In certain cases, light may be viewed as a stream of particles...

The magnitude of the initial momentum of the photon is $p = \dfrac{h}{\lambda}$.

Let the normal to the mirror be along the y-axis. The incident photon makes an angle of incidence $\theta$ with the normal.

The components of the initial momentum $\vec{p}_i$ are:

$p_{ix} = p \sin\theta$

$p_{iy} = -p \cos\theta$

After reflection, the angle of reflection is equal to the angle of incidence $\theta$. The components of the final momentum $\vec{p}_f$ are:

$p_{fx} = p \sin\theta$

$p_{fy} = p \cos\theta$

The change in momentum is given by $\Delta \vec{p} = \vec{p}_f - \vec{p}_i$.

$\Delta p_x = p \sin\theta - p \sin\theta = 0$

$\Delta p_y = p \cos\theta - (-p \cos\theta) = 2p \cos\theta$

The magnitude of the change in linear momentum is:

$|\Delta \vec{p}| = 2p \cos\theta = \dfrac{2h\cos\theta}{\lambda}$