Pepper's Ghost

Clear acrylic sheet reflects almost as well and will not shatter. If you do use glass, tape the edges and never cut it without eye protection.

Make a rectangular box roughly 300 mm on a side, open at the front where the viewer looks in and open at the top. The top opening is where the hidden object will sit.

Paint or paper every internal surface dead matte black. This single step decides whether the effect looks magical or looks like a reflection in a box. Any bright surface reflects into the acrylic as a ghost of its own.

Cut a sheet wide enough to fit the box interior and long enough to run from the front bottom edge to the back top edge. Peel the protective film only when it is time to install.

Fix it running from the bottom front edge up to the top rear, held in slots or on small blocks. 45° puts the reflected image at the same apparent position as the real scene; a few degrees off and the ghost sits visibly askew.

Support it without clamping it hard. A bowed sheet acts as a weak curved mirror and distorts the ghost — usually seen as a face that stretches when the viewer moves.

Set the object the audience is meant to see — a chair, a model room — in the back of the box, beyond the acrylic. This is what they look at directly.

Put the object to be ghosted on the top opening, face down toward the sheet, hidden from the viewer by the box edge. Nothing about it should be directly visible from the front.

The reflection reverses left and right. Any text or asymmetric object must be placed mirrored to read correctly — the classic mistake, and immediately obvious once someone photographs it.

Measure from the sheet to the ghost object, and from the sheet to where the ghost should appear in the scene. Make these equal, or the ghost will float at the wrong depth and refuse to look solid.

Put a small bright LED on the hidden object and shield it so none of its light spills into the main scene or reaches the viewer's eye directly.

Give the rear scene its own light, kept dimmer than the ghost's. The relative brightness of the two is the only control over how solid the ghost looks.

Bring the ghost light up and the figure materialises; take it down and it dissolves into nothing. The sheet never moves. This is exactly how the 1862 stage effect made spirits appear and vanish.

Look in from the viewing position in a dark room and hunt for stray bright edges, the LED itself, or your own face. Black-flock or tape each one out. This is most of the build time.

What the glass is actually doing. At every boundary between air and a transparent material, some light reflects and the rest passes through. For ordinary glass or acrylic at modest angles that reflection is only about four per cent per surface — weak, which is exactly what makes the effect useful. The viewer sees the bright rear scene through the sheet at ninety-odd per cent strength, and a four-per-cent reflection of the hidden object laid over it. A dim reflection of a brightly lit object can match a well-lit scene, and the result is a figure you can see straight through.

The 1862 debut, and an honest credit. John Henry Pepper demonstrated it at the Royal Polytechnic Institution in London in 1862 and it became a sensation, but the technique was patented by the civil engineer Henry Dircks, who had devised it as "Dircksian Phantasmagoria". Dircks's version required a theatre to be rebuilt around it; Pepper's contribution was to modify it so it could be installed in an existing stage, which is why it spread — and why it carries his name rather than Dircks's. They shared the patent, and Pepper reportedly tried and failed to get Dircks's name attached to it in popular use.

Why calling it a hologram is wrong. A hologram records the phase of a light wave on a medium and reconstructs a genuinely three-dimensional wavefront: move your head and you see round the object. Pepper's ghost is a flat reflected image of a flat or real object, and it has one correct viewing position — move far enough and the illusion breaks. The widely publicised "holograms" of deceased performers at concerts and of politicians on news broadcasts are, without exception, this 160-year-old reflection trick, usually with a projection screen above and a tensioned foil in place of glass.

Where you have used one today. A teleprompter is a Pepper's ghost: the presenter reads text reflected off angled glass while the camera shoots straight through it. A car's head-up display is the same arrangement using the windscreen. Both work for exactly the reason your box does — partial reflection, matched path lengths and careful control of what is lit.