Question

A convex lens is a transparent device that uses refraction to twist and focus light from an object, forming a sharp image. We'll first investigate how lens produces an image from a point source of light (a lamp).

Choose Screen at upper right, which makes the object a lamp and gives a black screen that can be dragged around. 

Choose many rays at upper left to see how the lens bends some of the rays of light from the lamp.

With lamp positioned far to the left of the lens, you must see that rays that go through the lens converge to a point. If the screen is placed where beams converge, the image on the screen will be in focus (it will be a small dot of light because that is what the object looks like in this case). The screen is then at focal plane. As the lamp is moved closer to the lens, the distance between focal plane and lens.