# Young Double Slit experiment, single photons

Hello, I was wondering if I could get any help with the following question/thing:

Why does a stream of photons (let's say that one is fired every 3 seconds) at a double slit as shown in Young's double slit experiment create an interference pattern although there is no interference because the photons are being fired with intervals, and they are already absorbed/something like that onto the photographic plate before the next photon is fired?

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DrChinese
Gold Member
Hello, I was wondering if I could get any help with the following question/thing:

Why does a stream of photons (let's say that one is fired every 3 seconds) at a double slit as shown in Young's double slit experiment create an interference pattern although there is no interference because the photons are being fired with intervals, and they are already absorbed/something like that onto the photographic plate before the next photon is fired?
The interference effect is primarily from self-interference. The photon acts as a wave in this setup.

The interference effect is primarily from self-interference. The photon acts as a wave in this setup.
How can a wave interfere with itself?

Bill_K
How can a wave interfere with itself?
That is exactly what waves do. Sound waves, water waves, any kind of waves. Did you ever see a ripple tank demonstration of this experiment? It clearly shows a wave interfering with itself.

How can a wave interfere with itself?
Exactly Bill, I always find a picture paints a thousand words in this case.

http://www.upscale.utoronto.ca/PVB/Harrison/DoubleSlit/DoubleSlit.html

Note that after the wave passes through the two slits it will produce interference fringes with itself. It's even easier to show with an actual water tank where you can actually see a wave front interfere with itself after it passes through the slits. Although make sure you understand that water waves and energy waves are merely an analogy not an exact representation of physical fact. It's even more interesting that we actually have no idea what the wave really looks like but that is a whole 'nother issue, we can at least inductively assume from its behaviour it is a wave.

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Exactly Bill, I always find a picture paints a thousand words in this case.

http://www.upscale.utoronto.ca/PVB/Harrison/DoubleSlit/DoubleSlit.html

Note that after the wave passes through the two slits it will produce interference fringes with itself. It's even easier to show with an actual water tank where you can actually see a wave front interfere with itself after it passes through the slits. Although make sure you understand that water waves and energy waves are merely an analogy not an exact representation of physical fact. It's even more interesting that we actually have no idea what the wave really looks like but that is a whole 'nother issue, we can at least inductively assume from its behaviour it is a wave.
Thanks, this helped me out a lot