Register to reply 
Why is information about the electron described with information of a photon [...]? 
Share this thread: 
#19
Nov1012, 08:51 AM

HW Helper
P: 3,440

p.s. sorry I've taken a while to reply, I have been under the weather.



#20
Nov2212, 11:26 AM

P: 554

Okay, I think I'm beginning to get it more.
Right now, I need to understand two more things to my knowledge.: 1) I'm beginning to understand the Δx = λ/sinθ more. Basically, a larger wavelength takes more physical space and needs a larger slit so it can pass through. Also, sinθ will yield a value between 0 (excluded) and 1 (included) meaning that the slit will be larger or equal to the wavelength of the photon and that the photon is somewhere within the slit. In other words, it makes sense to me why Δx and λ are proportional. What doesn't make sense to me is how the sinθ part is there. I would like your help to dive into the singleslit diffraction nonformal “proof” to figure out how to get the sinθ part if you don't mind. 2) I would like to understand how the momentum uncertainty is the Δp_x = 2h/λ sinθ equation. I understand it geometrically so, what I am asking is basically when you said QUOTE]p.s. sorry I've taken a while to reply, I have been under the weather.[/QUOTE] That's fine, I'm not rusing you. In fact, I am very grateful for your answers. Also, sorry for taking long to respond myself. I've seen your post for a while now but, I was either too busy or too tired to post and it's pointless to post if I haven't thought through what I am writing (since this thread is about understanding a physical concept rather than just responding to a regular email). Technically, I am tired now too but, I'm still wellrested enough to be able to understand what's going on if I think about it slowly. 


#21
Nov2812, 03:17 PM

P: 554

Actually, I think I get the theory now.
The only minor confusion I have now is simply: "Shouldn't Δx ~ λ/sinθ be Δx ~ λ/sinθ  λ/sinθ = 2λ/sinθ instead such that the final answer is Δx Δp_x ~ 8πħ?" 


#22
Dec112, 07:28 PM

HW Helper
P: 3,440




#23
Dec512, 05:53 PM

P: 554

Can you relate what you said in your most recent post (the one before this one that I am writing now) to the concept of a Diffractionlimited system ( http://en.wikipedia.org/wiki/Diffractionlimited_system ) for me please since it seems to be strongly related to what I am asking?
Based on the equation from the Wikipedia link I'm now posting, it seems that Δx ~ λ/(2nsinθ)  λ/(2nsinθ) = 2λ/(2nsinθ) = λ/(nsinθ) which is λ/sinθ when n = 1 for the problem that is the focus of this thread. 


#24
Dec612, 05:18 AM

HW Helper
P: 3,440

yeah, definitely. The equation Δx ~ λ/sinθ comes from the theory of farfield diffraction. And in this problem, we are saying that our uncertainty of the position of the electron is due to the equation for diffraction.
In the wikipedia page, they are saying the smallest resolution size is due to diffraction. This is really the same thing as our problem, where the resolution size is the uncertainty of the position of the electron. 


#25
Dec612, 12:13 PM

P: 554

Okay now, I need to discuss the index of refraction. There exists negative indices of refraction which would just make Δx negative (which doesn't change anything meaningful since it just means that we are referring to Δx as the distance from point B to point A rather than from point A to point B) but what if the index of refraction is a positive value that is not 1? That would affect the uncertainty of the photon's position (and by extension, the electron's).
According to the problem that is the focus of this thread, the photon will be observed through a lens and, as mentioned here ( http://www.wolframalpha.com/input/?i...index+of+glass ), the index of refraction of the lens is a positive value that is not 1. If it was just some slit with no glass, we could say that the index of refraction is 1 in a vacuum or very close to it in air. Is the problem saying “we're assuming the index of refraction is a positive value that's not 1 but since it's not important to the argument, we'll just ignore since it is an approximation after.”? By the way, is it correct for me to believe that the reason why a lens is being used is because it allows us to converge the (monochromatic) light at a specific point allowing us to treat it more like a particle than a wave when measuring it? 


#26
Dec712, 04:46 AM

HW Helper
P: 3,440

In this problem, they get this Δx ~ λ/sinθ because they are assuming the refractive index of the substance between the electron beam and the lens is roughly equal to one. They probably chose this just because it is the simplest option. Also, it would fit with the idea of the experiment being done in a vacuum.
And yes, the lens will have greater than 1 refractive index. This doesn't enter into the equation above, because after the light goes through the lens, it's wavefunction is no longer intimately linked to the wavefunction of the electron. Can you think why? And yes the lens is used to focus the light given off (just so that we can see it really). 


#27
Dec712, 09:20 PM

P: 554

The wavefunction seems to require more knowledge that I do not have including knowledge of probability. Could you tell me the bare minimum I need to know about it to understand your previous point please?



#28
Dec812, 06:10 AM

HW Helper
P: 3,440

The wavefunction of a particle (e.g. a photon) contains all the physical information we have about the particle (e.g. its momentum). And in the collision of photon and electron, we have certain rules about conservation of momentum, e.t.c.
Therefore, immediately after collision, the wavefunctions of the two particles are intimately related. And while the two particles are moving away after the collision, their wavefunctions are still related. But when the photon goes into the lens, the lens will interact with the photon, changing its wavefunction, so the wavefunction of the photon is no longer intimately related to the wavefunction of the electron. I hope I explained that ok. Just ask if I didn't explain something. So anyway, the whole point of what I was trying to say in the last post, is that after collision, we get a simple equation for the angle within which the photon is most likely to go. But when the photon interacts with the lens, then the motion of the photon will also depend on what kind of lens we are using, right? 


#29
Dec812, 12:26 PM

P: 554

This means that the wavefunction of the photon will change since the momentum is part of the data held by it and that will be changed therefore, the wavefunction will also change as a consequence. I was going to say that this confuses me further because, the experimenter is using the lens to focus the light at a point to detect it as a particle and that, this would introduce additional uncertainty but that is not the case since the refraction is measurable to a theoretical 0% uncertainty so we could “correct” the “screwedup” wavefunction where we only have the theoretical uncertainty from the uncertainty principle, right? 


#30
Dec812, 07:24 PM

HW Helper
P: 3,440




#31
Dec1412, 08:27 PM

P: 554




#32
Dec1512, 05:59 AM

HW Helper
P: 3,440

The entire problem is based around this equation Δx ~ λ/sinθ Which is classical physics. You could argue that the problem contains no quantum physics.
Edit: That's why this is a nice problem for people who are starting quantum physics, because it shows that with classical physics we can get behaviour which is similar to quantum physics. 


#33
Dec1512, 09:11 AM

P: 554

Just to confirm though, the Abbe diffraction limit for a microscope and the dealing of light as a ray for the converging lens are both also classical, right? 


#34
Dec1512, 10:03 AM

HW Helper
P: 3,440




#35
Dec1512, 10:27 AM

P: 554




Register to reply 
Related Discussions  
Photonphoton collision  electronanti electron pair creation  Introductory Physics Homework  4  
Which path information for a photon reflecting from a mirror as in Feyman's QED  Quantum Physics  0  
K, L, M electron transitions, xray emission lines, conflicting information!  Atomic, Solid State, Comp. Physics  1  
Is photon a basic unit of information?  Quantum Physics  14  
Information Loss in Photon Absorption?  Quantum Physics  24 