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 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 lit by light rays that travel from a light source to the object and then bounce off the object towards our eyes.


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The light rays that arrive at our eyes after reflecting off of an object behave predictably - consistent with the law of reflection. How we perceive that light is influenced by the characteristics of the object's surface.

One characteristic of a surface that influences what we see is the surface's roughness because there are two general types of reflection: specular and diffuse. The smoother the surface is, the more specular the reflection.

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 the point the light ray hts, a set of parallel incident ray*s encountering a smooth surface reflect so that the angles of reflection are all the same. As a result, the reflected ray*s will retain the same organization present in the incident rays.

Since the reflected rays retain the organization of the incident rays, reflection off of smooth surfaces preserves the organization of the reflected light, allowing us to see images in mirrors.

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 a variation of 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 funhouse mirror.

Specular reflection is beneficial. Without it, we would not have cameras or mirrors. However, diffuse reflection 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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