Pepper's Ghost is real geometric optics, not trick photography. A large sheet of glass (or foil) is tilted between the audience and a brightly-lit performer hidden in a dark pit off to the side. A small fraction of the light from the hidden performer reflects off the glass toward the audience, obeying the law of reflection:
θ_incidence = θ_reflection (measured from the glass normal)
Because reflection preserves distance-from-mirror, the audience's brain reconstructs a virtual image exactly as far behind the glass as the real performer is in front of it — appearing to float on the empty stage:
Q' = Q − 2[(Q − P₀)·N] N
Q = hidden performer position
P₀ = a point on the glass
N = unit normal of the glass plane
The glass simultaneously transmits the ordinary stage lighting, so the audience sees both the real stage and the faint reflection superimposed — the "ghost" only reads as visible when its reflected light beats the stage's direct light in contrast. That contrast is governed by the unpolarized-light Fresnel reflectance of glass at the incidence angle θ (n ≈ 1.5, air n ≈ 1.0):
r_s = (cosθ − n·cosθₜ) / (cosθ + n·cosθₜ)
r_p = (n·cosθ − cosθₜ) / (n·cosθ + cosθₜ)
R(θ) = (r_s² + r_p²) / 2 [Snell: sinθₜ = sinθ / n]
- Glass tilt — sets the incidence angle and therefore both R(θ) and where the virtual image lands on stage.
- Performer distance — how far the hidden performer stands behind the glass; the ghost appears exactly that far in front of it.
- Hidden-chamber spotlight vs. stage ambient — the illusion vanishes the instant house lights compete with the reflected image; keeping the pit bright and the stage dim is the entire trick.
- Cue the Reveal — sweeps the two lights to the classic "ghost appears" stage cue.
This is the same principle used in modern "hologram" concert projections and heads-up cockpit displays — a controlled partial reflection layered over a transparent view.