Photograph Holder with Morphing Prism

Tim bought this toy in New York, back in 1993, from a toy shop that sadly no longer exists. The item is actually made in Italy, and it employs a nice piece of optics.

The photograph holder consists of a prism that is covered on all sides but one with black plastic. Two of the sides can be removed, in order to slot in pictures or photos.

When you look from the front, you will see one of the pictures, and then as you tilt the prism the picture morphs into the other image.

How does it work?

The scientific principle is one called total internal reflection. If you look directly at a glass window, you see through the glass, to what is on the other side. If however you look at the window at an angle, then the glass acts as a mirror.

So when you look into the prism at a shallow angle, you see the image that has been slotted in a the back of the prism, reflected off the bottom surface. For this reason, the picture that you place at the back of the prism has to be upside down.

At higher angles, you see through the bottom face of the prism, and you see the picture that has been slotted in on the bottom face of the prism.

Total internal reflection happens at a very precise angle, which is why the picture changes quite suddenly from one to the other.

By the same token, if you swim underwater at your local swimming pool and then look straight up, you will see a circular window at the surface of the water, where you can see through the surface, up to whatever is above the water, i.e. the sky, or the roof of the pool. However at a certain angle, total internal reflection kicks in, and all around that circular window you will see a reflection of the bottom of the pool.