If light travelling in a straight line bends by a small but fixed...

When light undergoes reflection, its path changes direction. For a given angle of incidence $i$, the angle of deviation is $\delta = 180^{{}^{{}^{\circ}}} - 2i$. By choosing an appropriate angle of incidence (such as grazing incidence where $i \to 90^{{}^{{}^{\circ}}}$), the deviation can be a small, fixed angle.

When light undergoes refraction through a thin prism, it deviates by a small, fixed angle given by $\delta = (\mu - 1)A$, where $\mu$ is the refractive index and $A$ is the angle of the prism.

Dispersion involves different wavelengths of light bending by different angles, meaning the angle of deviation is not fixed for a composite beam of light.

Diffraction involves the spreading of light around obstacles into a continuous range of angles to form a pattern, rather than bending a single ray by one fixed angle.

Therefore, the bending of light by a small but fixed angle can be an instance of either reflection or refraction.