Optics and Telescopes Help

By — McGraw-Hill Professional
Updated on Sep 18, 2011

Basic Optics

Until a few hundred years ago, the only instrument available for astronomical observation was the human eye. This changed in the 1600s when several experimenters, including such notables as Galileo Galilei and Isaac Newton, combined lenses and mirrors to make distant objects look closer. Since then, optical telescopes have become larger and more sophisticated. So have the ways in which the light they gather is scrutinized.

You have learned that visible light always take the shortest path between two points and that it always travels at the same speed. These are the cornerstones of relativity theory and can be taken as axiomatic 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 through it, these principles of relativity do not apply.

Let’s focus our attention on what happens when light passes through a medium such as glass or is reflected by mirrors. If a ray of light passes from air into glass or from glass into air, the path of the ray is bent. Light rays change direction when they are reflected from mirrors. This has nothing to do with relativity. It happens all the time, everywhere you look. It even takes place within your own eyes.

Light Rays

What is a ray of light ? Definitions vary. Informally, 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 can be considered to be the path that an individual photon (light particle) follows through space, air, glass, water, or any other medium.

Light rays have properties of both particles and waves. 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. Yet there are certain things we can say about the way in which rays of light behave. These things are predictable, both qualitatively and quantitatively. When we know these facts about light, we can build high-quality instruments for observing the Cosmos at visible wavelengths.


Prehistoric people knew about reflection. It would not take an intelligent creature very long to figure out that the “phantom in the pond” actually was a reflection 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. 17-1. The angle of incidence and the angle of reflection are both measured relative to a normal line (also called an orthogonal or perpendicular ). In the figure, these angles are denoted q . They can range from as small as 0 degrees, where the light ray strikes at a right angle, to almost 90 degrees, a grazing angle.

Optics and Telescopes Basic Optics Reflection

Figure 17-1. 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 .

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 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.

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