JEE Main 2008 — Dual Nature of Matter Question with Solution

Paragraph: Wave property of electrons implies that they will show diffraction effects. Davisson and Germer demonstrated this by diffracting electrons from crystals. The law governing the diffraction from a crystal is obtained by requiring that electron waves reflected from the planes of atoms in a crystal interfere constructively (see in figure).
Question: If a strong diffraction peak is observed when electrons are incident at an angle 'i' from the normal to the crystal planes with distance ' $d$ ' between them (see figure), de Broglie wavelength ${\lambda }_{\mathrm{dB}}$ of electrons can be calculated by the relationship ( $n$ is an integer)