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A light ray is refracted (bent) every time it passes between two media with different indeces of refraction. Lenses exploit this property to focus light. For example, if rays of light arrive at a lens parallel to its axis, they will leave in a non-parallel way (see the picture below). In the previous lab, you examined focusing with the two basic types of lenses: convex, also known as converging, and concave, also known as diverging. You saw that both types of lenses can be characterized by a focal point. With the converging lens, rays of light parallel to the axis converge at the focal point; with the diverging lens, parallel rays appear to come from the “virtual” focal point.
The distance from the lens to the focal point is called the focal length, f. By convention, f > 0 for a real focal point (as obtained with a converging lens) and f < 0 for a virtual focal point (as obtained with a diverging lens).
If an object is placed a distance in front of a converging lens, rays of light coming from each point in the object will meet at a corresponding point in the resulting real image. The image is located a distance from the lens. If a diverging lens is used instead, there is no place where the rays of light meet. However, the rays can be traced backward to where they appear to have originated; there is a virtual image of the object.
By convention, the object distance is always chosen to be positive; the image distance , is positive for a real image formed on the other side of (“behind”) the converging lens from the object, and negative for a virtual image formed on the same side of the diverging lens as the object (“in front of” the lens).
(covex)
(concave)
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