Trappa

Meet Trappa, light inside glass or water who, past a certain angle, cannot get out.

When light passes from a denser medium (higher refractive index n₁) to a less dense one (n₂), it bends away from the normal. The steeper the angle of incidence, the more it bends, until at the critical angle θ_c the refracted ray skims along the surface. Beyond θ_c no light is transmitted at all. Every bit is reflected back inside: total internal reflection. The critical angle comes from Snell's law: sin θ_c = n₂/n₁.

For water (n = 1.333) to air, θ_c ≈ 48.6°. A fish looking up sees the whole sky squeezed into a cone of about that half-angle, with reflections of the underwater world beyond it.

Optical fibres guide light over many kilometres by total internal reflection, carrying most internet traffic. Binocular prisms and the sparkle of cut diamonds use it too.

The reflection is total, but a weak evanescent field leaks a tiny distance past the surface. Bring another medium within about a wavelength and light can tunnel across.