 w 6 0 l 4 c 152 0 w 48 0 c 37737 0
Image credits