Behavior of Light Help (page 2)
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.
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.
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.
Early humans doubtless noticed refraction as well as reflection; a clear pond looks shallower than it actually is because of this effect. Refraction is associated with the fact that different media transmit light at different speeds. This does not violate the fundamental principle of relativity theory. The speed of light is absolute in a vacuum, where it travels at about 299,792 km/s (186,282 mi/s), but light travels more slowly than this in other media.
In air, the difference in the speed of light is slight compared with its speed in a vacuum, although it can be significant enough to produce refractive effects at near-grazing angles between air masses having different densities. In water, glass, quartz, diamond, and other transparent media, light travels quite a lot more slowly than it does in a vacuum. The refractive index , also called the index of refraction , of a medium is the ratio of the speed of light in a vacuum to the speed of light in that medium. If c is the speed of light in a vacuum and c m is the speed of light in medium M , then the index of refraction for medium M , call it r m , can be calculated simply:
r m = c/c m
Always use the same units when expressing c and c m . According to this definition, the index of refraction of any transparent material is always greater than or equal to 1.
The greater the index of refraction for a transparent substance, the more light is bent when it passes the boundary between that substance and air. Different types of glass have different refractive indices. Quartz refracts more than glass, and diamond refracts more than quartz. The high refractive index of diamond is responsible for the multicolored shine of diamond stones.
Behavior of Light Practice Problem
A certain clear substance has an index of refraction of 1.50 for yellow light. What is the speed at which yellow light travels in this medium?
Use the preceding formula and “plug in” the refractive index and the speed of light in a vacuum. Let’s express the speeds in kilometers per second and round off c to 3.00 × 10 5 km/s. Then the speed of the yellow light in the clear substance c m can be found as follows:
1.50 = 3.00 × 10 5 / c m
1.50 c m = 3.00 × 10 5
c m = 3.00 × 10 5 /1.50 = 2.00 × 10 5
Practice problems of these concepts can be found at: Optics Practice test
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