# Interference Fringes

by maverick280857
Tags: fringes, interference
 P: 1,770 Hi I just want to know....will interference FRINGES be observed when electrons are used instead of photons (light) in case of Young's Double Slit Experiment? What will be observed? Thanks and cheers Vivek
 Sci Advisor HW Helper P: 11,833 Yes,of course.An introductory book on QM should describe this in the first chatper together with a buch of arguments for wavefunctions,wave behavior of point particles (electrons,...),etc... We call it Double Slit Experiment,without "Young",because it's much more general wrt the "fauna" of particles used. Daniel.
 P: 1,770 Yes I know I was referring to the conventional Young's Double Slit Experiment. What happens if I replace the source of photons (Coherent) with a source of electrons? Even if I don't change the screen (for electrons I believe you need a detector or counter right?) The experiment I read with light used a simple screen (perhaps cloth or a photographic screen). Are you also implying that the formulae for fringe width will remain unchanged and all I have to do is use the debroglie wavelength of the electron "wave" for $\lambda$? For light, the fringe width is given by $\omega = \frac{D\lambda}{d}$ where D and d are the distance between the slits and the screen and the distance between the two slits respectively. The question actually is: what happens when in a conventional YDSE experiment, we replace the source of photons with a source of electrons (everything else is unchanged)? What happens when the velocity of the electrons is increased or decreased? (Please remember that this is a conventional YDSE setup...the screen is still cloth or photographic film. THE ONLY change has been that the photon source has been replaced by an electron source.) What is the observation on screen? Cheers Vivek
P: 1,770

## Interference Fringes

Hello again

I came across this:
http://zebu.uoregon.edu/~js/21st_cen...res/lec13.html

Yes I agree that interference "fringes" will be observed with electrons. But doesn't that depend on the nature of the screen/detector used?
Emeritus
PF Gold
P: 2,977
 Quote by maverick280857 Yes I agree that interference "fringes" will be observed with electrons. But doesn't that depend on the nature of the screen/detector used?
You would obviously need something that can detect electrons. Are you asking whether normal screens can do this? To my knowledge, they can't. I suspect that you'd have to coat it with a phosphor or something like they do with TVs.
 Sci Advisor HW Helper P: 11,833 The same kind of screen as an oscilloscope. Daniel.
 Mentor P: 11,038 The double-slit experiment with electrons has even been done under circumstances in which no more than one electron can pass through the apparatus at a time. The standard double-slit interference pattern literally builds up one random spot at a time: http://physicsweb.org/articles/world/15/9/1/1
 P: 1,770 Thanks for the replies. Are you implying that the fringe width in the Young's Double slit experiment (with the same old setup as used for a coherent light source...no electronic gadgets to measure things) when the photon source has been replaced by an electron source is still given by $\omega = \frac{D\lambda}{d}$ where $\lambda$ is the debroglie wavelength of the electrons....given by $\lambda = \frac{h}{mv}$ According to some sources, the observed pattern is not a set of fringes but specks which are observed only when the detector is a flourescent screen. Of course some people call them fringes (I would reckon most people do). These specks disappear after a while and a consistent fringe pattern (as observed with light) is observed only if the electron source emits electrons continuously (at a rate which is very large) and the "life of every speck" is more than the persistence of vision.
P: 1,770
 Quote by dextercioby The same kind of screen as an oscilloscope. Daniel.
What do you mean same kind of screen as an oscilloscope. The classic version of the young's experiment we're used to uses a simple screen because there is no electronic detector or counter.
 P: 1,770 What I want to know is: Is the SAME theory applicable to the interference of electron waves as is for photons when I replace the source in the setup usually used for photons with an electron source? The problem isn't whether single electron interference occurs. Its simply whether I can use the same relationship for fringe width. The problem isn't to know whether electron wave interference occurs at all. Of course I know it does. And it does create patterns most people call fringes. But the fringes are observed if and only if you have a proper detector. If you use the same setup as for photons, why will you see fringes with electrons unless you also change the screen to a detector or flourescent screen? This is my argument.
Mentor
P: 11,038
 Quote by maverick280857 According to some sources, the observed pattern [for electrons] is not a set of fringes but specks which are observed only when the detector is a flourescent screen. Of course some people call them fringes (I would reckon most people do). These specks disappear after a while and a consistent fringe pattern (as observed with light) is observed only if the electron source emits electrons continuously (at a rate which is very large) and the "life of every speck" is more than the persistence of vision.
The same thing happens with light, if you reduce the intensity enough so that only one photon at a time passes through the apparatus.

 Quote by maverick280857 Is the SAME theory applicable to the interference of electron waves as is for photons when I replace the source in the setup usually used for photons with an electron source?
Yes. If the wavelengths of the electrons and the photons (light) are the same, and the geometrical dimensions of the apparatus are the same (width and spacing of the slits, and distance from the slits to the screen or detector), you get interference fringes with the same spacing.
 P: 1,770 With the debroglie wavelength? With the same old screen?
 P: 1,770 Okay hello again...its been a while since we had the brief discussion regarding interference with electrons. I would like to point everyone to http://www.amazon.com/exec/obidos/tg...glance&s=books This book (Time Space and Things by BK Ridley) addresses precisely the problem I had posed at first and the diagram on page 22 of it gives a graph of the number of electrons arriving per second on the screen. This gives new meaning to the words interference pattern, fringes and intensity distribution.
Mentor
P: 40,696
 Quote by maverick280857 This book (Time Space and Things by BK Ridley) addresses precisely the problem I had posed at first and the diagram on page 22 of it gives a graph of the number of electrons arriving per second on the screen. This gives new meaning to the words interference pattern, fringes and intensity distribution.
I'm sure I have that book floating around somewhere in my home library. What new meaning does it give? As far as I can see, whether you do the YDSE with electrons or photons, the results are "the same". (What Young would not have known was that the light pattern is created one "quantum" at a time.)
 P: 1,770 Hello Doc Nice to hear from you...well if you do have that book then please do see that page (22). Cheers Vivek
 Mentor P: 40,696 I'll check it out tonight when I get home. Edit: Turns out I could view it online at amazon. The two patterns shown contrast treating the electron as a classical particle (no "wave" nature) versus treating it quantum mechanically. What about it?
 P: 1,770 Doc, did you see the comparison of the 'intensity' pattern (for electrons) when a single slit is used and when two slits are used? THAT was my question (and for everyone else who got involved in the debate with me, read the pages of this book and you'll understand what my first few mailings meant). So for now it seems the topic is closed....

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