Reflection and Transmission by Colloidal Metals
The Solar Panel Effect

Transmission and reflection vs. photon energy for solid and colloidal

Reflectivity of aluminium in the solid and colloidal forms

Colloidal metals are typically black: hence the use of colloidal silver in photography. In contrast solid metals are lustrous and do not absorb light. Metals are the materials of choice for high reflectivity mirrors. This contrasting behaviour is brought about by the dramatic change that colloidal structure brings to internal electromagnetic modes of the material: colloids have a high density of modes in the visible region capable of absorbing energy from incident light.

Our calculations confirm this picture: solid metal, though not highly transmitting, has a high reflectivity and relatively little of the light is absorbed. In contrast the colloid neither transmits nor reflects in the visible or near ultra violet; absorption falls off in the infra red and the colloid becomes transmitting.

Representation of a Solar Panel

This effect is exploited in solar panel technology. Panels are covered first with a highly reflecting surface, and next with glass containing a suspension of metallic colloid - usually copper. Most of the sun's energy is strongly absorbed by the colloid, but that in the infra red region of the spectrum is lost. However this is a small price to pay because a good reflector of infra red is also a poor emitter therefore the relatively cool solar panel (compared to the sun) loses little energy by radiation. At infra red wavelengths solar panels behave like inside-out thermos flasks.

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