Lenses work very much like mirrors. We will discuss two types of mirrors: the double convex and the double concave.
The double convex lense is a converging lense. When light waves parallel to the principal axis from an infinitely far object passes through the lense, it will converge at a focal point F on the principal axis. The distance between the focal point and the lens is the focal length, which is always a positive value for converging lenses.
The double concave lens is a diverging lens. When light waves from an infinitely far object passes through the lens, the light waves will diverge as if it originated from a focal point F on the principal axis. The focal length is always a negative value for diverging lenses.
Can you see where these two types of lenses could be applied?
Just like the mirrors, there are three principal rays for lenses used for locating the image. Notice that because the mirrors work the same in both directions, there are two focal points. Also, notice that all images created by lenses are real images.
The object distance, image distance, and focal length are also related by the image equation: 1/do + 1/di = 1/f
For this reason, the image equation is also known as the thin lens equation.
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