Dalo

Yes, but it is entirely irrelevant to the question: how does the distance between the fringes relate to the distance between the waves, assuming that all the waves hitting the screen at the same time originate from one and the same wave front going through the slits? (it would be even more complicated if they did not) Your diagram would be valid even if we changed the distance between two crests or troughs. The results would be the same, or so it seems to me.

could you please elaborate? As far as the fringes are concerned, I am not doubting the validity of the methods for measuring the distance between them

Let me see if I get it right: Green light, when measured the way described above, gives a certain figure. And so does each other color, each color having its own figure. And, that is what we call wavelength. If that is the case, then the mode of calculation does not fit the definition. You cannot define wavelength as the distance between two (monochromatic) waves, and then use another property of waves to calculate it. Because of diffraction, the distances from the grating to the screen are particular to each color, but that has nothing to do with the original definition. At least, I find it very difficult to link them together. The calculations are legitimate and give us a very useful way of identifying colors, but they do not seem to follow from the definition.

there is something as humor. Banishing it would be very sad. But I will auto-censure.

We are apparently not communicating very well. I want to know how the wavelength is calculated. I have read the information in textbooks and watched the videos, and I still have questions. My first question was answered, but it created its own questions. The difference in distances between two lines, from the diffraction grating to the screen, is used to calculate the wavelength. I find it strange because both lines come from the same wave, while the wavelength is the distance between two waves. How is that possible?

Then I simply do not understand the argumentation. How can it be a given? edit: I understand the definition being a given, but the question now is how it is calculated. John! Are you there? We were doing so well, and then you left me, and then this Terderator came out of nowhere and I think he is mad as hell!

Please do.

But we are still in the process of calculating the wavelength, defined as the distance between two crests/troughs. Your link already presupposes the calculation?

I was going to post exactly the same link! You beat me to it by a few seconds. My first reaction was: yes, it does make sense. It is not the horizontal distance (from the perspective of one standing on the beach) that is measured, but the difference in distance light has to travel from each slit to the screen. But then another problem came up, and I hope that it will also be as easily solved. The distances measured concern one and the same wave. My question is, how does that relate to the distance between two crests/troughs? Well, almost the same link:

Wavelength refers to the distance between two crests or between two troughs. Imagine standing on the beach and looking at the waves coming towards you. The waves can cover a long distance along the beach, let us call it their width, but that is irrelevant. What you want to know is how often they follow each other (frequency), how high they get (amplitude), and of course what we have referred to as their wavelength. Here is a genuine question: To measure the wavelength, we are told to measure the distance between the points on a screen of (monochromatric) light that has come through two or more slits. This is a horizontal distance, comparable to the width of the waves hitting the sand. But we want the distance between the crests or the troughs. Which is, seen from the perspective of someone standing on the beach, a vertical distance. Imagine erecting a barrier somewhere in the water, with a diffraction grating, and measuring the points where the water comes through. Apparently, you would be measuring points along the width of the wave, and not the distance between crests or troughs. So, what is the real explanation when dealing with light?

If you mean Wolfson's, then no, I cannot. You have to buy the course. The title is Physiscs and our Universe: How it All Works. By Richard Wolfson. I think he uses the same example in his textbook: Essential University Physics Here is a screen shot of a part of 14.5 Interference

of course.

the problem is that blind people do not see shadows. We need vision to see them. take two lights in a dark space, some distance from each other. The only way to keep both lights apart, is to register the dark space between them. We have, as it were, "to see darkness". If we did not, the dark space between the light would disappear, and we would see only one (bigger) light. Darkness is different from the absence of light. Closing your eyes is different from looking at darkness, Those are two different sensations. Also, the mind can create all kind of images when our eyes are closed, as in dreams, hallucinations and so on. I would certainly not deny that the absence of sunlight leads to darkness. I would like to point to the fact that light in physics, can be "invisible" to the naked eye. The absence of light, strictly speaking, would mean the absence of all wavelengths of light, visible and invisible. We do not know what that is. It would mean absolute vacuum. The idea that light can disappear and still leave a visual impression is somehow contradictory.

Also very interesting. We see the absence of light.

Yes, that makes perfect sense. But applying the same principle to light seems out of the question. We could not say that there is simply an absence of waves but light is still present, as the water is. Also, the water color is different of what it is there where there are still waves, and from what it was, before the experiment started.

I would find it very strange if it were anything else. But since we are talking about negative interference, it would seem that it only concerns the light waves. Water waves are unaffected by it. There is no "absence of water" as there is "absence of light". That means that we are observing a pure optical phenomenon.

This is certainly a nice video. But, as you might expect, I have some questions, the most important one being: are the dark parts still water?

I saw the example of two waves starting at opposing ends of a string in a video course by Richard Wolfson, a course which was based on his textbook on Physics. In the video, which is copyright protected, you can clearly see both waves moving towards each other, passing though each other without any visible effect, and continuing on their path. His explanation was that those waves were transporting energy, not matter. And that is why, according to him, they could pass through each other. That is what I find mysterious. Swansont's video is I am afraid not really representative of this example.

I must say that I was thinking not about light waves, but of waves on a string. Like when two people move a rope each from one end, and create two opposite waves?

What do you mean by "not reflect"? I suppose you mean that if wave A is stronger than wave B, when they meet, we see that wave A and wave B both continue on their path, instead of being thrown back. I would like to point out that it still does not necessarily mean that they go through each other. Maybe both inherit the strength of the wave they collide with.

I find the idea that waves transport energy and not matter very mysterious. There is of course the case of electricity running through wires, but I find somehow this phenomenon easier to understand. The perfect example of the principle "energy and not matter" is given by the example of two transversal waves running along a string. They seem to pass through each other without disturbing each other, reaching the end of the string and come back running again. I never understood why such waves were not simply considered as two standing waves, the place where they meet being the place of their collision and their recoil. In other words, why assume that they go through each other while it would be much easier to assume that each wave simply traces its path back after being stopped by the other? Are there deeper motivations and reasons behind the view that they do go though each other?

Of course. The problem is that there is no supreme authority that can claim to be neutral and objective. Truth, in such a context, is not scientifically definable, and is an object of struggle and competition. The scientific struggle for "truth" is luckily more institutionalized and rule-driven. That does not mean that we should ignore the human, economical and political factors. Only that they are not as preponderant as they used to be. We do not know of course how the future generations will look back on our era. Maybe they will see it as comparable to that of the Church dominated period of the Middle Ages. Or, hopefully, as the beginning of the Enlightening.

We already know how archeologists will deal with the opinions in this or other forums. At least, if the past is any indication of the future. Just look how scholars handle debates which have taken place centuries ago. They will tend of course to look for opinions which have turned out to be right while everybody considered them wrong (or maybe vice versa). Views of the Atomists through the centuries, writings of Giordano Bruno if he had been allowed to continue preaching his "insane ideas". Archeologists of different era and of different convictions will look for different things in the past. And they usually find them.

I find your explanation of how eye cells are activated really fascinating. I have read quite a lot on this subject and I never encountered such a theory. Again, would you you be so kind as to provide me with references and links?

this quote would have been more accurate. I am curious about the classical description you are mentioning. If it means the use of wave theory and vibrations to explain vision, I remain very interested indeed. In particular, I would be very interested in the process of transformation from wave to quanta you are referring to.