The finger rule for mirrors is a quick test for detecting two-way mirrors: place your fingertip directly against the mirror surface and observe the gap between your finger and its reflection.
On a standard mirror, the reflective silver coating sits behind a layer of glass, so a visible gap of roughly a quarter-inch appears between your finger and its reflected image. On a two-way mirror, the coating is on the front surface, which means your fingertip and its reflection will appear to touch with no gap. The finger rule works because glass thickness determines whether that gap exists — it is not specific to any mirror brand or finish.
- Standard mirror gap: approximately 0.25 inches (6 mm) between fingertip and reflection.
- Two-way mirror result: zero visible gap — fingertip and reflection appear to touch directly.
- The gap exists because standard mirror silver coating is applied to the rear glass surface, not the front.
- Xramfy mirrors use 5-layer float glass with a copper-free silver coating applied to the rear — a standard construction that produces the expected quarter-inch gap.