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Images and illusions.

During a teaching career, you accumulate in your head many ideas for future use in demonstrations and activities. Here are a few of those that I have used.


I saw this idea demonstrated when I was studying physics at university. As we walked into the lecture theatre, the lecturer said, "Look at the light globe on the box on the front bench." We looked at the globe; but when we sat down we realised that it was an illusion. There was no globe. A 'real image' of a lamp inside the box was being produced by a concave mirror.

Some years later, I found a suitable mirror to replicate this demonstration. I retrieved a large concave mirror from an old kerosene lamp. The mirror was 34 cm in diameter and had a focal length of 21 cm. When accurately mounted it produced, at its centre of curvature (double the focal length), an amazing real image of the hidden light globe.


This is based on the toy moneybox that hides the money inside. The invisible box is 31 cm by 31 cm by 42 cm. There is a finger hole on the top and one on the side. There is a window in the front. When you put your finger in one of the holes and look through the front window, your finger is 'invisible'.

The invisible box has a slanting mirror (at 45 degrees) inside. The visible interior is lined with striped wallpaper to help hide the edges of the mirror. The hidden interior, behind the mirror, is painted black. Black dots are accurately placed on the walls inside to create internal surrogates for the hole on the top and on the side.

While the invisible box is very amazing, it is at its most spectacular when a person drops a $20 note into the hole and the money disappears.


Before video cameras were miniaturised, endoscopes, used by doctors to view inside the human body, made use of fibre optics. The cable consisted of a bundle of glass fibres. When an image was focused on one end of the fibre bundle, the light transmitted by the fibres created a pixilated image that could be viewed by the doctor. Endoscopes of this type can be obtained after they have been retired by the medical profession.

However, images transmitted by fibres also occur in nature. It occurs in ulexite crystals; also known as 'television rock'.


Calcite crystals are birefringent. They have two refractive indices. Thus, if you sit a calcite crystal on a printed page, you see a double image.

Large blocks of acrylic can be easily cut and polished to dramatically show real and apparent depth.


A Stanhope lens was a simple, one-lens microscope invented by the Earl of Stanhope. It consisted of a small cylinder of glass, convex at each end. The object to be viewed was held near to one of the curved ends, and was viewed through the other end.

A popular souvenir for tourists in the past was a Stanhope viewer (invented by Rene Dagron in 1857). He modified the Stanhope lens by making one end flat. A microphotograph was attached to the flat surface and the lens was mounted in a souvenir, such as a cross or mini binoculars. The microphotograph was viewed by holding the lens up close to the eye.

The Stanhope lens in tourist souvenirs is usually only about 2 mm in diameter and about 4 mm long. A demonstration Stanhope lens can be made out of Perspex rod with one end polished flat and the other hand polished to convex.

The United Nations has declared 2015 as the International Year of Light and Light-Based Technologies

The Year of Light and Light-Based Technologies promotes widespread access of information and activities involving this science. Find out more at

Richard Rennie was a science teacher for 38 years. He now assists Western Australian schools with programs through the Light and Sound Discovery Centre.

All images courtesy of Richard Rennie unless otherwise attributed.
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Title Annotation:International Year of LIGHT and Light-Based Technologies; teaching materials
Author:Rennie, Richard
Publication:Teaching Science
Date:Dec 1, 2015
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