coherencediffractionelectromagnetic-radiation
For this picture Anna wrote: "Incandescent light is incoherent because it comes from many sources and the same is true for sunlight. By passing the light through the one slit he created a single coherent wave front."
What represent the drawn lines in the sketch about double slits? Are this wavefronts? What is a wavefront?
At the time of Young there were no lasers to provide coherent light. Incandescent light is incoherent because it comes from many sources and the same is true for sunlight. By passing the light through the one slit
he created a single coherent wave front . It is worth reading his description "on the nature of light and colors" in the link above.
Edit: I have not addressed the "why" a slit induces coherence. It has to do with wavelength : the size of the slit must be such that, similar to a diffraction grating, it diffracts specific wavelengths differently and also acts as a point source creating a coherent wavefrong. Here is an illustrated description of coherence. Here is a blog entry discussing coherence from thermodynamic sources .
Let's recall the easy way to make a diffraction pattern from a double slit. You just shine a laser on it. Why does this create a diffraction pattern? Well we know that the intensity of the light is determined by the electric field, so to figure out why the intensity of the light is the way it is, we just need to figure out why the electric field is the way it is.
To determine what the electric field is on the far side of the double slit (away from the laser) we only need to know what the electric field is at the two slits. This is because the only way information about the electric field can get to the other side is through the slits. Since the laser produces an electric field which oscillates sinusoidally at a specific frequency, we know that the field at each slit must be oscillating sinusoidally with the same frequency as the laser and hence each other. Also since the distance from the laser is the same, the electric field oscillations at the two slits are in phase (even if the distances to the laser were different, they would still be out of phase by a constant phase, and you would still see a diffraction pattern). So two things are necessary: the light needs to be oscillating sinusoidally at a single frequency (I will call this the monochromicity condition), and the oscillation at both slits needs to be offset by a constant phase (I will call this the phase coherence condition).
I will not explain why these two conditions are sufficient for diffraction (I assume you know that already), but let's see if they are necessary.
Suppose first that the monochromicity condition is met, but the phase coherence condition is not met, so that the phase difference between the two silts varies as a function of time. This could be accomplished for example by holding your double slit up to the sun but using a band-pass filter to filter the sunlight. In this case, I claim you would see no diffraction pattern because as the phase difference between the two slits varies, the location of the diffraction maxima and minima will quickly vary, and the diffraction pattern gets washed out. Thus the phase coherence condition is met.
Is the monochromicity condition necessary. Let's suppose you shine a green laser and a red laser on a diffraction grating at the same time. The electric field at the slits will be a sum of two sine functions. What pattern do you see? You can decompose the electric field on the screen into a piece from the red laser and a piece from the green laser. Each piece individually would give you a nice diffraction pattern, but they interfere with each other creating a complicated pattern of maxima and minima. Moreover, they oscillate at different frequencies, so these maxima and minima will move around very quickly and the pattern will wash out to something that doesn't look much like a diffraction pattern. As the number of lasers increases, this effect would get worse and worse. So the monochromicity condition is necessary.
Now let's take our favorite incoherent light source, an incandescent light bulb, and shine it on a double slit. What happens? Well the light bulb is emitting essentially as a black body so it is emitting radiation at all frequencies and so does not satisfy the monochromicity condition. Also since the light bulb is a large object composed of many independent sources with different path lengths to the slit, the phases at the slits will be uncorrelated and we will not satisfy the coherence condition.
After all this reading we are ready to see the magic. By first passing the light from the light bulb through a single slit, it is possible to arrange things so that the electric field in the double slits satisfies both conditions.
Let's start with the monochromicity condition. We know that a single slit produces a single slit diffraction pattern. Actually the electric field is a superposition of electric fields oscillating at all frequencies, and so the electric field will be a superposition of many interference patterns. But we can put the slits of the double slit at the maxima of the interference pattern for the wavelength we are interested in. Then the electric field from most the other frequencies will be filtered out because they will be closer to their diffraction minima, and we will be left with a sufficiently monochrome electric field to produce a diffraction pattern.
What about phase coherence? Well that is easy. The light takes the same amount of time to propogate from the single slit to either double slit, so the electric field at either of the double slits will always be the same, so they must be in phase.
Since the electric field at the double slit satisfies both the monochromicity and phase coherence conditions, you will see the double slit diffraction pattern.
Best Answer
A wavefront is the crest of the wave. When you go down to the beach, and see those things called waves, the front is the whole line that is at the same height.
In electromagnetics, it's the same thing. It's the points that are at the same height. In your diagram, the black curves represent a unit of cycle, and the two waves through b and c can either add to each other (to get double height), or can be out of phase (so b=-c and it's nothing).
Basically, S1 produces a single source, and there is only one set of ups and downs. S2 uses two points of S1's radiation to make two points that are using the same cycles. They don't have to be in the same phase, just that their period is the same. So b and c produce waves, and these intersect, and add together based on how they are in phase.
You can see a result by draging a sine wave over another one. The crest of the sine waves are the black arcs, the intensity is found by adding, eg one wave shifted by 0.5 cycle to the other. They cancel out.