# Young's double slit experiment

1. ### copernicus1

99
Why did Young specifically perform a double-slit experiment to demonstrate the wave nature of light, since light will diffract and interfere even through a single slit? Is there something special demonstrated by the double-slit experiment that's not demonstrated by a single-slit experiment? (I'm specifically interested in the 19th century experiment...I understand the importance of the modern versions of the double-slit experiment.)

2. ### razal_ak47

8
in single slit experiment,light wil diffract but will not interfere as there is no path difference.
we will get a bigger circle of light than the slit on the shadow region.

inorder to have interference there should be path bath difference,so we have two slits.

wave nature could only explain interference.

3. ### copernicus1

99
Thanks, but don't you get a sinc function through a single slit? And doesn't that demonstrate interference? You'd have a bright central maximum and then smaller maxima on either side. Doesn't this indicate that interference is occurring?

4. ### razal_ak47

8
if we observe shadow region after diffraction, we have a bright circle and comparitvely darker circles around it,until it becomes dark...
we dont have maxima,minima and then again maxima.

just make a small hole in a thick paper,pass light..you can observe

### Staff: Mentor

Have you seen Young's original description to the Royal Society in 1803? He did not do a "double slit" experiment as we would describe it today. The first experiment that he describes, uses a single obstacle of width 1/30 inch, which is basically the complement of a single slit.

http://books.google.com/books?id=7AZGAAAAMAAJ&pg=PA1#v=onepage&q&f=false

6. ### copernicus1

99
Thanks! This is helpful. I also found this webpage which discusses the difference between the single and double slit experiments as really just a matter of degree, with the interference pattern being much easier to see with two slits instead of one.

http://www.studyphysics.ca/newnotes/20/unit04_light/chp1719_light/lesson58.htm

7. ### MikeGomez

252
I believe your question on Young's original experiment has been answered.

Here is some information regarding your other question...

Yes, there is a difference. Look at this wikipedia article under the section "Example: Difraction patterns"

http://en.wikipedia.org/wiki/Envelope_(waves)

There they say "The first factor, the single-slit result I1, modulates the more rapidly varying second factor that depends upon the number of slits and their spacing."

8. ### MikeGomez

252
Ok., but then here is what Richard Feynman had to say about it.

"No-one has ever been able to define the difference between interference and diffraction satisfactorily. It is just a question of usage, and there is no specific, important physical difference between them."

### Staff: Mentor

There is no path difference only in the limit of an "ideal slit" that has zero width. A real slit has a non-zero width, and there is a path difference for light rays that arrive at a single point on a viewing screen, from two different points along the width of the slit. This is what gives you the usual single-slit diffraction pattern.

http://hyperphysics.phy-astr.gsu.edu/‌hbase/phyopt/sinslit.html

As for the difference between interference and diffraction, I think of interference as the more general phenomenon: the superposition of waves with different phases; whereas diffraction is one consequence of interference, namely the "bending" of light around obstacles.

In setups with slits, we tend to associate the term "interference" with effects due to the superposition of waves from different slits, and "diffraction" with effects due to the superposition of waves from different points in the same slit.

10. ### sophiecentaur

14,297
The reason that the two slits experiment is used, these days and probably pretty soon after the Young's firest demo will almost certainly be because the results are so visually stunning and can't just be put down to the 'natural fuzziness' of light coming from a single one. The sinc function is just not very impressive, compared with a dozen or more obvious fringes of equal brightness.

imho, 'everything' in optics is diffraction - it's just that some images show the wave nature of light more than others.
What we describe as interference is more of your Σ, whereas diffraction is more of your ∫.

11. ### Philip Wood

1,176
Sophiecentaur. Agree: it's more impressive.

The maths of predicting the pattern is also easier (if we neglect (treat as constant) the 'modulating factor' due to diffraction at each single slit). So we have a demonstration which is both impressive and simple to explain.

[I think that Young, though much better educated than Faraday, was not especially interested in treating various cases of interference and diffraction mathematically. Fresnel was. In this respect Fresnel's work in wave theory might perhaps be compared to Young's as we compare Maxwell's work in electromagnetism to Faraday's.]

12. ### brando2

1
So many difficult and painstaking approches to solve the simple problem of how to make 2 small slits .A "candle to smoke glass"?Come on man, why not a 2\$ can of spray paint and scratch the lines? Nerds have no comin sence

13. ### sophiecentaur

14,297
And some people have 'no spellin'. ;)
Have you ever tried getting a good , even slit by scratching through a layer of paint? It is designed to resist that sort of abuse and it forms a jagged line, with tears along it. Otoh, a thick layer of carbon has a very fine structure and does not bond well to itself. The slit you will get with carbon will be so much better than with paint. It is just very fragile, so you have to look after it.
The best slits can be made with photographic film, scaled down (in the camera) from a well drawn large (negative) version. Give the "nerds" a break. It's nerds who push back the frontiers of Science.

### Staff: Mentor

Who said anything about that, in this thread, or in any of the linked web pages?

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