When light hits a surface, it bounces off the surface. This causes a reflection. The light will reflect off that surface at the same angle it came in. For example, if a wave of light hits a surface at a 90-degree angle, it will bounce off at a 90-degree angle. When a reflective surface is flat and even, the light reflects in all direction perfectly, creating a smooth coherent, but inverse, image of the light. This process can be seen in simple household mirrors.
The speed of light is constant in a vacuum, but when it travels through objects such as glass, that speed is change. If light waves pass into an optically dense substance, the light will be bent and refracted outward. Unlike a reflection, which creates an inverse copy, refraction produces a distorted image. You can see the process whenever you look at a person or object that is standing in the water. The part above water appears normal, while the part below water looks strange and distorted.
Some surfaces, such as paper, are not smooth and even. Light does not bend as it passes through these surfaces nor do they reflect the light at a constant angle. Instead, the light is scattered by the surface of the material. Light moves from a state of high concentration at its source to a state of low concentration. This weakens the light and makes it much more difficult to see, especially over large distances.
Interference occurs when waves of light are combined. Light occurs in waves with crests and troughs. When the crests and troughs match up, they combine to form a stronger wave of light. When they are in opposite states it has a destructive effect, and the light waves cancel each other out. This process can be seen when light hits oil or soap film. The surface interferes with the light and combines it to emphasize different colors at different angles.