Specular vs Diffuse Reflection

Our ability to see and make sense of the world with our eyes depends on the reflective properties of light. Without reflection, we would only be able to see luminous objects like the sun, light bulbs and computer screens.

The light rays that allow us to see non-luminous objects such as our hands, the floor and the people around us, are rays that have traveled from a light source and then have reflected off of an object towards our eyes. There are two types of reflection: specular and diffuse. Specular reflection* occurs on smooth surfaces. Diffuse reflection* occurs on rough surfaces. The rougher the surface, the more diffuse the reflection.

Number of Rays

We can perceive the direction of and distance to the things we see because reflected light behaves predictably. We trust our eyes because light always reflects in accordance with the law of reflection. This means that the path of a reflected light ray can be determined given knowledge of the path of the incident ray* and the orientation of the surface.

On a very smooth surface, lines normal to neighboring points along that surface are parallel to each other. Since the angle of reflection* depends on the orientation of the normal, a set of parallel incident rays encountering a smooth surface will reflect so that the angles of reflection are all the same. During specular reflection, the reflected ray*s will be parallel if the incident rays are. The organization of the incident rays is retained by the reflected rays and this preservation of organization allows us to see images in mirrors.

In contrast to specular reflection, diffuse reflection occurs on rough surfaces. In diffuse reflection, all of the reflected rays still behave in accordance with the law of reflection, but the roughness of the surface results in variation in the normals along the surface. With this variation, normals at neighboring points are no longer parallel to each other. Since the angle of incidence depends on the normal line at the exact point a ray hits, the incident angles for a set of parallel rays will not be the same and each reflected ray will have a different angle of reflection. In other words, the rays scatter.

The orientation of the normal lines at neighboring points along a surface differentiates specular from diffuse reflection. If the normal lines are parallel, reflection will be specular. If they are not parallel, reflection will be diffuse. Note, this is roughness at the microscopic level. Diffuse reflection occurs on surfaces that are smooth to the touch, such as paper. Specular reflection occurs on curved surfaces such as a fun house mirror.

Specular reflection is very useful. Without it, we would not have cameras or mirrors. However, diffuse reflection that is central to our ability to see the world. Aside from the limited number of luminous objects, such as light bulbs and the sun, everything we see around us is visible because of diffuse reflection.

Test your understanding of the concepts covered with the specular vs diffuse reflection problem set.

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