14 Producing Images with Geometric Optics
Terminology of Images and Optical Elements
Instructor’s Note
By the end of this section you should:
- Be able to define several terms associated with images and optics.
- Know and apply the sign conventions associated with objects and images in optics.
An is any lens or mirror. A few examples include the lens in your eye that you’re using to read this or the aforementioned shaving or makeup mirror that many people have to help them get ready in the morning, it makes your face look a little bigger than it is. The lens or the mirror are examples of optical elements. Anything that’s a lens or a mirror is an optical element.
Optical elements or combinations of them can be used to make . An image is the apparent reproduction of an object formed by an optical element, or collection of them, through the reflection and/or refraction of light. In these two examples, the person is our object and the images are the image on the back of your retina formed by your eyeball, or the image of your reflection in the mirror.
Images can either be , with the same orientation as the object, or, upside down with respect to the object. In the makeup mirror your face is right-side up, and so the image is erect. The images on your retina are actually upside down your brain corrects them to put them right side up, and this is an example of an inverted image.
Before we move on to talk about image and object distances, we need to introduce two more terms: the and the .
The is an imaginary line that passes through the optical element in a way that’s perpendicular to it. Below, we have a converging lens at the top, with a diverging lens below that, with a converging mirror below that, and a diverging mirror at the bottom. This dashed line that always meets the lens or mirror perpendicular is what we call the optical axis.
Looking at a few examples, we can see that the optical axis meets the lens perpendicular here in the middle, same for the diverging lens. Moving on to the mirrors we see the optical axis meets the two mirrors in a way that’s perpendicular. The point where the optical axis meets the optical element is called the vertex.
The reason we needed these terms is because the image distance and object distance are measured along the optical axis from the vertex and these distances have signs that can be positive or negative and the sides are relative to the path of the light if the object is on the same side as the incoming light, then the object distance will be positive, otherwise the object distance is negative. If the image is on the same side as the outgoing light, Then the image distance is positive otherwise the image distance is negative. Note that for our lens the incoming and outgoing sides are different, light passes through a lens, so the light comes in one side and goes out the other. For a mirror on the other hand, the incoming and outgoing sides are the same, light bounces off of a mirror. These may seem like a r