How a Convex Lens Forms an Image
We have seen in the last video that a convex lens indeed makes an image of our object, the little lit arrow. But how exactly is this image formed? This animation shows you that light rays are going off in all directions from every point of the object. Let's look at one specific point of the object, say the very tip of the arrow. We know that the lens makes all light rays that are coming from this one point converge in one other point, the image of the tip of the arrow. But how do we know where this image point is located? There are three special rays for which we actually know the path. These three rays are called the principal rays. The first one is the one that hits the lens in the middle and goes right through without changing direction. The second one is the ray that goes parallel to the optical axis, so it hits the lens right on, the same way the sun rays hit it in the first experiment. Of course we know where such a ray goes: through the focal point! Now we can see where the image point of the tip of the arrow is located: at the intersection of these two principal rays! We can draw in a third principal ray just for confirmation: A ray that goes through the focal point of the lens leaves the lens parallel to the optical axis. And indeed, this ray also crosses with the two others in the same point! We can now find the image point for every point of the object in the same way - we can construct our image. What we find for the configuration that is shown here is: The image is smaller than the object, the original arrow, and it is upside-down, just as we observed in the video. So for this particular distance between the object and our lens, we are getting a reduced, inverted image.