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refraction and reflection. See that? From certain points of view
you would see quite clearly through it. Some kinds of glass would
be more visible than others, a box of flint glass would be brighter
than a box of ordinary window glass. A box of very thin common
glass would be hard to see in a bad light, because it would absorb
hardly any light and refract and reflect very little. 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 would vanish almost
altogether, because light passing from water to glass is only
slightly refracted or reflected or indeed affected in any way.
It is almost as invisible as a jet of coal gas or hydrogen is in
air. And for precisely the same reason!"
"Yes," said Kemp, "that is pretty plain sailing."
"And here is another fact you will know to be true. If a sheet of
glass is smashed, Kemp, and beaten into a powder, it becomes much
more visible while it is in the air; it becomes at last an opaque
white powder. This is because the powdering multiplies the surfaces
of the glass at which refraction and reflection occur. In the sheet
of glass there are only two surfaces; in the powder the light is
reflected or refracted by each grain it passes through, and very
little gets right through the powder. But if the white powdered
glass is put into water, it forthwith vanishes. The powdered glass
and water have much the same refractive index; that is, the light
undergoes very little refraction or reflection in passing from one
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