# Interference of light

In a double slit experiment, is diffraction caused by interference or interference is caused by diffraction?

• Philip Koeck

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ZapperZ
Staff Emeritus
In a double slit experiment, is diffraction caused by interference or interference is caused by diffraction?

Go back and look at the single slit. Do you think the diffraction from it is "caused" by "interference"? If you think it is, then you need to explain where this interference is coming from.

So now, translate that to the double slit.

BTW, just a friendly hint, don't ever just post one single, terse sentence when you ask question like this. Provide some context, and in this case, provide a sensible reason why you would ask a question like this so that we know where you're coming from. Otherwise, when I look at a "naked" question like this, I have no idea if you even know what the concepts of "diffraction" and "interference" are. Often times, many of us trying to respond to such questions waste our time just trying to understand the question and trying to decipher what you already know.

Zz.

• nasu and berkeman
sophiecentaur
Gold Member
In a double slit experiment, is diffraction caused by interference or interference is caused by diffraction?
This is just the dreaded 'use of words'. Diffraction is the general term to describe any effect on a wave front. It can be as complicated or as simple as your choose. Interference (imho) is a simplified model which allows us to get the main features of a diffraction pattern by treating the source as a small number (perhaps just two) of points / omnidirectional sources. Young's slits came first in History because it's a simple problem.
We are taught about interference first because it's easy to describe and doesn't require Integral Calculus which the full 'diffraction' treatment uses.

Thanks a lot for your help!! However, is it true that in the single slit experiment, diffraction happens due to interference at thr slits, whilst in the double slit experiment, the light rays from both slits interfere constructively/destructively as a result of diffraction ?
Thanks a lot !!
Josielle

sophiecentaur
Gold Member
Thanks a lot for your help!! However, is it true that in the single slit experiment, diffraction happens due to interference at thr slits, whilst in the double slit experiment, the light rays from both slits interfere constructively/destructively as a result of diffraction ?
Thanks a lot !!
Josielle
A slit of finite width has its own diffraction pattern. Two identical slits each have identical diffraction patterns and the resulting pattern (in a simple case like this, at least) can be calculated fairly accurately by 'multiplying' the diffraction pattern by the (simplified) two slit interference pattern.
I am referring to the pattern at 'infinity' - say on a distant screen. The near field is more complicated to work out.
I think you should step back at this point and re-focus. It's ALL Diffraction which can be calculated by assuming a vast number of points across a slit and doing an Interference calculation between all those points and summing the result (Integral Calculus). You can work out simple diffraction due to narrow slits by just doing the Interference calculation assuming two infinitely narrow slits ('point sources', if you like).

ZapperZ
Staff Emeritus
Thanks a lot for your help!! However, is it true that in the single slit experiment, diffraction happens due to interference at thr slits, whilst in the double slit experiment, the light rays from both slits interfere constructively/destructively as a result of diffraction ?
Thanks a lot !!
Josielle
You have not understood either of them. There are signatures of the single slit in the double slit pattern.

Look at the picture that I posted in this post:

Zz.

Thanks a lot for your help!! However, is it true that in the single slit experiment, diffraction happens due to interference at thr slits, whilst in the double slit experiment, the light rays from both slits interfere constructively/destructively as a result of diffraction ?
Thanks a lot !!
Josielle
Hi Josielle.
I think you're asking a really good question. I get the impression that you are not really satisfied with the model of Huygens elementary waves in text books and I agree sort of, although I also have to say that the model works quite well.

I'd like to start with a different example: If a plane wave (light for example) hits some very small objects then every object can become the source of a spherical wave. I would call this process scattering, but I guess diffraction is also correct. In any case, these spherical waves and the plane wave all interfere with each other and you'll get a diffraction pattern on a screen or detector placed at some distance from the objects. So you see that interference is important for getting the diffraction pattern.

Now let's replace the small objects with small holes in a screen. Now the plane wave cannot pass through anymore and the diffraction pattern you see is due to interference between spherical waves coming from the holes. It's not completely clear why a hole would lead to a spherical wave the same as a small object does. Maybe a good explanation is that oscillations in the edge of the hole cause the spherical wave.

If you now think of a single large hole or slit the model becomes even stranger. Now every point in the hole or slit is supposed to be the source of an elementary wave. If you add up these waves correctly (taking into account constructive and destructive interference) you get the diffraction pattern of a hole or a slit at a large distance. Again I would say interference between the elementary waves is necessary to get the diffraction pattern.

What's really strange is the idea that elementary can come from inside the hole. There is nothing there, so what is producing the waves? There are other explanations where the interaction between light and the hole or slit only happens at the edge, but these so-called boundary diffraction models are more difficult to work with.
Thanks a lot. This means a lot for me  