www.nortonkit.com 18 अक्तूबर 2013
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Direct Links to Other Optics Pages:
Basic Concepts: [What Is Light?] [Light as a Wave] [Light as a Particle] [The Characteristics of a Photon] [The Photoelectric Effect] [The Transverse Electromagnetic Wave (TEM)]
Reflection and Refraction: [Introduction] [Reflection, Part 1] [Reflection, Part 2] [Refraction, Part 1] [Refraction, Part 2]
Lenses: [Introduction] [The Convex Lens]
Fiber Optics: [Introduction] [Fiber Optics, Part 2] [Fiber Optics, Part 3] [Fiber Optics, Part 4] [Fiber Optics, Part 5] [Fiber Optics, Part 6]
Reflection, Part 2

Now that we have seen what reflection means and how light behaves as it reflects, let's take a look at a couple of special cases. Here, we look at reflecting surfaces that are smooth but curved. As you look at these examples, think of the distortions caused by the mirrors you might see in a fun house. Mirrors bent like this are entertaining, but can also be quite useful.

Reflection from a convex surface.

If the reflecting surface is convex as shown to the left, parallel rays of light striking the surface will diverge from each other evenly. This type of reflector might be used to help illuminate a wider area from a single source of light, or to reflect light onto a shadowed space and allow a wider spread of illumination.

If you look at your reflection in a Fun House mirror of this type, you will find that the farther away you are from the mirror, the larger your reflection appears, but your reflection is always right side up.

This type of mirror, with only a mild curvature, is used to allow a magnified view of a limited area. A typical application is for makeup application, since it allows closer and more accurate control of exactly where and how the makeup is applied.

When the surface is concave as shown to the right, light coming towards the mirror tends to converge towards a small area before continuing outward and away from the mirror. If you look at your own reflection in such a mirror, standing close you will see a normal but smaller reflection. As you back away, your reflection gets smaller yet, until you find a point where your reflection shrinks to a very small spot and nearly disappears! The point at this distance from the mirror is called the focus of the mirror, because the mirror tends to concentrate, or focus, all incoming light towards this point.

As you step farther back from the mirror, your reflection gets larger again, but is inverted (upside down), because light from the bottom of the mirror is now above your eyes, while light from the top is below your eyes.

If the mirror is shaped precisely as a solid, three-dimensional parabola, light arriving in parallel rays will converge to a very tight point, which is the focus of the parabola. This is the technique used in reflecting telescopes. It is also used in the other direction in spotlights. By putting a powerful light source at the focus, we can get a tight, parallel shaft of light to illuminate or highlight, for example, a single actor on a stage.

Reflection from a concave surface.

We'll halt the discussion of reflection for now, and go on to refraction. That topic is somewhat less intuitive than reflection, but is still reasonable once you identify in your own mind the essential factors that cause this phenomenon.

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