In Australia Christmas occurs in midsummer and is associated with long hot days, shimmering heat, cicadas' chorus and trips to the beach, yet many of our Christmas trappings and traditions relate to frosty midwinter weather. The experience of a snowy winter is certainly entrancing with its biting, crisp air; the crunch of snow underfoot; an indescribable clear scent and the muted shades of a landscape dominated by gleaming white snow.
Ice is translucent, yet snow, which is composed of millions of ice crystals, appears white. To understand why, we have to consider light and how we perceive it.
Light is electromagnetic waves - travelling energy in the form of vibrating electric and magnetic fields. The electromagnetic spectrum in principle is continuous and infinite but the only band we can see is the visible spectrum of light.
Sunlight appears to be white, but by passing sunlight through two prisms Isaac Newton demonstrated that white light is actually a mixture of colours called the visible spectrum. We can see this in rainbows, when water in the atmosphere splits white light into the colours of the visible spectrum – red, orange, yellow, green, blue, indigo and violet (in order of longer to shorter wavelengths). The electromagnetic bands on either side of the visible spectrum are infrared light and ultra violet light; these bands may be visible to other species but are not to us.
When a stream of light photons, the elementary particles of light, hit an object, the atoms and molecules respond by absorbing and emitting energy. Types of atoms and molecules respond differently, but consistently, to certain light frequencies by changing energy levels.
It is strange to think that no object actually has any colour. Colour is merely a combination of a multitude of reflected frequencies of light waves that are reflected and absorbed by the atoms and molecules on the surface of objects. Eg green plants contain chlorophyll, which absorbs blue and red colours, but reflects green; therefore our eyes perceive the plants as green.
Our eyes and brain have the ability to process this complex multitude of reflected frequencies into a coherent impression of our environment.
If we return to the ice versus snow, clear versus white question we have to consider how ice and snow react to light.
If an object is transparent its atoms and molecules re-emit light photons to the next atoms and molecule so that light passes through directly. Ice is translucent not transparent. The distance between its atoms is so close to the wavelength of light that the photons interact with the structure of the ice, the photon can move through the ice but its path is altered.
Snow consists of an arrangement of millions of ice crystals. When a light photon enter the first crystal its direction is changed and it is re-emitted to the next crystal, the process continues and the ice crystals scatter and bounce all frequencies of light equally so we see this as reflected white light.