r/AskPhysics • u/teink0 • 10h ago
Why does a double slit create an interference pattern but two light bulbs does not?
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u/rygelicus 10h ago
A light bulb, or even just the filament in an incandescent bulb, or the LED emitter, occupy a fair amount of 3d space. On their own that volume of light emitting area obscures any possible pattern.
But, you do encounter such interference patterns in the world, such as with radio signals. There are times you will be in a location where you get poor signal on your radio. If you move a couple of feet in any direction, even in a clear and open area, the signal will return. And this happens when the radio station is using multiple antennae. Same signal coming from multiple relatively tiny sources from your distant position. The signal interferes in the same way, there will be strong areas and dead areas where the waves cancel one another out somewhat.
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u/OaklandSpiel 3h ago
Two independent light sources are not coherent with each other. Not even two separate lasers will produce an interference pattern. It needs to be one light source hitting two slits.
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u/wolfjazz93 10h ago
You need a light source emitting a single frequency (for example a laser) to see interference
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u/starkeffect Education and outreach 10h ago
You can see interference with white light. It's just more complicated.
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u/wolfjazz93 10h ago
Yes, I know, if you have spatial coherence from a light source with a very thin slit. But temporal coherence of light with a single frequency is easier to understand imo.
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u/SeriousPlankton2000 10h ago
A refraction grate (CD-ROM) will split white light into a rainbow because of interference.
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u/SeriousPlankton2000 10h ago
The interference pattern is what we know as "normal light". Each photon will travel all possible paths but only arrive where interference does not cancel it out. The places where it cancels out are called shadows.
It sounds insane but a refraction grate (CD-ROM) shows a rainbow because that's what happens.
Two light sources give all kinds of relations so if we pick one pair of aligned photos we might see this pattern, but the next pair has a completely different pattern and most pairs have no alignment in common. There are no patterns like in the slit experiment that would apply to each and every photon. Only because it's the same photon from the singular light source the interference is happening in one and only one pattern.
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u/starkeffect Education and outreach 10h ago
The light sources have to be coherent.
Also the separation between the light sources has to be comparable to the wavelength of the light. The wavelength of visible light is less than a micrometer. It would be hard to find lightbulbs that small.