31.0 - Introduction

“Image is everything,” one well-known advertisement blared for years. (An ad from

another company then trumpeted the contradictory message that “image is nothing.”

Go figure!)

For physicists, image is as much a matter of mirrors and lenses as it is of

appearance and athleticism. By arranging mirrors and lenses, they can magnify or

shrink images and place them where they are needed. The design of a wide range

of devices, from reading glasses and cameras to telescopes, depends on a

thorough understanding of how to manipulate images created with light.

This chapter begins with a discussion of light rays and mirrors, two essential

elements of your understanding of images. The simulation to the right features a

concave mirror. The mirror creates images of objects. In this case, the object is a

penguin. You can move the penguin left and right, and observe how its positioning

changes the location and size of the image created by the mirror. The image is

shown as a faded out version of the object itself. At some locations of the object, the

image will be off the screen, but it will reappear when you drag the object to another

location.

As you move the object back and forth, consider the following questions. Where can you place the object so that the image and object are on

the same side of the mirror? On opposite sides? Where can you place the object so that the image is smaller than the object? Larger? The

variety of images produced by different types of mirrors is a major topic in this chapter.

If you press the SHOW RAYS button in the simulation, you will see three light rays that emanate from the penguin, reflect off the mirror, and

converge to define the top of the image. These rays are used extensively in the study of mirrors and lenses, and this is your chance to begin to

experience their properties. (The rays do not always converge perfectly; this is a realistic depiction of the way curved mirrors work.) You can

turn off the rays by pressing the HIDE RAYS button.

31.1 - Light and reflection

Reflection: Light “bouncing

back” from a surface.

When you look at yourself in a mirror, you are seeing

a reflection of yourself. When you look at the Moon at

night, you are seeing sunlight reflecting off that distant

body.

Not all the light that reaches a surface reflects. In fact,

you see an object like a tree as having different colors

because its varied parts reflect some wavelengths of

light and absorb others. Light can pass through a

material, as it does with a glass window. It can also be absorbed by a material, as

evidenced by how a black rock warms up during a sunny day. All this can happen

simultaneously: Light will reflect off the surface of a lake (which is why you see the

lake), penetrate the water (otherwise, it would be completely dark below the surface),

and be absorbed by the water, warming it.

To understand reflection, it is often useful to treat light as a stream of particles that

move in a straight line and change direction only when they encounter a surface. Each

light “particle” acts like a ball bouncing off of a surface, and like a ball, it reflects off the

surface at a rebound angle equal to its incoming angle. You see yourself in a mirror

because the light bounces back to your eyes from the mirror.

The term “reflection” likely conjures up images of light and perhaps mirrors. Studying

mirrors is a good way to learn about reflection because they are designed to reflect light

in a way that creates a clear visual image. However, it is worth noting that reflection

does not apply only to light. Some creatures use the reflection of sound (echoes) to help

them perceive their surroundings and stalk their prey. For example, bats, seals and

dolphins emit high frequency sound and then listen for the reflected waves. By analyzing these reflections, they can “see” with great precision.

Radar, used to track airplanes, is based on the reflection of radio waves. A sophisticated understanding of reflection can be used to design

“stealth” aircraft that are difficult to detect with radar. Stealth aircraft register on radar screens as being about as large as a BB, in part because

of their ability to reflect incoming waves in “random” directions.

To form a mirror image, light bounces back from the mirror's surface.

Reflection

Light bouncing back from a surface

(^574) Copyright 2007 Kinetic Books Co. Chapter 31