Materials: two regular drinking glasses, a small saucer and a penny (or other coin)
1. Fill one of the glasses almost full with water.
2. Cover the empty glass with the saucer and place the covered glass on the coin. Let your guest observe the only from the side of the glass. (Coin can be seen in sketch A).
3. Fill this glass with water, tilting the glass towards the observer (see sketch B), and place back the saucer.
4. Ask: "Where did the penny go?" Move the glass to the side and reveal the penny.
1. What made the penny invisible?
2. Where do the light rays of the coin travel?
3. Why do we need a saucer to cover the glass?
4. Why does the saucer have to slant toward the observer?
5. What other objects may be used in place of the coin?
The coin is still visible with the empty glass and saucer on top of it, because the light rays are not refracted by the air filled glass, and the coin can be seen from the sides of the glass. By filling the glass with water, the material through which the rays are traveling is denser and thus, when they hit the side of the glass they are being totally reflected or refracted very closely to the side of the glass. The saucer on top of the glass prevents seeing these rays and thus seeing the coin. The saucer has to be tilted towards the observer, while filling the glass with water, for the same reason. Other objects in place of the coin would be, anything which is flat and thin, and that can be covered by the bottom of the glass; like a stamp, a thin button, a piece of paper, a washer, etc.