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| Introduction to Reflection and Refraction | Reflection, Part 1 | Reflection, Part 2 | Refraction, Part 1 | Refraction, Part 2 |

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.

In the reverse direction, converging light rays reaching a convex mirror will be reflected to be more nearly parallel to each other. If you look at a large image reflected in such a mirror, it will seem to be smaller to your eye, because of that convergence. 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 the portion of your body reflected to you appears in the convex portion, 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. Another application is the right side mirror on an automobile. The convex surface allows the driver to see a wider angle, for better coverage. However, as the warning on that mirror says, the result is that objects reflected in the mirror appear smaller than normal, and so seem to be farther away than they really are. This is a reverse application of a convex mirror, since it takes converging light rays and bundles them together.



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 a large concave mirror (larger than your own body), 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.

If you look at your reflection in a small concave mirror (for example, a mirror only 4 inches in diameter), the image you see will be distorted. Light rays from your face are not arriving in parallel, or even nearly parallel, at the mirror's surface, but are already converging. As a result the point at which they converge to a point is much closer to the mirror than its actual focus point. It also means that the convergence of these light rays is not complete — light from the edge of the mirror will converge at a different point than light from the center. As a result, as you move your face back toward the focus point, the image you see will actually become limited a distorted view of your own eye, which seems to fill the mirror.

As you step farther back from the mirror, your reflection gets larger again, but is inverted (upside down and backwards), because light from the bottom of the mirror is now above your eyes, while light from the top is below your eyes. Light from the left and right sides has also crossed over to produce this reversal.

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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