We know that according to the law of reflection, the angle of reflection is always equal to the angle of incidence.  

If i is the angle of incidence and r is the angle of reflection then,
According to the rule of reflection ∠i = ∠r  

Here reflected ray is at an angle of ${90}^{°}$ to the incident ray.

So, ∠i + ∠r = ${90}^{°}$

$\angle \mathrm{i}=\frac{{90}^{°}}{2}$

Hence, ∠i = ∠r = ${45}^{°}$

The angle of incidence of a ray if the reflected ray is at an angle of ${90}^{°}$to the incident ray is ${45}^{°}$.