Disappearing Prism
refractive index - refraction of light - invisibility
What it shows:
Light is refracted as it passes between two transparent materials of different
refractive indices. If the materials are different, but the refractive indices
are not, then the light rays are undeviated and the materials are optically
indistinguishable.
How it works:
"And if you put a sheet of common white glass in water, still more if you put
it in some denser liquid than water, it will vanish almost all together, because
the light passing from water to glass is only slightly refracted or reflected or
indeed affected in any way."
— H. G. Wells, The Invisible Man
A glycerin/water solution is prepared (see setting it up) with the same refractive
index as quartz (n=1.46). Into a beaker of this solution is placed a quartz prism.
The refractive indices being identical means that there is no refraction at the
glycerin/quartz boundary and light rays pass through the beaker undeviated. The
prism is invisible. The effect is particularly noticeable on the prism face where
total internal reflection has occurred while the prism is in air. To avoid sticky
fingers, the prism is attached to a glass rod with epoxy resin; this adds to the demo,
as the glass rod, with its different refractive index, is still visible in the glycerin.
Setting it up:
The refractive index of glycerin can be altered by diluting it with water. The use
of a beaker of pure water as well as the glycerin/water mixture will emphasize the
requirement of identical refractive index.
Comments:
The prism's disappearance is not complete, as the sharp vertices of the prism are
still visible as pencil lines. Rating***
Reference:
H. G. Wells (1892) The Invisible Man, Bantam Books, 1988