The above applet shows: two arrows, a converging lens, and rays of light being emmitted by the red arrow. The red arrow is the object, while the green arrow is the image that results after the rays have passed through the lens. The applet also displays two focus shown as blue dots.
The image formed by a converging lens can be made using only three principal rays.
The above is a useful tecnique, but it usually involves a drawing of some sort. A more practical way is not as complete but is much simpler. You can find the distance of the image from the lens by the following equation:
Interesting things happen when do is equal to or greater than f. When you place the object directly at the focal point, solving the above equation for di we get:
As the object is moved closer toward the lens, the image distance tends to zero from the negative side. This is called a virtual image represented by a gray arrow in this applet. Although the rays of light do not intersect, the mind perceives them to be coming from a point on the other side of the lens (this is shown by the dark green lines), which is infact the location of the virtual image.
Last modified: June 20, 1997
