Two or more views of a scene are printed on a thermoplastic sheet of lenticular (concave) lenses that look like parallel ridges (see below). In order to align the image to the lens array ...

Draw a ray which passes from the object through the centre of the lens. Some ray diagrams may also show a third ray. Peep holes contain concave lenses. They are set into doors so the occupant can ...

the lens being too thick and curved - so that light is focused in front of the retina. Short-sightedness can be corrected by placing a concave lens in front of the eye, as shown in the diagrams below.