Optical illusion with concave mirror

Demonstration

This experiment is well worth the effort of setting it up.

Apparatus and Materials

• Concave spherical mirror, large, with holder
• Red marble
• Blue marble
• Small cardboard box (5 cm or larger) e.g. matchbox
• Stand for box
• Small reading lamp
• Plasticine or Blu-tack

Health & Safety and Technical Notes

Read our standard health & safety guidance

The concave mirror should be as large as possible and preferably have an aluminized front surface. Suitably sized mirrors are obtainable from the supplier: Ocean Direct.

For the additional effect with a lamp, the aperture diameter across the mirror's face should be at least as big as the mirror's radius of curvature.

Photo courtesy of Mike Vetterlein

Procedure

1. Fix the red marble, R, on the under side of the top of the small box using Plasticine or Blue-Tack. Place the box with its open side facing the mirror at such a height that the red marble is near the centre of curvature of the mirror, a very small distance below the mirror's axis. The red marble is thus hidden from the observer.
2. Place the blue marble, B, as a decoy, to catch the observer's eye. This is also fixed with Plasticine or Blue-Tack to the box, but on the upper side of the top. B is the same distance from the mirror as R, but a little above the axis and a little to one side. (As B is above the box it is clearly visible to an observer.)
3. Face the mirror and look at the visible marble, B. You can see an equally real red marble beside it.

Teaching Notes

• It is essential to place the blue marble carefully so that the two marbles do not seem to overlap but appear side by side. A little empty bed of Plasticine, or Blue-Tack stuck beside the blue marble, will look like a support for the red marble's image.
• With the object placed at the centre of curvature of the mirror, there is no spherical aberration.
• You can look at the image with a magnifying glass. This then becomes a Newtonian telescope.
• For an additional effect, place a small lamp (spotlight) beside the observer's head so that it lights up the visible marble B. Any rays from the lamp which hit the image beside the visible marble will pass right through that image, continue to the mirror, and be reflected to the concealed marble R. This means that the imaginary marble is illuminated as well.
• Both marbles must be located correctly. Shields of black paper or card can be arranged to limit the observer's view. The demonstration works best as a set piece with a frame constructed especially for it.
• With a mirror of smaller aperture, the concealed marble needs to be illuminated directly by a lamp concealed somewhere between the marble and the mirror.
• Scientific toy shops have a version of this consisting of two concave mirrors facing each other, one above the other. The top mirror has a hole cut in it, and an object placed just below the hole on the bottom mirror will produce an image which appears to float in space.

This experiment was safety-tested in January 2007