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# Young's double slit experiment watch

1. When Young first performed the experiment with a candle or whatever that emitted white light he used a narrow single slit before the double slit, why?
2. (Original post by G.Y)
When Young first performed the experiment with a candle or whatever that emitted white light he used a narrow single slit before the double slit, why?
Simple answer is "to make a coherent light source".
3. the use of a single slit makes the light source coherent and it makes the light diffract more. The closer the gap size is to the wavelength, the better the diffraction. Hope this helps.
4. (Original post by Odetosophxo)
the use of a single slit makes the light source coherent and it makes the light diffract more. The closer the gap size is to the wavelength, the better the diffraction. Hope this helps.
Isn't it going to diffract going through the double slit anyway?

And could you expand on how it makes it coherent?
5. When the light diffracts through the first slit, the light waves become in phase. Since the single slit has a certain sized gap, it will diffract certain wavelengths of the lgith by certain amounts. So effectively a certain frequency of light will be incident on the double slits.

A coherent light source is one in which there is a constant path difference between each wave front (waves are in phase), and waves have the same frequency.
6. (Original post by G.Y)
Isn't it going to diffract going through the double slit anyway?

And could you expand on how it makes it coherent?
Well if you just aim the light beam without diffracting it first then it probably won’t diffract through the slits as there is a section in the middle that is solid.

By using a single slit first it insures that the light source is coherent and in phase. If the source wasn’t in phase then you wouldn’t get constructive interference and so you wouldn’t see an interference pattern in the screen
7. (Original post by Odetosophxo)
Well if you just aim the light beam without diffracting it first then it probably won’t diffract through the slits as there is a section in the middle that is solid.

By using a single slit first it insures that the light source is coherent and in phase. If the source wasn’t in phase then you wouldn’t get constructive interference and so you wouldn’t see an interference pattern in the screen
So what would you observe with the fringes if the single slit was wider?
8. (Original post by G.Y)
So what would you observe with the fringes if the single slit was wider?
If the slit was wider then the interference pattern wouldn’t be as clear because there would be less diffraction. If the slit was too wide then there would be no pattern at all.

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