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Directional transparency

(published 10 January 2011)

Transparency (and, as it transpires, this is also true of honesty) is not, after all, an absolute thing. It is a matter of the energy gap, apparently. My energy gap is such that I am transparent to X-rays and cosmic rays, but light and heat do not penetrate too far into me.

The passage of light (and other radiations) through things, or their absorption by things, relies on spaces of the right size being available for photons to be acquired as they interact with the thing. If they are not absorbed these radiations either pass on through (for example high energy cosmic rays) or bounce back off the thing being subjected to them. Light in the visible range bouncing off and scattering from things results in us being able to see them – the basis of visual observation. The leaves of the geranium on my windowsill absorb all but the green portion of the visible spectrum (and use this light for photosynthesis); the unused green light is reflected and scattered away, giving the appearance of a green leaf. The flowers of this fragrant wonder, however, absorb all but a beautiful deep crimson, a colour presumably used to entice aerial suckers to come and interact with the soft, velvety petal, to feed on the nectar offered up, and thus to pollinate the plant.

Optical fibres are made of silica (SiO2), which is transparent to visible light. Pure silicon is transparent to electromagnetic radiation of wavelengths greater than 1.1 micron, which is in the infrared, but is not see through in the visible range. Silicon is electrically ‘very good’, but this lack of visible transparency can, I believe, cause problems for those hoping to integrate electronics and optics in silicon.

Glass is transparent in the visible spectrum, so we can see through it (and thus use it for windows), and has a manipulating transparency to infra-red light, as a result of which we make greenhouses out of it and use it for passive solar heat gain in houses. (I must admit that I am unclear as to how heat can pass through a window one way but then not be able to get back out again – directional transparency? Imagine if the same thing happened with the visible light, my greenhouse would become brighter and brighter internally through the course of a sunny day (although I’m not sure we’d be able to see that light from outside the greenhouse – it would be blinding to walk into though), and then slowly the radiance would dissipate and diminish over night, as the light seeped back out into the surrounding dark).

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