In Young's double-slit experiment, what is the purpose of the single slit? Watch

optalk
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In order to produce a stable interference pattern, you need two coherent waves, meaning they both have a stable phase difference and are the same frequency. Now, after Young applied the colour filter, the light passing through was now monochromatic as it's just a single frequency of light. Now that there aren't all the frequencies of light mixed up, doesn't that mean that after using just a double slit, the two waves diffracted through are coherent?

What is the point of a single slit?
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VGM
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Yes, the light is coherent. The double slit is made using 2 single slits very close together.
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optalk
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(Original post by VGM)
Yes, the light is coherent. The double slit is made using 2 single slits very close together.
I know that monochromatic light would be coherent when diffracted through a double slit because it's just the same wave, just diffracted through equal slits. But for some reason there is a single slit behind it and infront of the colour filter. I don't understand the purpose. All it does is diffract the light through that single slit, and then the double slit infront of it diffracts the light again so that there are two equal frequency waves.
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Starra
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It's because Young didn't use a colour filter, that's just the textbook being weird. Young used a lightbulb, so had to make it coherent by putting it through a single slit before the double slits. I think.
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VGM
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I’m pretty sure it’s just the light emitter -> filter -> slit 1 -> slit 2 -> screen. Slits 1 and 2 together make up the double slit
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optalk
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(Original post by Starra)
It's because Young didn't use a colour filter, that's just the textbook being weird. Young used a lightbulb, so had to make it coherent by putting it through a single slit before the double slits. I think.
What you said is also weird, because when a wave is diffracted, frequency and wavelength do not change. So then the interference pattern wouldn't be stable. The textbook clearly states that he used a colour filter.
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optalk
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(Original post by VGM)
I’m pretty sure it’s just the light emitter -> filter -> slit 1 -> slit 2 -> screen. Slits 1 and 2 together make up the double slit
The diagram shows the lamp>colour filter>single slit>double slit>screen

The single slit has one gap and the double has two gaps.

They are both labeled single and double slit respectively.
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VGM
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(Original post by optalk)
The diagram shows the lamp>colour filter>single slit>double slit>screen

The single slit has one gap and the double has two gaps.

They are both labeled single and double slit respectively.
That’s strange. My textbook only has a single double slit, we only used a double slit in the practical in class
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BobbJo
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(Original post by optalk)
In order to produce a stable interference pattern, you need two coherent waves, meaning they both have a stable phase difference and are the same frequency. Now, after Young applied the colour filter, the light passing through was now monochromatic as it's just a single frequency of light. Now that there aren't all the frequencies of light mixed up, doesn't that mean that after using just a double slit, the two waves diffracted through are coherent?

What is the point of a single slit?
The colour filter will make the light monochromatic (i.e single frequency) but it will not make the phase difference between the light waves constant. So it will not produce a permanent (or sustained) interference pattern.

The purpose of the single slit is to ensure that the light is coherent
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optalk
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(Original post by VGM)
That’s strange. My textbook only has a single double slit, we only used a double slit in the practical in class
I suppose I'll just disregard it in my notes, because it's bloody confusing to think about.
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BobbJo
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(Original post by optalk)
I suppose I'll just disregard it in my notes, because it's bloody confusing to think about.
Your book is correct. It is light source -> filter -> single slit -> double slit -> screen

The reason why a single slit may be omitted is that the light source may be a laser. Laser light is coherent so there is no need of a single slit.
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VGM
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(Original post by BobbJo)
Your book is correct. It is light source -> filter -> single slit -> double slit -> screen

The reason why a single slit may be omitted is that the light source may be a laser. Laser light is coherent so there is no need of a single slit.
That makes sense. My textbook uses a laser, not a lamp or a lightbulb
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optalk
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(Original post by BobbJo)
Your book is correct. It is light source -> filter -> single slit -> double slit -> screen

The reason why a single slit may be omitted is that the light source may be a laser. Laser light is coherent so there is no need of a single slit.
I am so confused. Monochromatic light is coherent. What's the point of a single slit if you already have a colour filter.

edit: Just because it has a single frequency doesn't mean that it has a constant phase difference. Thank you BobbJo for clarifying.
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BobbJo
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(Original post by optalk)
I am so confused. Monochromatic light is coherent. What's the point of a single slit if you already have a colour filter.
Coherent light is light of the same frequency and having a constant phase difference. Colour filter does not ensure constant phase difference.

It is not true that light having a constant frequency is coherent. A sodium vapour lamp emits light at a characteristic wavelength of 589.6 nm (and 589.0nm) but it is not coherent. You need a single slit to ensure a constant phase relationship between the waves
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optalk
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(Original post by BobbJo)
Coherent light is light of the same frequency and having a constant phase difference. Colour filter does not ensure constant phase difference.

It is not true that light having a constant frequency is coherent. A sodium vapour lamp emits light at a characteristic wavelength of 589.6 nm (and 589.0nm) but it is not coherent. You need a single slit to ensure a constant phase relationship between the waves
That makes sense! Thank you for clarifying.
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