Two electrons are moving with non-relativistic speeds perpendicular to each other. If corresponding de brogile wavelengths are ${\lambda }_1$ and ${\lambda }_2$, their de brogile wavelength in the frame of reference attached to their centre of mass is:

• A

  $\frac{1}{{\lambda }_{CM}}=\frac{1}{{\lambda }_1}+\frac{1}{{\lambda }_2}$
• Correct answer
  B

  ${\lambda }_{CM}=\frac{2{\lambda }_1{\lambda }_2}{\sqrt{{{\lambda }_1}^2+{{\lambda }_2}^2}}$
• C

  ${\lambda }_{CM}={\lambda }_1={\lambda }_2$
• D

  ${\lambda }_{CM}=\frac{{\lambda }_1+{\lambda }_2}{2}$

Hence, the correct answer is option: 'B'.