How Do You Calculate the deBroglie Wavelength of a Relativistic Electron?

[Chocobo7]

Quantum Phys Questions PLEASE HELP THANK YOU!

DeBroglie postulated that the relationship lambda=h/p is valid for relativistic particles. The Planck's constant is 6.63x10-34 Js. What is the deBroglie wavelength for a relativisitc electron whose kinetic energy is 6.77 MeV? Answer in units of m.
What I did was v=c sqrt (1-(Er/K+Er)^2), solved for v, plugged that into relativistic p=mv/sqrt(1-v^2/c^2), then plugged that into lambda=h/p to solve for lambda, but I got it wrong. I also tried using KE=1/2mv^2.

One more ques: A beam of neutrons with a kinetic energy of .00061 eV falls on a slit of width .0001m. What will be the angular spread of the beam after it passes through the slit? Answer in units of radian.
I have no idea how to do this...the only thing can possibly think of is getting wavelength from the kinetic energy and somehow using the single slit equation to solve for theta? But then I don't have distance from source to slit screen or y (width of bands?)

ANY HELP WOULD BE GREATLY APPRECIATED! I NEED THESE ASAP! THANKS IN ADVANCE!

 

[Office_Shredder]

I would post this in the advanced physics forum. You'll get more people who know exactly how to answer all of this there (I could make an educated guess on the first one, but I don't think that's what you're looking for :) )

 

[Chocobo7]

nevermind i got it. -_-

 

[thiotimoline]

DeBroglie postulated that the relationship lambda=h/p is valid for relativistic particles. The Planck's constant is 6.63x10-34 Js. What is the deBroglie wavelength for a relativisitc electron whose kinetic energy is 6.77 MeV? Answer in units of m.
What I did was v=c sqrt (1-(Er/K+Er)^2), solved for v, plugged that into relativistic p=mv/sqrt(1-v^2/c^2), then plugged that into lambda=h/p to solve for lambda, but I got it wrong. I also tried using KE=1/2mv^2.
QUOTE]

Using KE=1/2mv^2, get the v. Then put v into L=h/mv.