Specular and Diffuse Reflection

Most things that we encounter every day do not themselves emit visible light but reflect incident natural sunlight and artificial light. A tomato appears red because it has a relatively smooth surface that reflects red light and absorbs other non-red wavelengths of light. Reflection of light can be categorized into two types as specular reflection and diffuse reflection. Reflection of light from smooth surfaces such as mirrors or a calm body of water leads to specular reflection while reflection off of rough surfaces such as clothing, paper, leads to diffuse reflection. The roughness and smoothness of the surface have a tremendous impact on the reflection of a beam of light.

Why do rough surfaces scatter a beam of light?

A beam of light is a bundle of individual light rays that are parallel to each other. Each individual ray of light follows the law of reflection. However, on a rough surface, individual light rays meet the surface that has a different orientation. In other words, the normal line at the point of incidence is different for different rays. Subsequently, when the individual rays reflect off the rough surface according to the law of reflection, they scatter in different directions.

Visualizing the difference between specular and diffuse reflection

To visualize the difference consider two different surfaces: a smooth mirror and a rough reddish surface. The mirror reflects all the components of white light equally and the reflected light follows the same angle as the incident light. The rough surface does not reflect all wavelengths because it absorbs blue and green components and reflects red light. The light reflected is scattered in all directions.

Applications of Specular and Diffuse Reflection

There are several interesting applications of diffuse reflection and specular reflection. However, here we shall discuss two of the most noteworthy applications.

  • Most of us are aware of the fact that driving at night on a wet road is difficult because of the glare caused due to the oncoming headlights. The glare is the result of the specular reflection of the beam of the light of the oncoming vehicle. Normally, the rough surfaces of the road cause diffuse reflection, but if the surface is wet, water fills up the crevices and smoothen the surface. The rays of light hit this surface and undergo specular reflection.
  • The second application pertains to the field of photography. Most of us have seen a photograph of a beautiful nature scene captured by a photographer with a calm body of water in the foreground. We can witness the specular reflection of light from the subject of the photograph if the water is calm. Light from the subject can reach the camera lens directly or it can take a longer path in which it reflects off the water before traveling to the lens. Since the light reflecting off the water undergoes specular reflection, the incident rays remain concentrated (instead of diffusing). The light is thus able to travel together to the lens of the camera and produce an image (an exact replica) of the subject which is strong enough to perceive in the photograph

Frequently Asked Questions on Specular and Diffuse Reflection

Is it easier to read from glossy magazine pages or the usual microscopically rough pages? State your reason.

It is easier to read from a rough page than it is to read from a glossy page because glossy pages result in specular reflection causing glare. The reader typically sees the image of the lightbulb that illuminates the page. Most magazines that use glossy pages are usually the type that people spend viewing pictures on rather than the type used for reading facts and news.

If the parallel incident rays that undergo diffuse reflection follow the laws of reflection, then why do they scatter in different directions after reflections?

Each individual ray strikes the surface at different points with different orientation. Since the normal is different for each ray of light, the direction of the reflected ray will also be different.

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