Young's double slit introduction (video) | Khan Academy
Thus, the question of the relation between the wave function and “God does not play dice”, the so called standard, or Copenhagen An alternative approach to the quantum trajectories is developed based on the Feynman's path integral. . In view of such definition, the absolute phase keeps its absolute meaning even if. Thus, we say that, there is a path difference between the two waves of about λ ( wavelength). It has a direct relation with phase difference. This path difference guarantees that crests from the two waves arrive This means that the light sources must maintain a constant phase relationship.
These lines represent lines where every point along there is a peak of the wave. What's in the middle? In the middle would be the trough of the wave, or the valley. That's what I'm going to use.
I'm going to use this representation for the wave. This will let me show this wave spreading out in two dimensions better than this one could. I couldn't draw it very well with this one. This wave comes in here, this laser light comes in here. That part hits that barrier, it doesn't get through.
This part hits that barrier, it doesn't get through. This part hits the barrier, it doesn't get through. The only portion that's going to get through is basically this portion here and this portion here.
These are going to be the ones that make it through. What do you see on the wall over here? If this was a screen that you could project the light on, what would you see?A Level Physics - Coherence and Path Difference
Naively, what I would have thought would have been, okay, shoot, light comes through here, bright spot. Light comes through here, bright spot. You just get two bright spots, right? Well, no, that's not what you get.
That's why this experiment is interesting, because you don't just get two bright spots. You get a pattern over here, because waves don't just travel straight through this hole. When a wave encounters a hole or a corner, it spreads out. That spreading out we call diffraction.
You're going to get a wave spreading out from down here. This is not going to go in a straight line. It spreads out in two dimensions.
That's why I had to use this wave drawing representation. It's going to spread out from the top one, too. Uh-oh, look what's going to happen. You're going to have two waves overlapping.
Phase Difference and Path Difference | Physics Things
These two waves are going to start overlapping, and where they overlap constructively, you'd get a bright spot, and where they overlap destructively, you'd get a dark spot.
Where it's sort of half constructive, half destructive, you might get a mediumly bright spot. How do we figure out what's going to be?
Well, I can't draw this precise enough to show you that, so let me get rid of all of this mess real quick, get rid of that. Out of the bottom hole, what would you get?
What is the coherence length? What is the coherence time?
You'd get this, a nice spherical pattern coming out of here. It might not exactly be the same intensity throughout here, but I can't draw it with the exact right intensity. Up here, this intensity of this portion would be smaller than this portion here, the degree to which it's spreading, but this will help me visualize it.
You've got this wave spreading out, out of the bottom hole. You also have a wave spreading out of the top hole. Now these are going to overlap. Let's draw them both, boom. In the same region you're going to have constructive and destructive interference. If you look, remember, these lines represent peaks, so every time a peak lines up right over a peak, or in the middle, a valley over a valley, every time the wave is exactly in phase, when it gets to the same point, these are all constructive points, so right in the middle you'd get a big bright spot.
That's kind of weird. Right in between these holes there'd be a big bright spot. Well, look at this. This is constructive, constructive, all constructive. They form a line, they get these lines of constructive interference. Same with this line, constructive, constructive, all the way over to here. So on the wall, you'd see multiple bright spots. Down here, these are all constructive because peaks are lining up perfectly.
I'd get another one here. You'd keep getting these bright spots on the wall. They wouldn't last forever. At some point, it'd start to die off.
It'd be hard to see, but you'd be getting these bright spots continuing on. At some point, they're so dim you can't see them. In the middle, well, wherever Let's see, what's a good point to look at?
Wherever a peak lines up with a valley, so this wave's a peak right here, but for the other wave, lookit, we're in between the two green lines, so in that point you'll have destructive, because the peak is matching up with the valley. This would be destructive and this would be destructive, so in between here you get a destructive point. The same is true, in between each of these perfectly constructive points, you'd get a perfectly destructive point, and in between those it'd be kind of half constructive half destructive, would merge into each other, and what you'd get, sometimes physicists draw a little graph to represent this, you get a bright spot in the middle.
This is sort of representing a graph of the intensity zero, and then another bright spot, and it goes down to zero again, another bright spot. They get weaker and weaker as you go out.
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- Young's double slit introduction
At some point, it's hard to see. Same on this side. Zero, bright spot, zero, bright spot. Thomas Young postulated that light is a wave and is subject to the superposition principle; his great experimental achievement was to demonstrate the constructive and destructive interference of light c.
The light passing through the two slits is observed on a distant screen.
Phase Difference and Path Difference
When the widths of the slits are significantly greater than the wavelength of the light, the rules of geometrical optics hold—the light casts two shadows, and there are two illuminated regions on the screen.
However, as the slits are narrowed in width, the light diffracts into the geometrical shadow, and the light waves overlap on the screen. Diffraction is itself caused by the wave nature of light, being another example of an interference effect—it is discussed in more detail below. This path difference guarantees that crests from the two waves arrive simultaneously. Young used geometrical arguments to show that the superposition of the two waves results in a series of equally spaced bands, or fringes, of high intensity, corresponding to regions of constructive interference, separated by dark regions of complete destructive interference.
Young's double-slit experimentWhen monochromatic light passing through two narrow slits illuminates a distant screen, a characteristic pattern of bright and dark fringes is observed.
This interference pattern is caused by the superposition of overlapping light waves originating from the two slits. Regions of constructive interference, corresponding to bright fringes, are produced when the path difference from the two slits to the fringe is an integral number of wavelengths of the light. Destructive interference and dark fringes are produced when the path difference is a half-integral number of wavelengths.
Using narrowly separated slits, Young was able to separate the interference fringes.
In this way he determined the wavelengths of the colours of visible light.