REFLECTION – Light bounces. We can reflect light. We use this all the time in mirrors. But by reflecting light we are reflecting energy. It is possible to concentrate light energy by using several mirrors to reflect light to the same spot. Or by using a curved mirrored surface.Concentrating Solar Power (CSP) technologies “…use mirrors to concentrate (focus) the sun’s light energy and convert it into heat to create steam to drive a turbine that generates electrical power.” Here, a good overview of the different types.Parabolas: Turns out that the curve most suited to concentrating solar energy (that is, reflecting it back to a focal point) is a parabola, not a half-circle.
Arba Minch Solar Cooking Initiative: “…experiences on promoting the use of a simple parabolic solar cooker to save firewood in Ethiopia.”It has been recently re-discovered that the early Chinese used polished brass parabolic mirrors to start evening cooking fires.
See National Geogrphic’s Energy Blog, “Seven of the Greatest Solar Stories over the Millennia”convex
REFRACTION – Light bends. Another way to concentrate light energy is to bend the rays of light so that they gather together (focus) at one particular spot. We use this most often in lenses.
An interesting lense design is the fresnel lense. August Fresnel, a French mathematician and physicist of the early 19th century, duplicated the effect of a huge convex lense using glass prisms. In this age of plastic, thin circular prismatic lines can be stamped out on a plastic sheet. Fresnel lenses are now sold in office supply stores in 8″ x 11″ sheets as page magnifiers, for approximately $2.00.
We perceive leaves as green because they are reflecting radiant energy of a wavelength that our eyes and brains “see” as the colour green. The other light energy wavelengths are being absorbed by the leaves.
At the extremes of colour reflection are the “colours” white (all colours combined) and black (no colours at all). A white object is reflecting nearly all of the light energy falling on it.
A black object is absorbing nearly all of the light energy falling on it.
Black collector plates and black cooking pots are most often used in solar heat collectors or cookers.
Light energy absorbed is re-radiated as lower wavelength heat radiation. An object of any temperature will radiate energy as electromagnetic radiation. The hotter an object is, the shorter the wavelengths of electromagnetic radiation that it emits (and the higher the energy per photon). The Sun radiates energy from its heated outer gases at about 5800 degrees Kelvin, and so emits energy peaking in the short wavelengths of our visible light range.
When an object is heated up on the Earth’s surface, by absorbing solar energy, it is never as hot as the sun. The electromagnetic radiation it emits will be of lower energy and longer wavelengths.
See also, as mentioned before : Electromagnetic Radiation, Temperature, Colour, Light
But the heat stays back. There is a feature of glass that allows us to trap solar energy. Glass is somewhat like a one-way valve to solar radiation. It allows the wavelengths of light and near-infra-red from the Sun to pass through it, but it is not as transparent to the longer wavelengths of farther-infra-red radiant energy emitted by solar-heated objects behind the glass. Glass reflects back most of these longer wavelengths of infra-red radiation.
What this means is that energy can build up in a glass solar energy trap. While energy is coming in all the time, it can’t re-radiate back out directly through the glass. Most of us have experienced this effect in a car in summer-time, or in front of a house window through which the sun is shining. It’s the working principle of greenhouses, and solar box cookers. Selective films allow us to control to some extent which wavelengths enter, which are reflected. Different glasses can allow different wavelengths of radiation to enter.
Principles of Solar Cooker Design: [This is the best I’ve found for explaining solar energy principles clearly. There is simply nothing like building a solar cooker to get hands-on experience with solar energy, building techniques…and cooking!]