What if windows were made of diamond?

Look through a window. You can see outside because light goes through the glass.

Diamond is clear too. What if windows were made of diamond?

A window lets light in because glass is clear. But light does something sneaky as it enters the glass: it slows down and changes direction. Scientists call that bending refraction.

Diamond is also clear, and it is famous for its sparkle. So what would happen to light if a window were made of diamond instead of glass? Would the light bend the same way? In this lab you can swap the material and see.

Every transparent material slows light by a factor called its refractive index, n. Crown glass is about 1.52; diamond, at about 2.42, has the highest index in OpenStax's table of common materials, ahead of zircon.

This lab keeps the light, the angle and the flat surface the same and changes only the material. How much more does the beam bend in diamond, and how much more of it reflects?

Olly shines a beam of light at glass. The beam is tilted.

Most of the light goes into the glass. It bends a little as it goes in. A tiny bit bounces off.

In the model, a light beam travels through air and hits a flat glass surface, tilted 40 degrees from the dashed line that points straight into the surface. Inside the glass, the beam bends toward that dashed line and travels at about 25 degrees.

A weak beam also bounces off the surface, at 40 degrees, the same angle it arrived. Only about 4.6 percent of the light bounces back. The other 95 percent goes into the glass, which is why a window is so easy to see through.

With n₁ = 1.000 (air) and n₂ = 1.5 (glass), a ray at θ₁ = 40° refracts to θ₂ = asin(sin 40° ÷ 1.5) ≈ 25.4°, a bend of 14.6°. The reflected ray leaves at 40°.

The model's unpolarized Fresnel reflectance at this angle is 4.6%, so 95.4% of the light is transmitted. Straight on (0°) it is 4.0%, for this one surface.

Now Olly changes the glass to diamond.

The beam bends much more! And more light bounces off the shiny surface.

Now switch the window material from glass to diamond and keep everything else the same. The beam still arrives at 40 degrees, but inside the diamond it travels at only about 15 degrees from the dashed line. It bends about 25 degrees instead of about 15.

The reflection changes too. About 17.8 percent of the light now bounces back, nearly four times as much as from glass. Even shining straight at the surface, about 17 percent reflects. In the model, a diamond window would act a little more like a faint mirror.

With n₂ = 2.42 the same 40° ray refracts to 15.4°, a bend of 24.6°. Fresnel reflectance rises to 17.8% at 40° and 17.2% at normal incidence, where R = ((n₂ − n₁) ÷ (n₂ + n₁))² gives (1.42 ÷ 3.42)² ≈ 0.172.

These figures are for one surface only. A real window has two surfaces, and our model shows only the first one, so it does not give the total for a whole pane.

Light slows down when it goes into glass. It slows down even more in diamond.

The more it slows, the more it bends.

Light travels fastest in empty space. In any material, it slows down, because it interacts with the atoms there. Diamond slows light much more than glass does: light moves through diamond at well under half its speed in air.

When a tilted beam slows down suddenly, it changes direction, like a shopping cart whose front wheel hits grass first and swings around. A bigger slowdown means a sharper turn. Our model treats the light as a single color and the surface as perfectly flat and clean.

The refractive index is n = c ÷ v. With n = 2.419 (OpenStax's table value for diamond), light in diamond travels at about c ÷ 2.42, roughly 124,000 km/s. Snell's law, n₁ sin θ₁ = n₂ sin θ₂, follows from matching the wavefronts across the boundary, so a larger n₂ gives a smaller θ₂ and a sharper bend. Fresnel's equations show reflection grows with the index mismatch.

Model limits: one index per material, so it ignores dispersion (the color dependence that gives diamond its fire); a flat, clean, non-absorbing surface; unpolarized light; and ray optics. Weak reflected beams are drawn brighter than their true strength.

Light that goes into a diamond gets trapped and bounces around inside.

That is why a cut diamond sparkles. Scientists even make tiny diamond windows for special machines!

Diamond's strong bending has a big effect inside the stone. Light inside a diamond can only escape if it hits the surface at less than about 24 degrees from the dashed line; at steeper tilts it is all reflected back inside. Gem cutters place the flat faces, or facets, so light bounces around and comes out in particular directions, which makes a cut diamond sparkle.

Diamond windows are real, too, but small ones. Scientists grow thin diamond films from hot carbon-containing gas and use them as windows in some optical instruments. Ordinary window glass is made mostly from sand, limestone and soda.

OpenStax notes that diamond's critical angle against air is only 24.4°, and that total internal reflection together with its large refractive index explains why diamonds sparkle more than other materials; facets are placed so light can exit only in particular ways. Britannica adds that diamond's high dispersion splits white light into colors, the diamond's fire.

Diamond films grown by chemical vapor deposition serve as windows for optical devices, as cutting-tool coatings and as semiconductor substrates (Britannica). Sheet glass, by contrast, is a soda–lime–silica glass made mainly from sand, limestone and sodium carbonate; that class makes up most of the glass produced by weight.