Why do light rays obey the law of reflection?

Physics is a science subject trying to explain the phenomena of nature. Light rays when it strikes a surface, it will be partly reflected and partly refracted (depending on the nature of the object). If the object is an opaque object, only reflection occurs. The amount of light energy reflected depends on the surface (e.g. whether it is rough or smooth). When it reflects, it has certain kind of characteristics ( when it has been proved, these kind of characteristics is called law of reflection) . This is a phenomenon of nature - just like the sun will rise from the east, ice will melt when the temperature is greater than 0 degree celcius under normal atmospheric pressure, light rays has seven colours etc. Through experiment, we have proved that light rays obey the law of reflection. Up till now, by experiment, we still cannot find a counter example.

In science, when we see a phenomenon, we will make a hypothesis. Then, we will make experiments under different conditions to test our hypothesis. If under all possibilities human can think of, our hypothesis is still found to be true, it becomes a law. Law of reflection is derived in the above mentioned process.

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Light ray is one of the members of electromagetic wave. For all waves, they will have some similar phenomena, e.g. reflection, refraction, diffraction, interference, etc. It is just a definition that all waves obey the law of reflection, therefore there is no WHY to explain the DEFINITION.