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Teacher’s notes for the Thaumatrope and it’s Optical Illusion

Have fun with the thaumatrope! Kids really enjoy finding out about themselves and learning about the limits our own bodies place on our ability to observe reality is an important part of being a scientist.

Teacher’s Notes PDF

What is actually going on?

Kids will be very comfortable with the concept of a camera and are likely to apply this conceptual model to how they see – and certainly the eye itself follows the same rules of optics as a camera. However we “see” not just with the eye but with the brain.

The eye collects the light and focuses it on the retina at the back of the eye. Here, the light signals are turned into electrical signals that are transported to the brain by the optic nerve.

It is the brain that processes these signals and assigns meaning to them. This processing takes time and our brains can only ’see’ about 20 images a second. If images are presented faster than this, then they merge into one another. Movies consist of a sequence of still images captured by a movie camera at about 24 images (frames) every second. These are then projected on to the movie screen at the same rate and we see them as an uninterrupted moving picture. TV uses this same principle, but this works at 25 images a second.

Early (silent) movies used 16 images (frame) per second but this speed was not really fast enough to present as smooth movement. These old movies are now shown at 24 frames per second and it is this speeding up that gives these movies their characteristic ‘faster than normal’ view of the world.

What is happening in the brain?

Originally, the illusion of the thaumatrope was explained by invoking a ‘processing delay’ in the brain. This delay was called persistence of vision to explain how a new image ‘runs into’ a previous image. This theory uses the idea of an after image which remains in our vision system which merges with the next image in a sequence of rapidly changing images.

As we gradually understand more about how the brain works, this simple model appears to be inadequate and modern theory suggests a pair of visual illusions are working together. These are Phi phenomenon and Beta movement.

Teacher’s Notes PDF