The circle represent the object. You can drag any point in the circle to change it's shape. You can change the focus length f with slider or drag focus point direcly. This applet assume very thin lens.

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Enjoy the fun of Physics! You can access this simulation online from
object in front of concave/convex lens
Designed by Fu-Kwun Hwang (http://www.phy.ntnu.edu.tw/ntnujava/)
Deptartment of Physics, National Taiwan Normal University, Taiwan