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Light finds nothing to grab

Why Can You See Through Glass?

Glass is clear because its electrons cannot absorb visible light, and it has no crystal grains to scatter it.

Light arrives It passes straightthrough
Nothing in glass can take a bite out of visible light.

  1. Step 1 of 5

    Being solid has nothing to do with it.

    Brick Solid — and opaqueGlass Just as solid — andclear

    Glass is every bit as solid as a brick. What matters is not how packed the atoms are, but whether they can take energy out of the light.

  2. Step 2 of 5

    Electrons can only take certain sized bites.

    A packet of visible lightToo small for the electrons to acceptSo nothing absorbs it

    An electron can absorb light, but only in one exact amount. In glass, that amount is bigger than any visible light can offer, so the light is simply not taken.

  3. Step 3 of 5

    Glass does stop the light you cannot see.

    Ultravioletmostly blocked —no sunburnindoorsVisible lightpasses straightthroughInfrared heatpartly blocked

    Ultraviolet packets carry more energy, and those the electrons will take. That is why a window keeps you from burning but not from feeling warm.

  4. Step 4 of 5

    Most solids are cloudy because of their grains.

    A crystal solid Light scatters at everygrain edgeGlass No grains — nothing toscatter off

    A metal or a rock is a mass of tiny crystals pointing every way. Light bounces at every join. Glass has no crystals at all, so there are no joins.

  5. Step 5 of 5

    Crush glass and it turns white.

    1 One clear sheet2 Ground intopowder3 White — lightbounces offevery speck

    Grind a clear pane to powder and it goes opaque, because now you have made millions of surfaces for light to bounce off. Same glass, no longer clear.

    A modern fibre-optic cable is so pure that light travels several kilometres through it before it dims by half. Ordinary window glass would swallow it in a few metres.

The short version

Glass is clear because its electrons cannot absorb visible light and it has no crystal grains inside to scatter it.

Try it yourself

Look at the narrow edge of a glass shelf or a thick window. It is green — over that distance even good glass absorbs a little red light.

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