Properties of Images

Before we go much further, it's useful to define some terms.

Device

When an image is created, we will use the term device to refer to the object that diverted the rays before they reached the eye. For a penny underwater, the surface of the water is the "device" that creates the image. The devices we will primarily investigate are lenses (transparent objects which create images using refraction) and mirrors (which use reflection to create images). When we introduce mathematics, we will always measure distances from the device.

Magnification

Images often have a different size than the object they are created from; we call this magnification:
$$M={\hbox{size of image}\over\hbox{size of object}}$$

By "size" we mean any linear dimension: height, width, distance between the eyes of a face, etc. For some devices, magnification is hard to define precisely: a funhouse mirror may increase an image's height and decrease its width, for instance. For the simple devices we will consider, however, magnification is consistent in all dimensions. A plane mirror, for instance always creates an image with the same size as the object, so its magnification is $M=1$. (If objects in the mirror look smaller, it is due to perspective: they are farther away from you than the object is.)

Some devices will invert an image, so that it appears to be rotated by 180°. We will say that those have a negative magnification, such as in the figure.

Types of Image

There are two basic types of images. Plane mirrors and water surfaces create virtual images. The eye traces the diverted rays of light back to a point, but the rays of light did not come from that point. Our eyes see an image behind a flat mirror even though the mirror is opaque.

There are cases, however, when the rays of light are brought together so that they all meet at a single point again, as shown in the figure. When the eye traces those rays backward, they don't just appear to come from a single point, they really come from that point. This is called a real image. Real images have a couple properties which make them easy to identify. First, real images are inverted as compared to the object. If you look into the bowl of a spoon and see your upside-down face in it, you are looking at a real image. Also, if you place something (like a screen or a piece of paper) at the place where the rays come together, the image will be projected onto that object. A movie projector creates a real image of an (upside-down) LCD display onto the screen in the front of the room. Your eye projects a real image of the world onto the retina where your brain can analyze it (and turn it right-side up!)