Behavior of Light Help

Introduction

Until a few hundred years ago, the only instrument available for observation of visible phenomena was the human eye. This changed as experimenters developed telescopes, microscopes, and other devices.

Behavior of Light

Visible light always take the shortest path between two points, and it always travels at the same speed. These two rules hold as long as the light stays in a vacuum. However, if the medium through which light passes is significantly different from a vacuum, and especially if the medium changes as the light ray travels, these axioms do not apply. If a ray of light passes from air into glass or from glass into air, for example, the path of the ray is bent. A light ray also changes direction when reflected from a mirror.

Light Rays

A thin shaft of light, such as that which passes from the Sun through a pinhole in a piece of cardboard, can be called a ray or beam of light. In a more technical sense, a ray is the path that an individual photon (light particle) follows through space, air, glass, water, or other medium.

Visible light has properties both wavelike and particle-like. This duality has long been a topic of interest among physicists. In some situations, the particle model or corpuscular model explains light behavior very well, and the wave model falls short. In other scenarios, the opposite is true. No one has actually seen a ray of light; all we can see are the effects produced when a ray of light strikes something. However, there are certain things we can say about the way in which rays of light behave. These things are predictable, both qualitatively and quantitatively.

Reflection

Prehistoric people surely knew about reflection . It would not take an intelligent creature very long to figure out that the “phantom in the pond” was actually a visual image of himself or herself. Any smooth, shiny surface reflects some of the light that strikes it. If the surface is perfectly flat, perfectly shiny, and reflects all the light that strikes it, then any ray that encounters the surface is reflected away at the same angle at which it hits. You have heard the expression, “The angle of incidence equals the angle of reflection .” This principle, known as the law of reflection , is illustrated in Fig. 19-1.

Fig. 19-1 . When a ray of light is reflected from a shiny, flat surface, the angle of incidence is equal to the angle of reflection. Here both angles are denoted q .

In optics, the angle of incidence and the angle of reflection are both measured relative to a normal line (also called an orthogonal or perpendicular ). In Fig. 19-1, these angles are denoted q and can range from 0°, where the light ray strikes at a right angle with respect to the surface, to almost 90°, a grazing angle relative to the surface.

If the reflective surface is not perfectly flat, then the law of reflection still applies for each ray of light striking the surface at a specific point. In such a case, the reflection is considered with respect to a flat plane passing through the point and tangent to the surface at that point. When many parallel rays of light strike a curved or irregular reflective surface at many different points, each ray obeys the law of reflection, but the reflected rays do not all emerge parallel. In some cases they converge; in other cases they diverge. In still other cases the rays are scattered haphazardly.