An age-old debate which has persisted among scientists is related to the question, "Is light a wave or a stream of particles?" Very noteworthy and distinguished physicists have taken up each side of the argument, providing a wealth of evidence for each side. The fact is that light exhibits behaviors which are characteristic of both waves and particles.
Light exhibits certain behaviors which are characteristic of any wave and would be difficult to explain with a purely particle-view. Light reflects in the same manner that any wave would reflect. Light refracts in the same manner that any wave would refract. Light diffracts in the same manner that any wave would diffract. Light undergoes interference in the same manner that any wave would interfere. And light exhibits the Doppler effect just as any wave would exhibit the Doppler effect. Light behaves in a way that is consistent with our conceptual and mathematical understanding of waves. Since light behaves like a wave, one would have good reason to believe that it might be a wave.
A wave doesn't just stop when it reaches the end of the medium. Rather, a wave will undergo certain behaviors when it encounters the end of the medium. Specifically, there will be some reflection off the boundary and some transmission into the new medium. The transmitted wave undergoes refraction (or bending) if it approaches the boundary at an angle. If the boundary is merely an obstacle implanted within the medium, and if the dimensions of the obstacle are smaller than the wavelength of the wave, then there will be very noticeable diffraction of the wave around the object.
The wave theory predicted that light waves could interfere with each other like sound waves (as noted around 1800 by Thomas Young), and that light could be polarized. Young showed by means of a diffraction experiment that light behaved as waves. He also proposed that different colors were caused by different wavelengths of...