Constructive Interference In Light

[-Job-]

The double-slit experiment seems to show wave interference patterns, the dark strips being the result of destructive interference. My question is about the lighted regions, do they display constructive interference? Is the color of light under constructive interference shifted? I realize we're shooting a photon at a time, but if it shows destructive interference then mustn't it show constructive interference as well?

 

[DrChinese]

The double-slit experiment seems to show wave interference patterns, the dark strips being the result of destructive interference. My question is about the lighted regions, do they display constructive interference? Is the color of light under constructive interference shifted? I realize we're shooting a photon at a time, but if it shows destructive interference then mustn't it show constructive interference as well?

Good point! In fact there is constructive interference. It does not affect the color (frequency) but it does affect the intensity of the region.

It may not surprise you then to learn that a single photon - even one not going through a double slit - also exhibits a "small" amount of constructive and destructive interference in its path from a source to detector. This has been demonstrated amply in reflection experiments in which the reflective surface is modified to prevent reflection at various points (usually through etches on the surface). If destructive reflection points are eliminated, the net intensity increases. If constructive reflection points are eliminated, the net intensity decreases.

 

[-Job-]

Is this intensity something that we can observe with just a single photon, or do we need plenty more? Are these more intense photons more energetic?

 

[DrChinese]

Is this intensity something that we can observe with just a single photon, or do we need plenty more? Are these more intense photons more energetic?

You are correct that you must observe intensity using a collection of photon detection events. However, the intensity can be demonstrated with photons sent one at a time or as a group. The point is simply that interference exists with or without the presence of other photons. The reason is that normally many paths exist from source to detector. If that is the case, then these paths interfere with each other both constructively and destructively.

 

[-Job-]

I think i would pay to be able to see this experiment performed and observe how changes in its parameters affect the pattern observed. I have some particular questions:

In water, as we place the slits farther apart the waves will be able to travel farther before they interfere. Assuming that the waves lose amplitude as they propagate through the water, when they do interfere the amplitude at the troughs and crests in areas of constructive interference will be smaller than if the slits were closer together (right?). With light, as we place the slits farther and farther apart, does the intensity measured at the lighted regions vary? Does anything vary with the distance of the slits at all, like the width of the lighted/dark stripes or measured intensity?

Can we place the two slits far enough so that there is no interference?
Assuming the detector is some distance from the slits, what is the maximum distance between the slits that still produces the interference pattern?

May be a naive question, but just to make sure, does the photon always make it to the other side? I sure hope it doesn't.

 

[DrChinese]

With light, as we place the slits farther and farther apart, does the intensity measured at the lighted regions vary? Does anything vary with the distance of the slits at all, like the width of the lighted/dark stripes or measured intensity?

Can we place the two slits far enough so that there is no interference?
Assuming the detector is some distance from the slits, what is the maximum distance between the slits that still produces the interference pattern?

May be a naive question, but just to make sure, does the photon always make it to the other side? I sure hope it doesn't.

The light does not have to make it through the slits - it can simply illuminate some other part of the apparatus or the material that the slits are cut into.

The separation of the slits definitely comes into play. The shape is a factor too, although most versions feature rectangular slits. The width of the slits and their separation, and the wavelength of the light, form some of the common parameters which are varied. If a light beam is used (rather than individual photons), then normally you need coherent light (such as a laser beam) so that the interference between photons does not become a critical factor.

 

[-Job-]

What would you expect to be the outcome of the experiment if it were set up as displayed in this image I've attached?

 

[madhusudan]

i need a video of interference of light it double slit . . . . .if anyone has send it to madhu91@gmail.com . . . . . please

 

[jtbell]

I think i would pay to be able to see this experiment performed and observe how changes in its parameters affect the pattern observed.

Interference is a standard lab experiment in introductory and intermediate physics courses at colleges and universities. Probably even in many high schools. The equipment is not very elaborate. If you visit a nearby school and ask nicely, you can probably arrange to sit in on a lab, or even get a quick on-the-spot demonstration. It takes only a few minutes to pull out a laser and a slide with engraved slits.

The one-photon-at-a-time version is another matter, though.

 

[nanoWatt]

Oh, this is a great topic. It gave me a question.

Can the frequency of light be shifted due to interaction with the magnetic field of another frequency?

I believe frequency shifts occur when light propogates through a medium, with regard to the index of refraction. I'm wondering about a purely magnetic/electric field shift.