Physics Stack Exchange is a question and answer site for active researchers, academics and students of physics. Join them; it only takes a minute:

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

The problem statement:

Narrow beam of electrons hits an aluminium plate under an angle $\alpha=30^\circ$ according to the flat surface. We know the distance between two crystal planes in aluminium is $d=0.2nm$. At some voltage $U_1$ at wich electrons are accelerated we get a "mirror reflection" of electrons. If we increase the voltage to $U_2 = 2.25U_1$ we get same "mirror reflection" in the same direction. Calculate $U_1$.

What I have managed to do so far:

First of all I had to draw this sketch and i found out that in order to get the "mirror reflection" $\alpha=\beta$ crystal planes have to be parallel to the surface plane. So i concluded that $\alpha = \vartheta$. In any other way it would happen that $\alpha \neq \beta$ i think.

I hope my conclusion is correct and that $\vartheta = 30^\circ$.

enter image description here

What i haven't managed to figure out:

Now I can use the Brag's law to calculate $N\lambda_1$ and $\underbrace{(N+1)}_{\llap{\text{I hope this is correct,}}\rlap{\!\!\!\text{but please someone confirm}}}\lambda_2$ where I don't know the number $N$.

\begin{align} \left. \begin{aligned} 2d\sin\vartheta &= N\lambda_1 \\ 2d\sin\vartheta &= (N+1)\lambda_2 \end{aligned}\quad \right\} \quad N\lambda_1 &= (N+1)\lambda_2\\ \frac{\lambda_1}{\lambda_2} &= \frac{N+1}{N}\\ \frac{\lambda_1}{\lambda_2} &= 1 + \frac{1}{N}\\ \frac{1}{N} &= \frac{\lambda_1}{\lambda_2} - 1\\ N &= \frac{1}{\lambda_1/\lambda_2 - 1} \end{align}

At this point I can see that I can calculate $N$ if I can find a ratio $\lambda_1/\lambda_2$. Does enyone have any idea how to find it?


I have been trying to get the ratio using the invariant interval, but it didn't work out for me. I can't get the ratio because factor $2.25$ is giving me trouble... Because i can't factor it out I can't get rid of variable $U_1$:

\begin{align} \frac{\lambda_1}{\lambda_2} &= \frac{h p_2}{p_1 h} = \frac{p_2}{p_1} = \frac{\sqrt{ { {E_{k2}}^2 + 2E_{k2}E_0} }\cdot c }{\sqrt{ {E_{k1}}^2 + 2E_{k1}E_0 }\cdot c} = \sqrt{ \frac{ e^2{U_{2}}^2 + 2eU_{2}E_0}{ e^2{U_{1}}^2 + 2eU_{1}E_0}} =\\ &= \sqrt{\frac{2.25^2\, e^2 {U_{1}}^2 + 2.25\, 2eU_{1}E_0}{e^2{U_{1}}^2 + 2eU_{1}E_0}} \end{align}

share|cite|improve this question
up vote 1 down vote accepted

The energies involved should be small compared to the rest mass of the electron, so you can use Newtonian mechanics to get the momentum. We have $p_1=\sqrt {2mU_1},p_2=\sqrt {2mU_2}$, so $\frac {\lambda_1}{\lambda_2}=\frac {p_2}{p_1}=\sqrt{\frac{U_2}{U_1}}=1.5$

share|cite|improve this answer
I like this but is there a correct way to do it relativistically. – 71GA Jul 25 '13 at 18:41
I always have a problem, because i totaly forget that we can deal with a problem in a nonrelativistic aproximation. But if a relativistic derivation is possible it would be really nice. – 71GA Jul 25 '13 at 18:49
The problem poser chose 2.25 because it came out nicely. I don't think there is a selection in the relativistic case that works cleanly. You would probably be left with a numeric solution instead of algebraic. – Ross Millikan Jul 25 '13 at 19:06
It really works fabulous if nonrelativistic approach is used. Can you just tell me how, did you know that nonrelativistic should be used here? When did you first think of this? What was the trigger... ? – 71GA Jul 25 '13 at 21:24
Just that scattering like this is done at X-ray energies, while the electron rest mass is 0.5 MeV. You can check that the energies you find are well below that. – Ross Millikan Jul 25 '13 at 21:28

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.