Why does light travels in a straight line?

Answer 1

Answer
Seemingly innocent questions like this gave rise to Quantum Mechanics.
It turns out that light explores ALL paths between the source and the observer. And they all cancel out except one....you guessed it.
Except that there is no such thing as a 'straight line'. The light is a wave in the first place, and since there is no such thing as a 'constant refractive index' in reality, light only travels in a straight line in 'conceptual' terms, not in reality.

Answer 2

That's a result of the Huygens principle. At ANY point reached by a light wave, a new wave goes out IN ALL DIRECTIONS OF SPACE. These waves will then interfere with one another; the interference can be constructive of destructive. The net result is that (under many common situations), the wave (including light wave) travels in a straight line - i.e., that's where there is the most constructive interference.The details of how all this is calculated, of course, are quite complicated.

Answer 3

Light travels in a straight line because it follows the path of least resistance, known as the principle of least time. When light moves from one medium to another, it changes direction due to the different refractive indices of the two media.