The de Broglie wavelength of a particle is given by the equation λ = h/p, where h is Planck's constant and p is the momentum of the particle. In non-relativistic calculations, the momentum of a particle is given by p = mv, where m is the mass of the particle and v is its velocity.
To calculate the percentage error in the de Broglie wavelength, we need to compare the non-relativistic calculation with the relativistic calculation, which takes into account the effects of special relativity at high speeds.
At low speeds, the non-relativistic calculation is accurate and the percentage error is negligible. However, as the speed of the particle approaches the speed of light, the relativistic effects become significant and the non-relativistic calculation starts to deviate from the actual value.
In the case of a 100-keV electron, the speed is relatively low and the percentage error in the de Broglie wavelength would be very small. It is only at much higher energies, close to the speed of light, that the relativistic effects become significant and the non-relativistic calculation would result in a larger percentage error.
Therefore, while the non-relativistic calculation of the de Broglie wavelength may not be completely accurate, it is still a useful approximation for particles with low energies. For higher energy particles, a relativistic calculation would be more accurate. Overall, the accuracy of the de Broglie wavelength calculation depends on the energy and speed of the particle, and it is important to take into account the effects of special relativity at high energies.