Calculating a table of deBroglie wavelengths for various electron energies
First, simplify: factor out \$C^4\$ out of square root:

\$\lambda(E_k) = h\left/C\sqrt{(\frac{E_k}{C^2}+m_e)^2-m_e^2}\right.\$

Simplify even more: factor out \$m_e^2\$:

\$\lambda(E_k) = h\left/C m_e\sqrt{(\frac{E_k}{(C m_e)^2}+1)^2-1}\right.\$

Do not put everything in lambda:

# These are all constants; no need to recompute them.
# Consider putting them into a namespace.
# I didn't think too hard to figure appropriate names; my physics is too rusty

CM = spc.c * spc.me
e1 = spc.ev/(CM * CM)
scale = spc.h / CM

# This is an actual computation.
# Can be converted to lambda, but I strongly advise against.

def deBroglieWaveLength(n):
    arg = e1 * n + 1.0
    return scale / np.sqrt(arg * arg - 1)