Even though all waves conform to these rules, we will constrict our study of reflection and refraction to light waves, with a few comments about sound along the way.

We consider waves to  move in straight lines. Even if the wave is circular in shape any small portion of it travels in a straight direction.  Light is really made up of photons (small bundles of waves)  moving along at the speed of light, but from any one spot those photons will move in a straight line away from the source.  A light ray, which is simply an arrow drawn on paper, is used to indicate its path.  In the old science fiction movies they talked of “ray guns” which would have to shoot paper arrows to be accurately named; not too exciting.

The law of reflection states that the angle between the in coming ray of light (incident ray) and the Normal (remember, perpendicular to a surface) is equal to the angle between the reflected ray and the normal.  The Normal is used since the surface may be curved which would interfere with an accurate angle measurement.

The blue line represents the reflective barrier (like a mirror), the red line represents the Normal, and the two black lines represent the incident and reflected light rays.

Specular reflection occurs when the reflective surface is a smooth surface, like a mirror, or polished metal.  All the reflected rays of light are uniform and predictable.

Specular reflection maintains the spatial orientation of the source which emitted the light.  This means that all of the light retains its relative position and you can determine the identity of the source.  If you can determine the identity of the source in the reflection, it is called an image.

Pages of a book often have glossy surfaces which provides a high amount of specular reflection.  As a result you get bright spots on the page which is the image of the light source.  When a lake or pond is smooth due to a lack of any wind, then the surface acts like a mirror, reflecting the background.  A great source of subjects for photographers.

When light hits an uneven surface, like the rough surface of a concrete road, the reflections are scattered all over. The law of reflection still holds true, but because of the rough surfaces the normal lines are not all parallel so the reflected rays are scattered about.  This is called diffuse reflection.

Notice how the incident light (green) strikes the surface in parallel directions, but the reflected rays(violet) reflect in many directions.  At each point on the surface the law of reflection is followed.

Since the light is scattered in diffuse reflection no image is possible.  A matte finish on a page makes it rough enough to scatter the light so you don’t get the glare. On the lake surface, if a slight wind whips up, then the surface is no longer smooth and the reflected rays are scattered every which way and the background image is no longer visible.

Just about every object we see is because of diffuse reflection.  Since there are very few primary sources of light (the sun, candles, lightbulbs, etc.) everything else is detected by reflected light.  Diffuse reflection allows the light striking an object to be scattered in all directions and be detected from any angle.

Think about this once. How can your headlights help you to see at night if the law of reflection states that angle i = angle r. All the light from your headlights should reflect away from the car and up into space and not come back to your eyes. You see the road because light is reflected back to your eyes. Light is reflected back because of diffuse reflection. The road's surface is rough and some of the light is bounced back due to the law of reflection. On a dark rainy night, when a film of water covers the road and makes the road's surface mirror like, then more light reflects away and less comes back to your eyes and the night seems extra dark and scary. I just added the scary part, it has nothing to do with Physics.

Specular reflection creates images.  Images are apparent sources of light for our eyes.  The light coming to our eyes from the location of an image carries all of the information as if the light were coming from the object itself.  There are two types of images and the distinction between them is very important.