Reflection

Polished metal surfaces reflect light much like the silver layer on the 66 back side of a glass mirror. A beam of light incident on the metal sur

Normal

Reflected rav

Reflected rav

Fig. 6.1. Light ray reflected from a metal surface with angle of incidence i equal to the angle of reflectance i'. The dashed line (normal) is perpendicular to the sample surface.

face is reflected. Reflection involves two rays—an incoming, or incident, ray and an outgoing, or reflected, ray. In Figure 6.1 we use a single line to illustrate a light ray reflected from a surface. The law of reflection requires that the two rays be at identical angles but on opposite sides of the normal, which is an imaginary line (dashed in Figure 6.1) at right angles to the mirror surface at the point where the rays meet. We show in Figure 6.1 that the angles of incidence i and reflection i' are equal by joining the two angles with an equal sign. All reflected light obeys the

Incident Reflected Incident /

Incident Reflected Incident /

Fig. 6.2a,b. Light reflection from (a) smooth surface (specular reflection) and (b) rough surface (diffuse reflection). In both cases the angle of incidence equals the angle of reflection at the point that the light ray strikes the surface.
Fig. 6.3a, b. Cross sections of paintings with (a) varnish-covered surface and (b) rough paint surface without a varnish overlayer. The rough surface (diffuse reflection) will appear matte.

relationship that the angle of incidence equals the angle of reflection. Just as images are reflected from the surface of a mirror, light reflected from a smooth water surface also produces a clear image. We call the reflection from a smooth, mirrorlike surface specular (as shown in Figure 6.2a). When the surface of water is wind-blown and irregular, the rays of light are reflected in many directions. The law of reflection is obeyed, but the incident rays strike different regions that are inclined at different angles to each other (Figure 6.2b). Consequently, the outgoing rays are reflected at many different angles, and the image is disrupted. Reflection from such a rough surface is called diffuse reflection.

The surface of paint layers reflects light in much the same way as water. The smooth varnished surface of the painting shown in cross section in Figure 6.3a causes specular reflection and appears shiny, while the rough surface of the painting in Figure 6.3b has diffuse reflective properties and appears matte, or dull.

Light is reflected when it is incident on a surface or interface between two different materials such as the surface between air and water, or air and paint, or varnish and paint. The reflection of light from varnish and paint layer interfaces is sketched in Figure 6.4. The ray of light incident on the layer of varnish is partially reflected and partially transmitted through the transparent material. At the interface with the paint layer, the transmitted incident ray is reflected and transmitted through the varnish to the varnish/air interface. This ray is then partially reflected internally and partially transmitted into the air. Each time a ray of light strikes a boundary between two materials, such as air/varnish or varnish/paint, some of the light is reflected. The laws of reflection are obeyed at all interfaces. The amount of reflected light at the interface depends on the differences in refraction between the two adjoining materials.

Fig. 6.4. A ray of light incident on a transparent varnish layer is both reflected from the air/varnish surface and transmitted through the layer, where it is reflected at the varnish/paint interface. The outgoing ray at the varnish/air surface is both reflected and transmitted.

Fig. 6.5. Light in air incident on a glass surface where it is partly reflected at the interface and partly transmitted into the glass. The direction of the transmitted ray is changed at the air/glass surface. The angle of refraction r is less than the angle of incidence i.

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Freehand Sketching An Introduction

Freehand Sketching An Introduction

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