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Lowrey Brown

©1997 Lowrey Brown

Lowrey Brown

©1997 Lowrey Brown

Both solar ovens and PV panels have glass between the outside air and the place where work gets done. This glass can interfere with the transmission of sunlight which is the lifeblood of both these technologies. As the sun moves overhead, the angle at which the solar radiation strikes the glass surface changes. The amount of energy which is reflected by the glass is a function of the angle at which the radiation strikes it. This article presents a simple calculation showing how much energy is reflected from a pane of glass as the angle at which the solar radiation strikes the glass changes.

Reflection and Refraction

When an electromagnetic wave (sunlight is electromagnetic radiation) travels from one medium to another, some of the radiation is reflected back into the first medium and some is transmitted to the second medium. The radiation which is transmitted into the second medium may be refracted (bent). How much radiation is reflected or transmitted depends upon, among other things, the wavelength of the radiation, the properties of the mediums, and the angle at which the radiation strikes the interface between the mediums. This angle is the angle of incidence and is measured in degrees from perpendicular to the interface.

In the case of a pane of glass, the electromagnetic radiation must pass from outside-air into glass, and from the glass into the working-space in order to be useful. Not only must the radiation pass through two material interfaces, but the light is refracted as it enters the glass so the angle of incidence as the light strikes the glass to working-space interface is different than the original angle of incidence when the light passed through the outside-air to glass interface. See figure 1.

Calculations

The percentage of radiation which is reflected or transmitted at a given angle of incidence is calculated using Maxwell's equations. Below are graphs of the solutions to Maxwell's equations for light (electromagnetic radiation of wavelength about 589 nm) entering glass from air, and light entering air from glass. See figures 2 and 3.

We must first figure out how much light is reflected and how much is transmitted as light hits the glass for a given angle of incidence (lets say 60 degrees). This is easy, we just look at graph 1 and read along the axis labeled, "Angle of Incidence" to 60 degrees and then read up, parallel to the axis labeled, "Energy in %," and see where the reflected wave curve crosses the 60 degree grid line (9%), and where the refracted wave curve (which is also the transmitted light ) crosses the 60 degree grid line (91%). These should add up to 100%, unless our radiation is disappearing somewhere.

Incident Wave

Reflected Wave

Transmitted Wave \

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