# Single Slit Diffraction Interference Pattern

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#1
Hi guys, I'm really struggling to understand how coherent monochromatic light can enter a single slit, then diffract and interfere with itself, producing a pattern of light and dark fringes on a screen.

I think what I struggle to comprehend is how more than one light wave can enter the slit at a time, and then diffract in different directions as shown below:

I always thought that light entered a gap one wave at a time, and each wave diffracted in the same direction as the previous one, kind of like this:

Can anybody locate where my understanding is flawed? As I just don't see how light waves from a single source, one after the other, can enter a slit and diffract in multiple directions.

Thanks guys, any help will be MASSIVELY appreciated!
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7 years ago
#2
It's not "one wave at a time".
If the slit is wider than one wavelength you have to consider a number of single waves passing through it.
In the case of light of wavelength around 600nm then the slit is much wider than this.
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#3
(Original post by Stonebridge)
It's not "one wave at a time".
If the slit is wider than one wavelength you have to consider a number of single waves passing through it.
In the case of light of wavelength around 600nm then the slit is much wider than this.
But why does it matter, because isn't the wavelength the distance between one wave and the wave following behind it?
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7 years ago
#4
(Original post by EconFan_73)
But why does it matter, because isn't the wavelength the distance between one wave and the wave following behind it?
It seems odd I know. But wave phenomena depend on the wavelength of the light. In the case of the water waves passing through the gap, does not the diffraction effect depend on the wavelength of the wave compared to the width of the gap? It's the same with light.
Waves will tend to diffract around an obstacle if their wavelength is around the same size as the obstacle. If you think about it, as far as a wave is concerned, and the way it interacts with its surroundings, it's only got its wavelength as a measure of how it fits into space. (The amplitude is a measure of the energy that it carries.)
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#5
(Original post by Stonebridge)
It seems odd I know. But wave phenomena depend on the wavelength of the light. In the case of the water waves passing through the gap, does not the diffraction effect depend on the wavelength of the wave compared to the width of the gap? It's the same with light.
Waves will tend to diffract around an obstacle if their wavelength is around the same size as the obstacle. If you think about it, as far as a wave is concerned, and the way it interacts with its surroundings, it's only got its wavelength as a measure of how it fits into space. (The amplitude is a measure of the energy that it carries.)
Ok cheers! So the number of waves that can enter a slit at once depends on how many wavelengths fit into the width of the slit? So if the slit was three times then a similar thing would happen to the top diagram, but with 3 separate waves all interacting?
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7 years ago
#6
(Original post by EconFan_73)
Ok cheers! So the number of waves that can enter a slit at once depends on how many wavelengths fit into the width of the slit? So if the slit was three times then a similar thing would happen to the top diagram, but with 3 separate waves all interacting?
It's not that simple, unfortunately, but it's the right idea.
The complete mathematical model of diffraction is actually quite complicated.
You just need to know that wave diffraction effects depend on the size of the obstacle or slit compared with the wavelength of the wave.
One way to think about it, which I have always found useful, is to think about radar and the bouncing of radio waves back of an object to locate it.
If the object was, say, an aircraft of size 50m, then in order to "see" the object, the waves you send towards it must have at most a wavelength of about 50m. Preferably shorter. Longer wavelengths will just pass around it and not reflect. If you check on Wiki, you will find that typical radar waves are maximum 100m, and for air traffic control, where you sometimes need to "see" much smaller objects in the sky, the typical wavelengths are 15 - 30cm.

Hope this helps.
It's an interesting subject.
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#7
Cheers Stonebridge! The AS Physics syllabus is so frustrating in that it brushes over so many areas without actually allowing for a full understanding of how things work, which if provided would make everything much easier to understand. It's like they want us to just accept that things are true, rather than to actually understand why, which is just a bit boring and pointless all round imo.
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7 years ago
#8
(Original post by EconFan_73)
Cheers Stonebridge! The AS Physics syllabus is so frustrating in that it brushes over so many areas without actually allowing for a full understanding of how things work, which if provided would make everything much easier to understand. It's like they want us to just accept that things are true, rather than to actually understand why, which is just a bit boring and pointless all round imo.
Very true.
Sometimes the explanations just involve a lot of complex maths, unfortunately.
However, you can always ask here, and search around the internet for a deeper understanding.
You may find hyperphysics useful as it is aimed at a level just beyond A2.
Some of it will be difficult, some will be very informative.
http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html

Here is the stuff on diffraction.
http://hyperphysics.phy-astr.gsu.edu...inslit.html#c1

Good luck.
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#9
Yeah I came across that site earlier and thought it looked very useful, just read in more detail and pleased it explained why the narrower slit gave a wider interference pattern, as that did previously seem counter intuitive. Will deffo use site in future when in need of more thorough understanding! Cheers man!
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