The velocity of light changes all the time. If the velocity of light were constant, eveyrtything would be perfectly transparent and you couldn't see anything. So let me list down when, where and how the velocity of light changes.

1. Reflection.

Let us choose a convinient orthonormal coordinate system where the reflector is sitting in the $xy$-plane and the $z$-direction is normal (orthogonal, perpendicular) to the reflector. Then, the $y$-direction, let us say, is redundant, so discard it. For convenience, also let the point where the the light ray hits the reflector be the origin of the coordinate system. Now, clearly, then, the $x$ component of the light ray's velocity is negated while the $z$ component remains the same.

2. Refraction

The speed and direction are both changed. Let us say the light ray is hitting the refracvtive medium from  a vacuum. Then, it will change its speed as $v=r^{-1}c_0$ and its angle will also be affected as $\theta_f=\arcsin\left(r\sin\theta_1\right)$.

3. Deflection

Here, light appears to bend due to gravity as per the Geodesic Equation, which can be shown using the Euler-Lagrange Equations.
$$\frac{\mbox{d}^2x_\lambda}{\mbox{d}\tau^2}=-\Gamma_{\mu\nu}^\lambda\frac{\mbox{d}x^\mu}{\mbox{d}\tau}\frac{\mbox{d}x^\nu}{\mbox{d}\tau}$$
Of course, switching your coordinate system back to  a curved one (using the inverse of the Christoffel symbols $\mathbf{\Gamma}$), the light is still following its geodesic in curved spacetime.

Luckily, photons are uncharged (electromagnetically) and thus are not deflected by electromagnetism.

For these reasons, light almost NEVER goes at a constant direction (in a straight line) or at a constant speed. Since there is gravity, opaque objects, non-vacuum media, etc. Also, in case you are talking about a constant being applied to light's direction, then there' is one more.:

4. Light is emmitted in different directions, and if it were only one, then that would single out a prefered direction,.