The focal length decides the relative positions of the object and image from the lens (and also the magnification). If the distance of the object from the lens is u, and that of the image (screen if the image is real,)
is v , and f is the focal length,then they are related by
1/f = 1/u + 1/v.
In a converging or covex lens, from the above formula if the object is at a very large distance, which we can treat as infinity, then we have v = f.
Thus if the image of the sun is formed on the screen, the *distance betwen the screen and the lens is the focal length.* |