Specular and Diffuse Reflection

Once a normal to the surface at the point of incidence is drawn, the angle of incidence can be determined. The light ray will reflect in such a manner that the angle of incidence is equal to the angle of reflection. This predictability concerning the reflection of light is applicable to the reflection of light off of horizontal surfaces, vertical surfaces, angled surfaces, and even curved surfaces. As long as the normal can be drawn at the point of incidence, the angle of incidence can be measured and the direction of the reflected ray can be determined. A series of incident rays and their corresponding reflected rays are depicted in figure 6.3. Each ray strikes a surface with a different orientation, yet each ray reflects in accordance with the law of reflection.

Fig.6.3. The law of reflection is always observed regardless of the orientation of the surface.  

The behavior of light is often studied by observing its reflection of flat mirrors. Mirrors are typically smooth surfaces, even at the microscopic levels. As such, they offer each individual ray of light the same surface orientation. But quite obviously, mirrors are not the only types of objects light reflects off. Most objects which reflect light are not smooth at the microscopic level. Your clothing, the walls of most rooms, most flooring, skin, and even paper are all rough when viewed at the microscopic level.

Imagine that you are looking at your reflection in a nice smooth piece of aluminum foil, fresh off the roll. You perceive a face, not a piece of metal. Perhaps you also see the bright reflection of a lamp over your shoulder behind you. Now imagine that the foil is just a little bit less smooth. The different parts of the image are now a little bit out of alignment with each other. Your brain can still recognize a face and a lamp, but it’s a little scrambled, like a Picasso painting. Now suppose you use a piece of aluminum foil that has been crumpled up and then flattened out again. The parts of the image are so scrambled that you cannot recognize an image, but your brain tells you you’re looking at a rough, silvery surface.

Mirrorlike reflection at a specific angle is known as specular reflection. It is reflection off smooth surfaces such as mirrors or a calm body of water.Random reflection off rough surfaces such as clothing, paper, and the asphalt roadwayin many directions is called diffuse reflection. Whether the surface is microscopically rough or smooth has an impact upon the reflection of a beam of light. Fig. 6.4 depicts two beams of light incident upon a rough and a smooth surface.

Fig. 6.4. Specular and diffuse reflection.

Fig. 6.5.

For each type of reflection, each individual ray follows the law of reflection. If the bundle of light rays is incident upon a smooth surface, the light rays reflect and remain concentrated in a bundle upon leaving the surface. But the roughness of the material means that each individual ray meets a surface which has a different orientation. The normal line at the point of incidence is different for different rays. When the individual rays reflect off the rough surface according to the law of reflection, they scatter in different directions. The result is that the rays of light are incident upon the surface in a concentrated bundle and are diffused upon reflection. Figure 6.5 depicts this principle. Five incident rays (A, B, C, D, E) approach a surface. The normal line at each point of incidence is labeled with an N. In each case, the law of reflection is followed, resulting in five reflected rays (A', B', C', D', E').

The differences among white, black, and the various shades of grey in between is a matter of what percentage of the light they absorb and what percentage they reflect. That’s why light-colored clothing is more comfortable in the summer, and light-colored upholstery in a car stays cooler than dark upholstery (from ‘Optics’).

Exercise 16. Look for the words in the text which have the following meanings.

1. ___________ __________ – reflection from a rough surface, in which a single ray of light is divided up into many weaker reflected rays going in many directions.

2. ___________ __________ – the angle between the incident ray and the normal to the surface.

3. ___________ _________ – a source of artificial light that works by releasing the thermal radiation from a body due to its temperature.

4. _______________ (n) – what happens when light hits matter and gives up some of its energy.

5. ___________ __________ – reflection from a smooth surface, in which the light ray leaves at the same angle at which it came in.

6. ___________ __________ – a ray of light which hits a surface.

7. ___________ __________ – the angle formed by a reflected ray and a normal to the surface at the point of reflection.

Exercise 17. Look through the text again and find the words with opposite meaning to the words below.

Natural, diffuse, smooth surface, horizontal, incident ray, horizontal surface, plane surface, white, dark-coloured, to crumple.

Exercise 18. Answer the questions to the text.

1. What is absorption?

Fig. 6.6.

2. How is the process of heating by absorption different from heating by thermal conduction?

3. What is a reflected ray?

4. What is the angle of incidence?

5. What is the angle of reflection?

6. Consider the diagram at the right (Fig. 6.6). Which one of the angles (A, B, C, or D) is the angle of incidence? Which one of the angles is the angle of reflection?

7. What does the law of reflection state?

8. What types of reflection do you know?

9. What is specular reflection?

10. What is diffuse reflection?

11. Why do we see the difference among white, black and shades of grey?

Exercise 19. Identify whether the following phenomenon are attributable to diffuse reflection (DR) or specular reflection (SR).

1. The image of a mountain can be clearly seen in the calm waters of a lake. _______

2. A lacquered tabletop produces a glare of the lamp bulb in the overhead light. _______

3. Water is sprayed onto a sheet of paper. A laser beam is directed towards the paper, reflects and produces a red dot on the ceiling. _______

4. Light from the overhead lights strikes your body and reflects towards all your classmate's eyes. ______

Exercise 20. Fill in the blanks with the words from the box. Summarize the information in three or four sentences.

diffuse, bounces off, reflection, called, enhanced, media, energy, mirror, propagation, transparent, light, coating, strikes, specular, glass, wave

____ is the change in direction of a ______ front at an interface between two dissimilar ______ so that the wave front returns into the medium from which it originated. Common examples include the reflection of ______, sound and water waves. Reflection of light may be ____ (that is, mirror-like) or _____ (that is, not retaining the image, only the ______) depending on the nature of the interface. A _____ provides the most common model for specular light reflection and consists of a _____ sheet in front of a metallic _____ where the reflection actually occurs. Reflection is _____ in metals by suppression of wave _____ beyond their skin depths. It is also possible for reflection to occur from the surface of ______ media, such as water or glass. When light ____ a rough or granular surface, it _____ in all directions due to the microscopic irregularities of the interface. Thus, an 'image' is not formed. This is _____ diffuse reflection.

Exercise 21. Complete the table with the appropriate information.

Reflection	Definition	Examples
Specular reflection
Diffuse reflection

Exercise 22. Read the text below and tell about applications of specular and diffuse reflection.

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