Figure shows lens the occurrence of lens flare and the usefulness of a proper lens hood. There is a regular light ray that belongs to the photographed scene, this is the blue ray that arrives within the lens angle of view indicated by the brighter gray. This ray is normally refracted and contributes to the image formation. (Note however that I did not attempt genuine ray tracing, the light paths in figure 1 are merely illustrative.) There is also a slanting ray of sunlight that impinges upon the front element when a lens hood is absent. A small part of the light is reflected from the rear surface of this element (red arrow). A fraction of this light is reflected again from the front surface, directed to the film (dashed red line). This is flare.