In 1670 Christiaan Huygens gave compelling evidence that light is a wave. He developed his Wavelet principle.

This theory helped to explain the polarization of light, diffraction, and interference patterns. Then about 20 years later Sir Isaac Newton supported the "corpuscular theory of light." This said light is particle and behaves as such. This theory explained the reflection and refraction of light in objects like glass lenses and curved mirrors. (Refraction means bending.) For nearly 200 years the scientific community was confused as to which theory to believe to be true. They argued back and forth.

In 1905 Einstein settled the debate by saying both theories are true. If you measure the particle property of light, then the wave property is destroyed. If you measure the wave property of light then the particle property is destroyed. Light has a duality where both sets properties are true. Light exhibits wave like properties when it's moving through a material and particle like properties when it hits something.

Reflection

A specular reflection occurs off of a smooth surface. A diffuse reflection occurs of of "rough" surfaces.

One light ray is incident on the surface and one leaves the surface at the same angle. This is called the law of reflection. It only applies to smooth surfaces.

When describing light rays, all angles are referenced from the normal line to the surface at the point of incidence to the light ray.

This an image of a reflection in a large, calm, lake. Notice anything peculiar about the image above? I posted it upside down. Click here to see the image right-side up.