Dispersion relation — UPSC Physics Optional CSE 2026 Paper II

The energy near the valence band edge of a crystal is given by E = -10^{-39} k^2\text{ J}\cdot\text{m}^2. An electron with wave vector 10^{10}\hat{k}_x\text{ m}^{-1} is removed from an orbital in the completely filled valence band. Determine its effective mass and momentum.