Any high-optical index lens that is "thicker in the center" than on the edges is generally described as a convex lens and will function as a converging lens when it is operating in air.
The point where all rays which enter the lens parallel to its axis are brought to a focus is called the principal focus.
This position is located behind the lens and is usually labeled as F in ray diagrams. A similar point the same distance in front of the lens is called the lens' secondary focus, F'.
The distance from the center of the lens to the principal focus is called the focal length of the lens and is represented by the variable, f.
Whenever the actual rays of light that refract through the lens converge behind the lens to form an image, that type of image is called a real image. Real images can be projected onto a screen, are always inverted and reversed left-to-right. For those of you who have ever loaded a slide projector, you know that you must first flip-over and then rotate each slide to insure that the image on the screen will be correctly oriented.
Since the actual rays of light passing through the lens form these real images, they are also known as "hot" images. Remember, that each ray of light is composed of photons which are packets of radiant energy. If you have ever tried to use a magnifying glass to burn a hole in a dried leaf or roast a small piece of a hot dog, then you have experienced this property of real images. You instinctively learned to place the leaf or hot dog at the principal focus of the magnifying glass' converging lens.
Converging Lenses
There are three primary rays which are used to locate the images formed by converging lenses. Each ray starts from the top of the object.
Ray #1 (aqua)
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runs parallel to the axis until it reaches the lens; then it refracts through the lens and leaves along a path that passes through the lens' principal focus
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Ray #2 (gold)
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runs straight through the center of the lens never bending
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Ray #3 (pink)
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first passes through the secondary focus until it reaches the lens; then it refract through the lens and leaves parallel to the lens' axis on the other side of the lens
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Remember, ALL rays must have ARROWS! When all three of these rays meet, they will form the top of the image.
Before continuing to a paper-and-pencil exercise in which you will construct six special cases for converging lenses, we are going to use the following PhET animation to examine the general properties of images formed by converging lenses in air.
When the animation opens notice that the author has listed for you the initial focal length, object distance and image distance. Move the object as far to the left as possible and then notice the position, orientation, and size of the image that is formed as you move the object towards the lens. |