What to see?
This optical illusion is called the Hallow-Face illusion and it leaves us the impression that a concave mask is a normal face. When we look at these images we can easily see the existence of two faces, if we try to look carefully at them to spot that thing that makes them different, all we’ll get will be two faces, one on each side of the figure.
What to do and how it works?
The convex face will always appear to look in just a direction, but the flat face like the Lord Kitchener Wants You seems to follow you with its eyes. The most used are the flat faces, how many times haven’t you entered in a museum and you haven’t got at least once the impression that “this man in the painting is watching me!” A hollow face on the other hand seems to move its eyes faster than the viewer; it seems that the face looks forward when you look directly ahead.
The reason why we perceive these faces as being normal and convex is because the depth cues such as shading and shadows interfere whit the visual system. A very well known illusion is the “Gathering For Gardener” dragon. This dragon head leave us the impression that it follows the viewers eyes from left to right and from up to down. If this is in the right light you can’t tell that this face it’s actually hollow because the stereoscopic cues aren’t strong enough. In this case if you will pay attention you will be able to see that not the head of the dragon moves but appears to turn twice as fast around its center than they do themselves.
A better effect can be obtained if the light source is in the proper position because the effect can be stronger for the eye. Somewhere in the world there is a room in a museum filled with hollowed faces. Every person that enters in this room first has the impression that the faces are normal but in reality they are more like some holes in the wall.
If you pay attention at the two faces below you may think that they are both normal faces, but one is pointing out and the other is hollow. Take a few moments to tell which is which…
Now let me tell you which is which. If you pay attention at the picture on the left you will see that is hollow and all you have to do is to follow the light, the shadow. Same goes for the picture on the right, only this one is a normal face.
What to see?