Question

why does light travel in same path?

Answer 1

We know that light is an Electromagnetic wave which consists of two components electric and magnetic fields that are oscillating at right angle with each other. These two components actually push each other at their point of intersection and the wave propagates in a direction perpendicular to both the direction of electric and magnetic component. So unless the E.M. wave is diffracted or refracted it travels in a straight line. The E.M. wave travels in a direction which is at right angles to the oscillation direction of both fields.
Therefore light always travels in the straight path.