(a) Calculate the de Broglie wavelength of an electron accelerated through 150 V. (b) Calculate it for a 0.05 kg tennis ball moving at 20 m/s. Comment. Solution: (a) λ = 1.227/√V nm = 1.227/√150 = 1.227/12.25 ≈ 0.100 nm. This is the order of atomic spacings — electrons at this energy diffract from crystals. (b) p = mv = 0.05 × 20 = 1.0 kg·m/s. λ = h/p = 6.63×10⁻³⁴/1.0 = 6.63×10⁻³⁴ m. This is ~10²⁴ times smaller than an atom — no slit or crystal could ever reveal it. Matter waves matter only for microscopic particles.