# The angle at which first order reflection will occur when X-ray of wavelength $1.54A^{\large\circ}$ are diffracted by the atoms of a crystal ; the interplanar distance is $4.04A^{\large\circ}$

$(a)\;22^\circ20'\qquad(b)\;10^\circ59'\qquad(c)\;33^\circ32'\qquad(d)\;15^\circ24'$

$\lambda = 2d\sin\theta$
$1.54 = 2\times4.04\sin\theta$
$\theta = \sin^{-1}(0.191)$
$= 10.98^{\large\circ}$
$=10^{\large\circ}59'$