Question

The equation $E=pc$ is valid for

• A. Electron as well as a photon.
• B. For an electron but not for a photon.
• C. For a photon but not for an electron.
• D. Neither for an electron nor for a photon.

Answer 1

The correct option is C For a photon but not for an electron.

The energy of a photon is given by,

$E=h\nu =\frac{hc}{\lambda }......\left(1\right)$

de-Broglie wavelength is given by,

$\lambda =\frac{h}{p}\Rightarrow p=\frac{h}{\lambda }$

Putting this in (1) we get, the energy of a photon as $E=pc$

The equation $E=pc$ is valid only for a particle with zero rest mass. The rest mass of a photon is zero, but the rest mass of an electron is $9.1\times {10}^{-31}\text{kg}$.

Hence, the equation $E=pc$ is valid for a photon but not for an electron.

<!--td {border: 1px solid #ccc;}br {mso-data-placement:same-cell;}--> Hence, $\left(C\right)$ is the correct answer.

Why this question? Caution: Be careful while applying the formula $E=pc$, as it is valid for photons only.