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Diffuse reflection is the property of light to scatter when reflecting off a surface. Light travels in a straight line, and when it strikes an object, its expected primary angle of reflection is called a specular reflection. In addition, the light also reflects in all possible angles and directions. The degree or magnitude of this diffused reflection is dependent upon the characteristics of the reflecting substance and surface.
Some surfaces, such as a highly polished metal mirror, reflect light at nearly 100% specular efficiency. Other surfaces, such as those of some crystals or liquids, may most or all of it to pass through the surface and medium. Most objects of the world, including the above examples, reflect light both specularly and diffusely to varying degrees. Diffuse reflection is the primary property of light that allows human eyes to see an object.
The main determinant of reflection is the surface’s absorption of light. Polished surfaces such as white marble stone, or the randomly irregular fibers of paper, reflect diffuse light with nearly equal efficiency. Black objects tend to absorb more light. Some naturally occurring as well as human made things emit light and overwhelm any diffuse reflection from their surface to distinguish them as objects.
Light is absorbed by and reflects off surfaces at the nano scale. Visible light, from violet to red, has a wavelength of 380 to 780 nanometers (nm). Surfaces with a molecular structure sympathetic to a given wavelength will reflect it. Others will pass through it until the ray encounters a reflective surface beneath it. At this scale, all surfaces are irregular to some degree.
The color of an object is determined by its diffuse reflection. Specular reflection, such as the sparkling highlights of an automobile’s sculptured form, is close to 100% of the lighting source. Oblique light rays from the rest of its body paint are partially absorbed, and only a narrower wavelength such as cherry red is reflected in all directions, including toward an admirer’s eyes.
Three types of materials do not have good diffuse reflectivity. They include molecularly compact substances, such as metals, which do not allow light to pass. Substances such as gases and glass with loose molecular structures which allow almost all light to pass are also in this category.
Additionally, very simple or very complex crystalline structures absorb light and refract it through its surface rather than reflecting off it. Diamonds, salt, and the hard shell or scales of some insects fall into this category. Diffuse reflection has also been used as a term of photographic technique to bounce a light source onto a subject for more even, less directional, illumination.