A wave is spread across. In a double slit experiment, if the wavefunction representing the electron, hits the slits, shouldn’t that be a measurement and shouldn’t that collapse the wavefunction? Now, you'll say it's not a physical wave, it's a probability wave. But then how does a probability wave split into two after the slits? Then, it should be like, the wave hits the slits, the electron says, dude I'm going through the slits, so don't collapse, but you can split into two and diffract. When does it decide that it's going through the slit?
I'm trying my best to explain you, but this is not easy on a TH-cam comment and English is not my native language. It is not easy to understand it on a 2D diagram, because those waves are in 3D, so they have a deepness (the 3rd dimension) that we can't see on a board because this deepness is "hidden" behind the board. Try to imagine the same situation in 3D, as if you are watching real waves on an ocean, which are in 3D (contrary to the waves on a guitar string for example). Imagine that each blue circle is the crest of an ocean's wave : between two crests, there is no "empty space", but still water whose level is descending until a minimum (the "trough") that is exactly in the middle between the two crests (this descending water between two crests is "hidden" behind the board). So, when a "trough" (minimum amplitude of water) of the first wave encounters a "crest" (maximum amplitude of water) of the second wave, they will cancel out, so the addition of the two crests gives us a "flat water", which is a "destructive interference". Now, imagine that we have the same situation for the light, but we have to be careful because is is a little different. Actually, each blue circle is equivalent to a "crest" on an ocean (maximum positive amplitude of the light), whereas the middle between each circle is a "trough" (minimum negative amplitude of the light). The straight lines of light that we can observe on the screen are not equivalent to these positive or negative amplitudes (let's call them "+A" and "-A"), but equivalent to the intensity (let's call it "I"). The relation between I and A is a follows : I = A² (for a single "wave"). So a "trough" (minimum negative amplitude -A) or a "crest" (maximum positive amplitude +A) gives exactly the same intensity of light, because (-A)² = (+A)² = I. So we have a constructive interference if we combine two "troughs" or two "crests". Indeed : - "trough + trough" = -A + (-A) = -2A => Imax = (-2A)*(-2A) = 4A² (maximum intensity of light, so it is "shiny") - "crest + crest" = +A + A = +2A => Imax = (+2A)*(+2A) = 4A² (maximum intensity of light, so it is "shiny") (Note that what I say is true only for calculating the maximum of the intensity, because if you want to calculate the intensity of light when you combine all other "transitional" amplitudes, certain "angles" or "phases" intervene : mode details for example in Richard Feynman's book, Mechanics, chapter 29 or chapter 37) And we have a destructive interference if we combine one "trough" with one "crest" : "trough + crest" = +A - A = 0 => I = 0² = 0 (minimum intensity of light, so it is "dark"). So, each of the three arrows that were represented on the board passes through each addition "crest + crest" (symbolized by the blue dots) and through each addition "trough + trough" (that are not represented on the board), to give a maximum intensity of light, ie to give us straight lines of light on the screen. On the other hand, each "x" mark on the board represents the additions "crest + trough" (the blue circles represent the crests of the first wave, whereas the middle of the "white space" between each crest of the second wave represent the "troughs" of the second wave), which give us the "dark" (absence of light) : it is equivalent to the addition "crest + trough = flat water" of my first example about the ocean's waves). I do not know if I am enough clear, it is difficult to explain it on TH-cam, not in my native language, but I tried my best. :-)
Thank you so much for putting these up, they are fantastic. I understand far better now. I really appreciate it, thank you.
Nick Kraw You're welcome Nick! Glad they're helpful!
thank you bro you've always helped me through countless topics
Such impressive lecture, I hope could sat in person to listen you lectures one day. Thank you.
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A wave is spread across. In a double slit experiment, if the wavefunction representing the electron, hits the slits, shouldn’t that be a measurement and shouldn’t that collapse the wavefunction? Now, you'll say it's not a physical wave, it's a probability wave. But then how does a probability wave split into two after the slits? Then, it should be like, the wave hits the slits, the electron says, dude I'm going through the slits, so don't collapse, but you can split into two and diffract. When does it decide that it's going through the slit?
This is an awesome video. Great explanation!
U the best man
U make physics easy
You are so good. U are the reason i understand physics
Thats awesome! keep at it
Madhumitha Ayyappan hey you said true
How do you know what is a trough and what is a crest? there is a lot of empty space in between each wave front, is that all a trough?
I'm trying my best to explain you, but this is not easy on a TH-cam comment and English is not my native language.
It is not easy to understand it on a 2D diagram, because those waves are in 3D, so they have a deepness (the 3rd dimension) that we can't see on a board because this deepness is "hidden" behind the board. Try to imagine the same situation in 3D, as if you are watching real waves on an ocean, which are in 3D (contrary to the waves on a guitar string for example). Imagine that each blue circle is the crest of an ocean's wave : between two crests, there is no "empty space", but still water whose level is descending until a minimum (the "trough") that is exactly in the middle between the two crests (this descending water between two crests is "hidden" behind the board).
So, when a "trough" (minimum amplitude of water) of the first wave encounters a "crest" (maximum amplitude of water) of the second wave, they will cancel out, so the addition of the two crests gives us a "flat water", which is a "destructive interference".
Now, imagine that we have the same situation for the light, but we have to be careful because is is a little different. Actually, each blue circle is equivalent to a "crest" on an ocean (maximum positive amplitude of the light), whereas the middle between each circle is a "trough" (minimum negative amplitude of the light). The straight lines of light that we can observe on the screen are not equivalent to these positive or negative amplitudes (let's call them "+A" and "-A"), but equivalent to the intensity (let's call it "I"). The relation between I and A is a follows : I = A² (for a single "wave"). So a "trough" (minimum negative amplitude -A) or a "crest" (maximum positive amplitude +A) gives exactly the same intensity of light, because (-A)² = (+A)² = I. So we have a constructive interference if we combine two "troughs" or two "crests". Indeed :
- "trough + trough" = -A + (-A) = -2A => Imax = (-2A)*(-2A) = 4A² (maximum intensity of light, so it is "shiny")
- "crest + crest" = +A + A = +2A => Imax = (+2A)*(+2A) = 4A² (maximum intensity of light, so it is "shiny")
(Note that what I say is true only for calculating the maximum of the intensity, because if you want to calculate the intensity of light when you combine all other "transitional" amplitudes, certain "angles" or "phases" intervene : mode details for example in Richard Feynman's book, Mechanics, chapter 29 or chapter 37)
And we have a destructive interference if we combine one "trough" with one "crest" :
"trough + crest" = +A - A = 0 => I = 0² = 0 (minimum intensity of light, so it is "dark").
So, each of the three arrows that were represented on the board passes through each addition "crest + crest" (symbolized by the blue dots) and through each addition "trough + trough" (that are not represented on the board), to give a maximum intensity of light, ie to give us straight lines of light on the screen. On the other hand, each "x" mark on the board represents the additions "crest + trough" (the blue circles represent the crests of the first wave, whereas the middle of the "white space" between each crest of the second wave represent the "troughs" of the second wave), which give us the "dark" (absence of light) : it is equivalent to the addition "crest + trough = flat water" of my first example about the ocean's waves).
I do not know if I am enough clear, it is difficult to explain it on TH-cam, not in my native language, but I tried my best. :-)
Correction: it's diffraction that makes the waves spread out, not dispersion.
I love u man
Why are you erasing the points you are trying to highlight? 3:36
Yea...marker kind of dried up :)