In non-inertial frame of wedge 

Using pseudo force,
Free body diagram is given as

Normal reaction is balanced by $mg\cos \left(\theta \right)$

Where m is mass of the body, 

g is the acceleration due to gravity

a is the acceleration produced by external force..
$ma\cos \theta =mg\sin \theta$

$a=\frac{g\sin \left(\theta \right)}{\cos \left(\theta \right)}$

$\mathrm{a}=g\tan \theta$